The Hydrophilic Amino-Terminal Arm of Reovirus Core Shell Protein λ1 Is Dispensable for Particle Assembly

ABSTRACT The reovirus core particle is a molecular machine that mediates synthesis, capping, and export of the viral plus strand RNA transcripts. Its assembly and structure-function relationships remain to be well understood. Following the lead of previous studies with other Reoviridae family members, most notably orbiviruses and rotaviruses, we used recombinant baculoviruses to coexpress reovirus core proteins λ1, λ2, and σ2 in insect cells. The resulting core-like particles (CLPs) were purified and characterized. They were found to be similar to cores with regard to their sizes, morphologies, and protein compositions. Like cores, they could also be coated in vitro with the two major outer-capsid proteins, μ1 and σ3, to produce virion-like particles. Coexpression of core shell protein λ1 and core nodule protein σ2 was sufficient to yield CLPs that could withstand purification, whereas expression of λ1 alone was not, indicating a required role for σ2 as a previous study also suggested. In addition, CLPs that lacked λ2 (formed from λ1 and σ2 only) could not be coated with μ1 and σ3, indicating a required role for λ2 in the assembly of these outer-capsid proteins into particles. To extend the use of this system for understanding the core and its assembly, we addressed the hypothesis that the hydrophilic amino-terminal region of λ1, which adopts an extended arm-like conformation around each threefold axis in the reovirus core crystal structure, plays an important role in assembling the core shell. Using a series of λ1 deletion mutants, we showed that the amino-terminal 230 residues of λ1, including its zinc finger, are dispensable for CLP assembly. Residues in the 231-to-259 region of λ1, however, were required. The core crystal structure suggests that residues in the 231-to-259 region are necessary because they affect the interaction of λ1 with the threefold and/or fivefold copies of σ2. An effective system for studies of reovirus core structure, assembly, and functions is hereby established.

The orthoreovirus (reovirus) core particle is a molecular machine that mediates synthesis (transcription), 5Ј-end modification (capping), and export of the 10 viral mRNAs (capped plus strand transcripts), as well as import of the substrates, nucleoside triphosphates and S-adenosyl-L-methionine, to produce those mRNAs (53).The core does not include machinery to polyadenylate the mRNAs, which instead terminate with the conserved 3Ј pentanucleotide UCAUC (4,5,28).The templates for plus strand synthesis are the minus strands of the 10 linear double-stranded RNA (dsRNA) segments that constitute the reovirus genome and that are retained within the core throughout the transcription cycle (1,51,55).Further studies of core structure and functions are needed to understand how the different components of this machine are organized to allow for controlled movements of the large template and product RNAs during the multiple rounds of mRNA synthesis that cores can routinely execute from each genome segment (55).Even less is known about how this machine undergoes assembly.The assembly of the three major proteins of reovirus cores to generate core-like particles (CLPs) in a foreign expression system is the subject of this report.
As demonstrated by the reovirus core crystal structure (50), the asymmetric unit from which the icosahedral capsid is built includes an asymmetric dimer of the 142-kDa 1 protein.Five of these 1 dimers surround each of the 12 fivefold axes in cores, and these decamers may represent subassemblies from which the full Tϭ1 capsid is later built (60 1 dimers in total) (29,50).Two proteins sit atop the 1 shell in cores: a pentameric turret of the 144-kDa 2 protein around each fivefold axis (60 total 2 subunits per core) and monomeric nodules of the 47-kDa 2 protein around or across each of the icosahedral symmetry axes, five surrounding and contacting the base of the 2 turret around each fivefold axis (2.5), three around each threefold axis (2.3), and one across each twofold axis (2.2) (150 total 2 subunits per core) (50).These 2 subunits may serve as clamps that hold the 1 shell together (31), with 2.5 bridging 1 subunits from the same decamer and 2.3 and 2.2 bridging 1 subunits from adjacent decamers (50).The two remaining proteins in cores are structurally minor in that they are present in lower copy numbers than the others: 12 copies of the 142-kDa 3 protein and an estimated 12 to 24 copies of the 83-kDa 2 protein (14).The precise locations of these last two core proteins remain to be defined, but a variety of evidence including that from X-ray crystallography (50) and transmission cryoelectron microscopy (cryoEM) (18) places both 3 and 2 near the fivefold axes, directly beneath and probably on October 13, 2019 by guest http://jvi.asm.org/Downloaded from anchored to the 1 shell.The 10 dsRNA genome segments are coiled within the core interior in a fairly dense, yet poorly understood, arrangement (18,19,50).
Not discussed above is information from the core crystal structure about the amino (N)-terminal 200 to 300 amino acids of 1.The two 1 subunits within each asymmetric unit are arranged such that one subunit approaches the fivefold axis (1.5), whereas the other subunit is excluded from the fivefold area and instead approaches the threefold axis (1.3) (50).As a result, five 1.5 subunits from the same decamer surround each fivefold axis and one 1.3 subunit from each of three different decamers surrounds each threefold axis.In the core crystal structure, the N-terminal 240 amino acids of the 1.5 subunits are not seen and thus must be irregularly placed relative to the core's overall icosahedral symmetry (50).In contrast, all but a few of these N-terminal amino acids of the 1.3 subunits are seen in the crystal structure and take the form of long threadlike arms that extend from their subunits of origin and progressively underlie, and make major contacts with, each of the two other 1.3 subunits around each threefold axis (50) (see Fig. 10).These arms, by linking the subunits from the three different decamers around each threefold axis, may play an important role in assembling the Tϭ1 1 shell.Their extended conformation is consistent with a sequence-based observation that the N-terminal 150 to 200 residues of 1 are much more hydrophilic on average than the rest of the protein (27).
It has been proposed that four of the five core proteins (1, 2, 3, and 2) act as enzymes during viral mRNA synthesis.3 is the RNA-dependent RNA polymerase that synthesizes the plus strand transcripts (17, 46, 57; Y. Tao, D. L. Farsetta, M. L. Nibert, and S. C. Harrison, submitted for publication), whereas 2 is the capping enzyme that mediates the RNA guanylyltransferase, 7-N-methyltransferase, and 2Ј-O-methyltransferase activities that complete formation of the 5Ј cap as each transcript is exported from the core (13,22,32,39,41,50,52).The roles of 1 and 2 are less clear.It has been reported that the N-terminal 440 residues of 1 include six motifs shared with helicase superfamily I proteins, including the nucleoside triphosphate-binding A and B motifs (7,48).Both genetic and biochemical studies have suggested that 1 has nucleoside triphosphatase (NTPase), RNA 5Ј-triphosphatase, and/or RNA helicase activities, presumably associated with the Nterminal region of 1 (7,8,48).From the core crystal structure, however, we know that this N-terminal region is not seen in the 1.5 subunits and assumes an extended conformation that makes it unlikely to play an enzymatic role in the 1.3 subunits (50).Genetic studies have suggested a role for 2 as well in modulating the NTPase and transcriptase activities of cores (49,63).Thus, the functional relationship between 1 and 2 and their relative roles in aiding the 3 polymerase in transcription and in mediating the first reaction in mRNA capping (RNA 5Ј-triphosphatase) remain in some question.
For several other viruses, including other Reoviridae family members from the Rotavirus and Orbivirus genera (15,20,33,38,47), coexpression of the viral capsid proteins from recombinant baculoviruses allows the assembly of CLPs or virion-like particles (VLPs), depending on which proteins were expressed.These particles have been purified and used to address important questions about their structure, assembly, and enzymatic activities (23,30,34,37,58,(64)(65)(66).The genomic dsRNA segments have not yet been incorporated into particles by following this approach.In reovirus-infected cells, the production of top component (TC) virions, which lack the dsRNA genome, suggests that assembly of the reovirus capsids can also occur independently of genome assembly (18,21,56).Indeed, mammalian reovirus CLPs were shown to assemble in HeLa cells when the three major core proteins 1, 2, and 2 were coexpressed by using recombinant vaccinia viruses (61).These reovirus CLPs, which seem likely to be useful for addressing several types of questions about the core, have however been subjected to only limited characterizations to date (61).
In this report, we used recombinant baculoviruses to express reovirus core proteins 1, 2, and 2 in insect cells.The CLPs that resulted were purified and characterized and found to be similar to cores with regard to their sizes, morphologies, and protein compositions.Like cores, the CLPs could also be coated with the two major outer-capsid proteins, 1 and 3, to produce VLPs.To begin to use this system to expand our understanding of particle functions and assembly, we addressed the hypothesis that the hydrophilic N-terminal arms of the 1.3 subunits play an important role in assembling the core shell.We showed that a large N-terminal portion of 1 is in fact dispensable for CLP assembly but that residues in the 231-to-259 region are required, possibly because they are necessary for the interaction between the 1.3 and 2.3 and/or 1.5 and 2.5 subunits.An effective system for studies of reovirus core structure, assembly, and functions is hereby established.
Virions, TC virions, and cores.Purified virions, TC virions, and cores of reovirus strain type 1 Lang (T1L) were obtained, stored, and quantitated as previously described (21).The protein compositions of the particles were confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (see below).
Dual recombinant baculovirus containing the reovirus T1L L3 and S2 genes.The cloning of the T1L L3 gene into the pBlueScript II KS vector (Stratagene, La Jolla, Calif.) to yield pBS-L3 L was previously reported (27).For cloning the T1L S2 gene, viral transcripts produced by T1L cores were first copied by avian myeloblastosis virus reverse transcriptase (Gibco BRL).For this reaction, the primer was complementary to the 3Ј end of S2 plus strand and included an XhoI site, not otherwise present in the T1L S2 gene, near its 5Ј end (5Ј-CCGCTC-GAGGATGAATGTGTGGTCAGTCG-3Ј; XhoI site underlined).The resulting single-stranded cDNA was subjected to second-strand synthesis by the Klenow fragment of Escherichia coli DNA polymerase I (New England Biolabs, Beverly, Mass.).For this reaction, the primer was complementary to the 3Ј end of S2 gene minus strand and again included an XhoI site near its 5Ј end (5Ј-CCGCTCGAG-GCTATTCGCTGGTCAGTTAT-3Ј for S2; XhoI site underlined).The resulting double-stranded cDNA was digested with XhoI, end filled with Klenow fragments, and subcloned into the SmaI site of pBluescript II KS to generate pBS-S2 L (due to unexplained difficulty in cloning into the XhoI site).The sequence of the S2 DNA clone used in these studies was shown to be identical to that previously reported for the T1L S2 genome segment (16).
To generate a dual-recombinant baculovirus to coexpress 1 and 2, the T1L L3 gene was excised from pBS-L3 L by EcoRI and subcloned into the EcoRI site of the pFastbacDual vector (pFbD) (Gibco BRL).In the resulting pFbD-L3 L plasmid, the L3 gene was positioned under the transcription control of the baculovirus (Autographica californica nuclear polyhedrosis virus [AcMNPV]) polyhedrin promoter.The T1L S2 gene was excised by EcoRV and NotI from pBS-S2 L and subcloned into the same sites of the pcDNAI vector (Invitrogen).The resulting pcDNAI-S2 L plasmid was digested with EcoRV and SphI, and the S2 gene-containing fragment was subcloned into the SmaI and SphI sites of pFbD-L3 L .In the resulting pFbD-L3 L S2 L plasmid, the S2 gene was positioned under the transcription control of the baculovirus (AcMNPV) p10 promoter.To generate pFbD-S2 L , the L3 gene was removed from pFbD-L3 L S2 L by excision with EcoRI.The pFbD-L3 L , pFbD-L3 L S2 L , and pFbD-S2 L plasmids were used to generate the corresponding three recombinant baculoviruses (Bac-1 L , -1 L 2 L , and -2 L ) with the Bac-to-Bac system (Gibco BRL).The baculoviruses were amplified into high-titer stocks by serial passage in S. frugiperda 21 cells.
Polyclonal antiserum against 1.The T3D L3 gene was excised from pBS-L3 D (27) at its BamHI and HindIII sites and subcloned into the same sites in the pET-21b expression plasmid (Novagen, Madison, Wis.) to generate plasmid pET-21b-L3 D .pET-21b-L3 D was transformed into BL21-DE3 cells (Novagen), followed by the expression protocol described in the pET system manual (Novagen).The expression of the 1 protein directed from pET-21b-L3 D was induced at the mid-log cell phase by adding isopropyl-␤-D-thiogalactopyranoside to 0.5 mM.After incubation at 37°C for 3 h, cells were harvested by centrifugation at 7,800 ϫ g for 5 min, resuspended in cold PBS containing 1ϫ protease inhibitor cocktail (Roche Applied Science, Indianapolis, Ind.), and lysed by sonication.The inclusion bodies containing 1 were crudely purified from the lysate by centrifugation at 8,000 ϫ g for 30 min through a 20% sucrose cushion.SDS-PAGE and immunoblotting with a rabbit antiserum raised against heat-inactivated reovirus cores (11; S. Noble and M. L. Nibert, unpublished data) demonstrated that 1 constituted around 70% of these inclusion bodies.To purify 1 further, an electroelution protocol was applied (25).The eluant was concentrated by solvent absorption with polyethylene glycol.The polyclonal antiserum was produced in a rabbit injected with the electroeluted and concentrated 1 protein at the Animal Care Unit of the University of Wisconsin Medical School (Madison).
Generation and purification of CLPs from S. frugiperda 21 cell lysates.To generate 122 CLPs, 2.4 ϫ 10 8 S. frugiperda 21 cells were coinfected with Bac-1 L 2 L and Bac-2 D , each at an MOI of 5 to 10.Because the 1, 2, and 2 proteins from reoviruses T1L and T3D can viably associate in reassortant viruses (17,48,49,63), the mixture of T1L 1 and 2 with T3D 2 in these experiments was not expected to interfere with particle assembly and did not do so.Cells were harvested at 72 h p.i., washed twice with cold PBS, and resuspended in cold A buffer (1 M NaCl, 50 mM Tris-HCl [pH 8.0]) containing a 1ϫ protease inhibitor cocktail.Either directly or after freezing and thawing, cells were lysed by sonication and incubated on ice for 30 min after sodium deoxycholate was added to 0.1%.Freon extraction was then performed three or four times, and the CLPs were purified from the supernatant through a 1.25-to 1.35-g/cm 3 CsCl equilibrium gradient.Centrifugation was performed for 3 h or more at 5°C with a Beckman SW41 or SW28.1 rotor at a speed of 35,000 or 25,000 rpm, respectively.The distinct particle bands were harvested and dialyzed against virion buffer for storage.To purify CLPs consisting of 1 and 2 (12 CLPs), similar numbers of S. frugiperda 21 cells were infected with Bac-1 L 2 L alone and subsequently processed as described above.Unsuccessful attempts to make and purify 1-only CLPs were performed in the same manner with Bac-1 L .
Negative-stain transmission electron microscopy (EM).Copper 400-mesh Formvar carbon-coated grids were used for particle adsorption.A grid was floated on a drop (ϳ50 l) of sample solutions for 5 min and then washed twice with PBS.In each step, excess fluid was absorbed with filter paper.The grids were stained in a drop of freshly filtered 1% uranyl acetate and observed with a Philips 120 transmission electron microscope at 100 kV as described previously (10).
Transmission cryoEM and image reconstruction.Purified CLPs were embedded in vitreous ice, and micrographs were taken below Ϫ176°C at a dose of ϳ21 electrons/A ˚2 in a Philips CM200 field emission gun microscope at a magnification of ϫ38,000 and defocus values ranging between 2.5 and 3.4 m underfocus.The T1L core images from a previous study (19) were reprocessed as described below.The particle orientations and centers were determined by using a modelbased method (2).Three-dimensional (3D) reconstructions were computed to 32-A ˚resolution as described previously (3) from 54 and 769 particle images for the core and CLP samples, respectively.The particle orientations in both reconstructions were randomly distributed in the asymmetric unit as evidenced by the eigenvalue spectra where all inverse eigenvalues were Ͻ0.10 and more than 99% were Ͻ0.01 (3).As the CLP data included images recorded at different defocus levels, the reconstruction of the CLP was computed with adjustments made to compensate in part for the effects of the microscope contrast transfer function (3).
Construction of 1 (L3) deletion mutants.For easy manipulation, the T1L L3 gene and surrounding sequences were excised from pFbD-L3 L S2 L at the SmaI and SpeI sites and subcloned into the SmaI and XbaI sites of pGEM-4Z (Promega, Madison, Wis.) to generate pGEM-4Z-L3 L .This plasmid was used as template for all inverse PCRs applied to generate a series of deletion mutants.With some modifications from the published protocol (62), inverse PCR was performed with the following conditions: step 1, 95°C, 30 s; step 2, 95°C, 30 s; step 3, 54°C, 1 min; step 4, 68°C, 15 min; there were 20 cycles of steps 2 to 4. The 50 l of reaction mixture contained 0.1 g of pGEM-4Z-L3 L , 10 pmol of each forward and reverse primer, 0.2 mM deoxynucleoside triphosphate, 2.5 U of Pfu polymerase (Stratagene), and 1ϫ Pfu buffer supplied by the manufacturer.Each PCR product was end filled with 5 U of Klenow fragment (New England Biolabs) in an effort to increase the population of full-length, blunt-end product and then treated with DpnI for 2 h at 37°C.After this, the product was cut with NcoI if required (see below), isolated on a 0.8% agarose gel, purified from the gel slice, and self-ligated to reconstitute the circular plasmid.
To introduce the 1⌬2-315 deletion into the T1L L3 gene, a regular PCR was carried out by using pBS-L3 L as the template, forward primer 5Ј-CGGTCCGG CTAATCGTCAGGATGGACTTTGACCGAGATTCG-3Ј (RsrII site single underlined, codons of methionine 1 and aspartate 314 double underlined), and reverse primer 5Ј-CGTGGCCACGTGTGAGGCG-3Ј (MscI site underlined).The PCR product was cut with RsrII and MscI and then substituted for the RsrII-MscI fragment of pBS-L3 L , generating pBS-L3 L ⌬2-315.Sequencing confirmed that the amplified fragment contained the expected deletion and that no other mutations had been introduced.The RsrII-MscI fragment from this plasmid was then excised and subcloned into pFbD-L3 L S2 L to generate pFbD-(L3 L ⌬2-315)S2 L , which was then used to generate recombinant baculovirus Bac- Purification of VLPs from the cell lysates.To obtain a cell lysate containing the two outer-capsid proteins 1 and 3, 8 ϫ 10 7 T. ni High Five cells were infected with Bac-1 L 3 L (11) at an MOI of 5 to 10.Cells harvested at 60 h p.i. were washed twice with cold PBS and resuspended in cold B buffer (100 mM NaCl, 50 mM Tris, pH 8.0) containing 1ϫ protease inhibitor cocktail.After being frozen and thawed, the cells were lysed and homogenized by sonication.This cell lysate containing 1 and 3 was then mixed with a cell lysate containing 122 CLPs, generated as described in a preceding section.The mixture was incubated at 37°C for 1 h with stirring every 15 min and then placed on ice to cool and incubated for 30 min further on ice after adding sodium deoxycholate to 0.1%.The mixture was next extracted with Freon and subjected to a CsCl gradient as described for CLPs in a preceding section.The resulting particle bands on the gradient were harvested and dialyzed against virion buffer for storage.Unsuccessful attempts to coat 12 CLPs were performed in the same manner and also by using a modification in which A buffer (see above) was substituted for B buffer.

RESULTS
Expression of the three major reovirus core proteins using recombinant baculoviruses.A previous study using recombinant vaccinia viruses for protein expression (61) indicated that both the core shell protein 1 and the core nodule protein 2 are required to assemble reovirus CLPs.For the present study 1 and 2 were therefore commonly coexpressed by using the dual-recombinant baculovirus Bac-1 L 2 L rather than by using the two single-recombinant baculoviruses Bac-1 L and Bac-2 L .The core turret protein 2 was expressed by using the single-recombinant baculovirus Bac-2 D as described previously (39,41).Although SDS-PAGE of lysates from the baculovirus-infected insect cells failed to show the recombinant 1, 2, and 2 bands clearly, due to their low expression levels (Fig. 1a), immunoblotting using the appropriate polyclonal antiserum or monoclonal antibody demonstrated each of these proteins in the lysates (Fig. 1b). 1 was detected only in cells infected with Bac-1 L or Bac-1 L 2 L (Fig. 1b, lanes 3, 6, and 7), 2 was detected only in cells infected with Bac-2 D (Fig. 1b, lanes 5 and 7), and 2 was detected only in cells infected with Bac-2 L or Bac-1 L 2 L (Fig. 1b, lanes 4, 6, and 7).
Isolation of 12 and 122, but not 1-only, CLPs.In an effort to purify the CLPs expected to be formed in baculovirusinfected insect cells expressing the reovirus core proteins, the method for purifying infectious virions from reovirus-infected L cells (21) was modified (see Materials and Methods).In the CsCl gradient that made up the final step of the protocol, four or five closely migrating bands, each similar in appearance to the bands representing purified virions or TC virions from infected L cells (18,21), were routinely obtained from the insect cell lysates in which either 1 and 2 or 1, 2, and 2 had been expressed (Fig. 2a and data not shown).The buoyant densities of these bands were measured by refractometer to be in the range of 1.29 to 1.31 g/cm 3 (data not shown), and the uppermost band was always the predominant one (Fig. 2a and data not shown).For the characterizations in this study, all of these bands were harvested together.
The protein compositions of the putative CLP bands obtained from insect cells expressing either 1 and 2 or 1, 2, and 2 were analyzed and compared with that of authentic core particles by SDS-PAGE and immunoblotting.In Coomassie brilliant blue-stained gels, the putative 12 CLPs (Fig. 3a, lane 2) and 122 CLPs (Fig. 3a, lane 3) each showed two major protein bands, which comigrated with the band (containing the 1, 2, and 3 proteins) and 2 band of core particles (Fig. 3a, lane 1).Immunoblots of the same gel confirmed that the slow-migrating band contained 1 in the putative 12 CLPs and both 1 and 2 in the putative 122 CLPs, whereas the fast-migrating band contained 2 in each (Fig. 3b).The densitometric ratio of the slow-migrating band to the fast-migrating band in 122 CLPs was somewhat less than that in cores, at least partly due to the absence of 3 from the CLPs (data not shown).Small amounts of 1 degradation products with slightly lower molecular weights than the fulllength protein were detected by immunoblotting in both types of CLPs as well as in cores (Fig. 3b).No detectable levels of other proteins were consistently evident.Together with the observation that 122 CLPs migrated in sucrose gradients at a rate similar to that for TC virions (data not shown), these results are consistent with the successful interaction of the major viral core proteins in insect cells and subsequent purification of both 12 and 122 CLPs to a high level of purity.
We made multiple attempts to assemble CLPs with 1 alone.The reovirus core crystal structure shows that the 60 1 dimers form a complete shell, whereas 2 and 2 bind atop it (50).It is thus reasonable to hypothesize that expression of 1 alone might lead to CLP formation.Xu et al. (61) reported, however, that expression of 1 alone did not yield CLPs in the recombinant vaccinia virus system.Like those authors, we obtained no evidence for CLP formation after expression of 1 alone in the recombinant baculovirus system: particle bands were not observed in the CsCl gradient used as the final step in the purification procedure (data not shown).This is consistent with the previous conclusion (61) that 1 and 2 constitute the minimal reovirus components to assemble CLPs.
Negative-stain EM of CLPs.To determine whether the purified CLPs have morphology similar to that of cores, EM was performed with 1% uranyl acetate as the negative stain.Core particles of reovirus T1L were used as a positive control (Fig. 4a).The purified 122 CLPs (Fig. 4b) appeared similar to cores in that they had a uniform diameter of 50 to 60 nm and their single protein layers were continuous and thinner than those of TC virions, which have both inner and outer capsids (Fig. 4c) (21,56).Together with the previous SDS-PAGE and immunoblot results (Fig. 3), these findings suggest the purified 122 CLPs have a single continuous protein layer consisting of 1 and 2, consistent with the X-ray structure of cores (50).However, the negative-stain images in this study failed to provide clear views of the 2 turrets on either CLPs or cores, which might be explained by the poor resolution of this analysis.It was also noteworthy that the stain penetrated into the interior cavity of the CLPs (Fig. 4b), as into TC virions that lack the dsRNA genome (Fig. 4c), but that cores packed with the dsRNA genome were more impermeable to the stain (Fig. 4a) (18,21,56).This indicates that the purified CLPs allow small molecules of stain to enter and be deposited in their interior cavities, which thus appear mostly devoid of other components such as RNA (but see Discussion).CLPs also behaved like cores in that they reversibly aggregated and disaggregated when placed in low-and high-ionic-strength solutions, respectively (data not shown).Negative-stain EM images of 12 CLPs showed that they have a morphology similar to that of 122 CLPs and cores but a stronger tendency to aggregate, as previously noted for spikeless cores from which CryoEM and three-dimensional (3D) image reconstruction of CLPs.Since the 2 turrets on neither 122 CLPs nor cores were well visualized by negative-stain EM in this study, we performed cryoEM and image reconstruction to identify more details of the structural similarity between CLPs and cores preserved in a vitrified state and observed in three dimensions.Surface views of the purified 122 CLPs (Fig. 5a  and b) looked almost identical to those of purified T1L cores (Fig. 5c and d), including the large pentameric 2 turrets that project around the fivefold axes and the monomeric 2 nodules that externally decorate the 1 shell around the fivefold and threefold axes as well as across the twofold axes.These findings provide evidence that the 1, 2, and 2 proteins were able to assemble into approximately the same structure-and in approximately the same copy numbers-during CLP formation in recombinant baculovirus-infected insect cells as during virion morphogenesis in reovirus-infected mammalian cells.
A notable feature of the CLP reconstruction is that the outer, "flap" domains of the T3D 2 protein (50) adopt a conformation resembling that of the region of the T1L 2 in T1L cores (19) (Fig. 5c and d) and TC cores (18) but distinct from that of the region of the T3D 2 in T3D cores (40,42).The basis for this difference remains unknown but may relate to differences in preparation conditions (J.Kim, X. Zhang, S. B. Walker, T. S. Baker, and M. L. Nibert, unpublished data).In any case, this finding indicates that the flap domains of T3D 2 are capable of opening the top of the 2 "reaction vessel" to a larger extent than previously seen, which may be important for release of capped transcripts (19; M. Yeager, S. Weiner, and K. M. Coombs, abstract from the 40th Annual Meeting of the Biophysical Society, Biophys.J. 70:A116, 1996).
CLPs formed with N-terminal deletions of 1.To address the hypothesis that the hydrophilic N-terminal arms of the 1.3 subunits are important for assembly of the 1 shell (see the introduction), we designed a series of deletion mutations within the first 315 amino acids of 1 (Fig. 6).These residues include the first four helicase-like motifs (7,48) as well as the zinc finger (6,27,50).The 1 mutants from which the five smallest numbers of residues were removed (1⌬2-26, 1⌬28-175, 1⌬2-176, 1⌬2-200, and 1⌬2-230) were all expressed well in insect cells, to levels at least as high as that for fulllength 1 (data not shown).CLPs from insect cell lysates in which each of these 1 mutants had been coexpressed with 2 and 2 were purified in parallel by the procedure for CLP purification described above.In each case, a single particle band was observed (Fig. 2b and data not shown); this band comigrated with the uppermost, predominant band of the CLPs formed with full-length 1 (buoyant density near 1.29 g/cm 3 ) (Fig. 2a).Thus, the minor bands observed in the CsCl gradient for CLPs formed with full-length 1 are attributable to sequences in the N-terminal region of 1 (see Discussion).SDS-PAGE revealed that the CLPs containing 1⌬2-26, 1⌬28-175, 1⌬2-176, 1⌬2-200, or 1⌬2-230 were purified with virtually no detectable cellular protein contamination (Fig. 7a and data not shown).The 1 deletion mutants migrated at the expected gel position in comparison with the full-length 1 protein of cores: the 1⌬2-26 mutant migrated similarly to full-length 1 because of its small deletion (ϳ3 kDa), whereas the other four mutants migrated faster than full-length 1, consistent with their larger deletions of approximately 16, 19, 22, and 25 kDa for 1⌬28-175, 1⌬2-176, 1⌬2-200, and 1⌬2-230, respectively.As a representative of this group of CLP-positive mutants, 1⌬28-175 was also used to demonstrate that it can form CLPs with 2 in the absence of 2 (Fig. 7b), thereby excluding the possibility that 2 is needed to assemble or stabilize the CLPs that are formed with these mutants.In sum, these results demonstrate that the N-terminal 230 amino acids of 1 are dispensable for assembly of the 1-2 shell, which is contrary to the original hypothesis suggested by the core crystal structure (50).These results also demonstrate that the N-terminal 230 amino acids of 1 are dispensable for incorporation of 2 on top of the 1-2 shell, which is consistent with the locations of these N-terminal sequences beneath the main 1 shell in both the 1.3 and the 1.5 subunits (50).Since the 1 zinc finger (residues 181 to 208) was deleted from the 1⌬2-200 and 1⌬2-230 mutants, the results further suggest that the unknown function of this zinc finger pertains to something other than core shell assembly.
The 1 deletion mutants from which the two largest numbers of residues were removed, 1⌬2-259 and 1⌬2-314, were also expressed well in insect cells, to nearly the same levels as full-length 1.However, in three trials with each, we failed to obtain particle bands in the CsCl gradient during the procedure to purify CLPs.These failures suggest that amino acids 231 to 259 are necessary for CLP formation.One possibility is that amino acids in this region are important for maintaining the overall conformation of 1 so that the deletions spanning this region caused 1 to be misfolded and thereby not compe-tent for CLP assembly.Another possibility is that amino acids in the 231-to-259 region are important for mediating or affecting specific 1-1 or 1-2 interactions that are essential for CLP assembly.
Complementation of 1⌬2-259 and 1⌬2-314 for CLP formation.To address whether the CLP-negative mutants 1⌬2-259 and 1⌬2-314 have normal conformations in the parts of 1 that they contain, we adopted a complementation approach.We hypothesized that if the conformations of 1⌬2-259 and 1⌬2-314 are approximately normal, then each may be complemented by full-length 1 or a CLP-positive 1 deletion mutant, so that they can be assembled into CLPs that contain both these forms of 1.In contrast, if 1⌬2-259 and 1⌬2-314 are misfolded, then they would not be expected to be assembled into CLPs by complementation.In initial experiments, 1⌬2-314 was coexpressed with 1⌬28-175 (a CLP-positive mutant; Fig. 6b), 2, and 2. As a control, 1⌬28-175, 2, and 2 were coexpressed in parallel.The resulting CLPs from the two samples were purified in parallel by the usual procedure, and a single particle band at the expected position for CLPs was observed in the CsCl gradient for each.SDS-PAGE (Fig. 8a) and immunoblotting (Fig. 8b) revealed an additional, 1derived band migrating between the 1⌬28-175 and 2 bands in the CLPs obtained from the coexpression of 1⌬2-314 and 1⌬28-175, but not in the CLPs resulting from the expression of 1⌬28-175 only (Fig. 8a).Moreover, the position of this extra band was consistent with the size of the 1⌬2-314 protein (Fig. 7).These findings suggest that the additional band is indeed 1⌬2-314 and that this mutant had been complemented by 1⌬28-175 to be assembled into CLPs.Similar evidence for incorporation of the CLP-negative 1⌬2-259 mutant into CLPs by complementation was obtained in subsequent experiments in which that mutant was coexpressed with full-length 1, 2, and 2 (data not shown).Because 1⌬2-230 was competent for CLP assembly and because both 1⌬2-259 and 1⌬2-314 could be complemented for CLP assembly, we conclude that amino acids in the 231-to-259 region are important for mediating or affecting specific 1-1 or 1-2 interactions that are essential for CLP assembly (see Discussion for further considerations).
122, but not 12, CLPs support in vitro assembly of reovirus outer-capsid proteins 1 and 3 to yield VLPs.As a further test of structural and functional similarities between reovirus CLPs and cores, we analyzed whether they can support in vitro assembly of the reovirus outer-capsid proteins 1 and 3 (11).An insect cell lysate containing 122 CLPs (resulting from coexpression of those three proteins) was mixed and incubated with an insect cell lysate containing recombinant 1-3 complexes (resulting from coexpression of those two proteins) (11).The resulting particles were then purified by using the same protocol as that for purification of CLPs.In the CsCl gradient, a predominant upper band accompanied by three or four more minor bands migrating closely beneath (data not shown), resembling the bands seen for the CLPs formed with full-length 1, was observed (Fig. 2a).For the characterizations in this study, all of these bands were harvested together.The protein compositions of the putative VLP bands were analyzed and compared with those of authentic virions (data not shown) and TC virions (Fig. 9) by SDS-PAGE.Coomassie brilliant blue-stained gels showed that the putative VLPs contain levels of the 1/1C and 3 proteins similar to those contained by TC virions (Fig. 9), as later confirmed by densitometry (data not shown).Negative-stain EM of the putative 12213 VLPs (Fig. 4d) and TC virions (Fig. 4c) corroborated the gel findings by showing that these two particles have similar sizes and morphologies, including the presence of a thick, double-layered capsid that is clearly distinct from the thinner, single-layered capsid of 122 CLPs (Fig. 4b).The EM images of 12213 VLPs showed some partially coated particles in addition to more completely coated ones.We believe that the partially coated particles represent intermediates in the process of outer capsid assembly that result from insufficient amounts of the 1-3 complexes that were used in these experiments.In addition, whereas both CLPs and cores have a tendency to aggregate in low-ionic-strength solutions (see the preceding section), the purified VLPs did not show this tendency and remained in FIG. 8. Complementation of a CLP-negative 1 mutant with a CLP-positive one.One set of S. frugiperda 21 cells was infected to coexpress 1⌬28-175 (as a representative of the CLP-positive 1 deletion mutants), 2, and 2, while another set of S. frugiperda 21 cells was infected to coexpress both 1⌬28-175 and 1⌬2-3141 (a CLPnegative mutant) in addition to 2 and 2. The two sets of cells were processed in parallel to obtain purified CLPs.The CLPs were then analyzed by SDS-PAGE followed by Coomassie staining (a) or immunoblotting using the polyclonal anti-1 serum (b).The bands corresponding to the 1 deletion mutants are marked at the right.FIG. 9. Coating CLPs with complexes of the major outer-capsid proteins 1 and 3 to generate VLPs.(a) An S. frugiperda 21 cell lysate containing 122 CLPs was mixed with a lysate of T. ni High Five cells containing complexes of 1 and 3 (11,36).The mixed lysate was incubated and processed to obtain purified particles.These putative VLPs were then analyzed by SDS-PAGE and Coomassie staining (lane 2).Reovirus T1L TC virions (lane 1) were included on the gel to mark the positions of the 1/1C and 3 proteins as well as to provide a standard for the relative intensities of these bands.The 1 protein is missing from the VLPs because it was not added in this experiment.solution at low ionic strength, like virions and TC virions (data not shown).VLPs were also assembled in the same manner from CLPs containing 1⌬28-175, but in that case a single band was seen in the CsCl gradient, similar to the single band seen with the CLPs assembled with this and other CLP-positive 1 deletion mutants (see above).From these results, we conclude that the conformations of 1, 2, and 2 on the outer surfaces of CLPs are similar enough to those of authentic cores to support the complete or nearly complete assembly of outercapsid proteins 1 and 3.
We made multiple attempts to form VLPs by coating 12 CLPs with 1 and 3. Since the 1-3 complexes are thought to make contacts with the sides of the 2 turrets as well as the tops of the 2 nodules in virions (36), it was conceivable either that the contacts with 2 are necessary for proper assembly of the 1-3 outer capsid or that the contacts with 2 are sufficient for this.Evidence from particles formed by reovirus temperature-sensitive mutants previously suggested that the contacts with both 2 and 2 are necessary (43,45).In the present experiments, we obtained no evidence for VLP formation following the attempted coating of 12 CLPs with 1 and 3: particle bands were not seen in the CsCl gradient used as the final step in the purification procedure (data not shown).We conclude that 2 is necessary for assembly of the 1-3 outer capsid to form VLPs.

DISCUSSION
Generation of CLPs and VLPs from viral proteins expressed in insect cells via recombinant baculoviruses has been previously developed and applied to studies of other members of the Reoviridae family including orbiviruses (e.g., bluetongue and Broadhaven viruses) (20,38,47) and rotaviruses (15,33).A powerful aspect of this approach is the capacity to engineer mutations into the different viral structural proteins in order to determine their effects on particle assembly and structure, as well as on at least some functional activities that can be analyzed with these particles (23,34,37,58,64).In this report, we established the baculovirus-based CLP/VLP system for reoviruses and demonstrated how studies with different subsets and mutant versions of the reovirus structural proteins allow us to address specific questions and hypotheses about particle assembly and structure.
Minimal components for assembly of the core capsid.Our results concur with ones from a previous study indicating that 1 and 2 constitute the minimal components for assembly of a stable core capsid that can be purified by established methods (61).We further showed that 2, 1, and 3 can be effectively assembled into particles in this system.Thus, the reovirus proteins that were not included in this study are not strictly required for assembly of these particles.The missing viral proteins comprise the structurally minor core proteins 3 and 2, the structurally minor outer-capsid protein 1, and the nonstructural proteins NS, NS, and 1s.This is not to say that these other viral proteins are dispensable for assembly of reovirus particles within infected cells.Of particular note is that the CLPs and VLPs do not contain the dsRNA genome segments, and thus one or more of these other viral proteins may play a role in genome packaging into particles.Last, the structurally minor proteins 3, 2, and 1 must themselves be incorporated into infectious particles, and preliminary data from our laboratory indicate that all three can indeed be incorporated into CLPs and VLPs by using the recombinantbaculovirus approach (J.Kim and M. L. Nibert, unpublished data).Cellular proteins, yet to be identified, that are sufficiently conserved between insect and mammalian cells may also be required for the particle assembly that we demonstrated in this study.
Our failure to purify CLPs after expression of 1 in the absence of 2 or 2 indicates either (i) that 1-only CLPs cannot form or (ii) that 1-only CLPs are fragile and cannot survive the purification procedure.Baculovirus-based expression of rotavirus VP2 (33,65) or Broadhaven orbivirus VP2 (47), the 1 equivalents present in 120 copies per core of each virus, is sufficient to give rise to purifiable, single-layer particles.On the other hand, baculovirus-based expression of bluetongue orbivirus VP3, the 1 and VP2 equivalent present in 120 copies per core of that virus, is not sufficient.Instead, coexpression of VP3 and VP7 (a trimer protein arranged with Tϭ13 [laevo] symmetry in virions and thus most closely analogous to the reovirus 1 protein) was necessary to generate double-layer particles, which could then be dialyzed at low ionic strength to cause release of VP7 and generation of singlelayer particles of VP3 (20,38).Thus, there are precedents from studies of other Reoviridae members that 1 may contain all the virally encoded signals it needs to assemble into the Tϭ1 shell (rotavirus and Broadhaven virus) or that it may need another, externally bound viral protein to assist in its assembly (bluetongue virus).Although current evidence strongly suggests the latter for 1, with 2 being the additional required protein, we are pursuing further experiments to address both these possibilities and to identify probable intermediates in the process of core shell assembly.
Dispensability of the hydrophilic N-terminal arms of 1 for CLP assembly.The core crystal structure showing the N-terminal arms of 1.3 wrapped around the threefold axes (Fig. 10a, arms in cyan) clearly suggested that they may be important for tethering together the 1 decamers within the shell (50).This tethering activity could be required to operate during assembly to bring the decamers together in a productive manner (29) and/or after assembly to stabilize the shell.The results in this study, however, unequivocally demonstrated that the first 230 amino acids of these arms, or almost their full lengths, are not required for assembly of CLPs; for isolation of the CLPs from CsCl gradients; or for additional assembly of the 2, 1, and 3 components of the outer capsid.It is notable that the 1 equivalents in bluetongue orbivirus (VP3) and group A rotavirus (VP2) appear to contain shorter versions of these N-terminal arms (50 to 100 amino acids) (24,64), which have been shown not to be required for CLP assembly in the rotavirus case (64).The N-terminal region of rotavirus VP2 is, however, required for incorporating the structurally minor core proteins VP1 and VP3 into CLPs (64).For reoviruses, there is still a question as to what role these N-terminal arms of either the 1.3 or the 1.5 subunits may play.One possibility is that in one or both types of 1 subunits in cores the Nterminal arms may be required for binding the structurally minor core proteins 3 and 2. The hydrophilic (positively charged, pI Ϸ 10 for amino acids 1 to 180 [27]) N-terminal region of 1, though substantially longer, is also suggestive of the RNA-binding, N-terminal domains found in several other viral capsid proteins including those of tomato bushy stunt virus, a tombusvirus, and pariacato virus, a nodavirus (26,50,59).Thus, this region of 1 may also be involved in interactions with the genomic RNA (see below).
Location of 1 amino acids 231 to 259 in the crystal structure of cores and role of 2 in CLP assembly.Results from the series of 1 deletion mutants in this report demonstrated that amino acids 231 to 259 of 1 play an essential role in CLP formation.In an attempt to understand how these residues may work in assembly, we identified their locations in the reovirus core crystal structure (50).From the view of 1 (1.5 and 1.3) from inside the particle (Fig. 10a), we can see that residues 241 to 259 of 1.5 and residues 231 to 259 of 1.3 are mostly exposed on the inside face of the 1 shell.(Recall that the first 240 amino acids of 1.5 are not visible in the crystal structure [50].)These residues are located under the middle part of the body of each respective 1 subunit and are thus not likely to be involved in interactions between 1 subunits.Although amino acids 241 to 250 of 1.5 and 231 to 250 of 1.3 project from the inside face of the 1 shell, residues 251 to 259 (Asp-Thr-Pro-Arg-Leu-Val-Thr-Trp-Asp) of both 1.5 and 1.3 are partially embedded within the shell in an extended conformation (Fig. 10).This location suggests that 1 residues 251 to 259 may be important for determining the local conformation of both 1.5 and 1.3.Interestingly, residues 251 to 259 in 1.5 and 1.3 underlie the respective 2 monomer that binds atop the 1 shell in that position (2.5 atop 1.5 and 2.3 atop 1.3) (Fig. 10).In neither 1.5 nor 1.3, however, do residues 251 to 259 interact directly with 2. Instead, there is a long ␣-helix of 1 (residues 457 to 473) that crosses between residues 251 to 259 and 2 (not clearly evident in Fig. 10a, but see Fig. 10b).This long ␣-helix makes several contacts with the bottom of 2, including contacts by residues 463, 464, 467, 470, and 471 in 1.5 (Fig. 10b) and residues 466, 470, 471, and 473 in 1.3.We hypothesize that the deletion of 1 residues in this 251-to-259 region was the primary determinant of the CLPnegative phenotype of the 1⌬2-259 deletion mutant because these residues must interact with the ␣-helix at residues 457 to 473 of 1 to maintain the proper local conformation of 1 for it to interact productively with the overlying 2 monomer.This hypothesis is consistent with the evidence that 1 and 2 are the minimal components for core capsid formation, presumably because the 2 clamps are essential for assembling or stabilizing the 1 shell by bridging the 1 subunits within or between decamers (31,50).
Role for 2 in assembly of the outer capsid.Previous genetic and biochemical data (29,44,45,54), as well as more recent data from "core-recoating" experiments (11,12), suggested that assembly of the outer capsid is, or at least can be, uncoupled from assembly of the core.Our present data support this hypothesis by providing evidence that 12213 VLPs can be generated by coating preformed 122 CLPs with separately preformed 1-3 complexes.The previous data also suggested that the 2 protein in cores is essential for outercapsid assembly (29,45), and our present data support that hypothesis as well by showing that VLPs cannot be generated by addition of 1-3 to 12 CLPs.Why is 2 needed for outer-capsid assembly?Don't the 1 interactions with 2 (36) suffice to allow the 1-3 lattice to be formed?Two possibilities, which are not mutually exclusive, are (i) that 2 is required to initiate 1-3 assembly around the turrets and (ii) that the interactions with 2 are needed to stabilize the 1-3 lattice because the interactions with 2 are not sufficiently strong.The fact that the 12 CLP bands disappeared from the CsCl gradient in the coating experiments (data not shown) suggests that 1-3 complexes indeed bound to these particles but did not complete assembly of the outer capsid to allow a discrete particle band to appear in the gradient.The available crystal structures of the 2 and 2 proteins in cores (50) and the 1 protein in 1-3 complexes (36), coupled with the model for how 2 and 1-3, as well as 2 and 1, interact in virions as indicated by a fit of the crystal structures into a cryoEM map of reovirus virions (36), will be useful for designing future experiments to determine the relative roles of 2 and 2 in outer-capsid assembly.
CLPs and VLPs with different buoyant densities.Observations in this study that we have yet to explain in full were the presence of different bands of CLPs and VLPs, representing particles of different buoyant densities, that we obtained from the CsCl gradients of the particles prepared to contain the full-length 1 protein (Fig. 2a).The fact that particles prepared with any versions of 1 with N-terminal deletions showed only the one, least-dense band (Fig. 2b) suggests that 1 amino acids in the 2-to-26 range as well as ones in the 28-to-175 range can independently affect this phenotype.Since the hydrophilic N-terminal domain of 1 has been shown to mediate nucleic acid binding (9,35), one reasonable possibility is that the different bands reflect incorporation of different amounts of RNA into the particles and that this RNA incorporation is eliminated by the 1 deletions.Preliminary evidence suggests that the CLPs and VLPs may indeed incorporate a small amount of RNA (J.Kim and M. L. Nibert, unpublished data), but differences among particles containing different 1 mutants remain to be explored.An opportunity to use the CLP/VLP system to study RNA packaging would be an exciting development.

FIG. 1 .
FIG. 1. Expression of the three reovirus core proteins in insect cells using recombinant baculoviruses.S. frugiperda 21 cells were infected with wild-type (WT) baculovirus (lane 2), Bac-1 L (lane 3), Bac-2 L (lane 4), Bac-2 D (lane 5), Bac-1 L 2 L (lane 6), or Bac-1 L 2 L and Bac-2 D (lane 7).The cells were harvested at 72 h p.i. and lysed.(a) The soluble cytoplasmic fractions were analyzed by SDS-PAGE and Coomassie staining.Reovirus T1L core particles (lane 1) were included on the gel to mark the positions of the core proteins.All three proteins (142 to 144 kDa each) comigrate in one band in this type of gel.(b) The same samples, including core particles, were analyzed in the same respective lanes of another gel by SDS-PAGE and immunoblotting using a polyclonal anti-1 serum, anti-2 monoclonal antibody 7F4, or a polyclonal anti-core serum to detect 2. Only those portions of the blot containing the respective proteins are shown.

FIG. 2 .
FIG. 2. Appearance of putative CLP bands in CsCl gradients.S. frugiperda 21 cells were infected with Bac-1 L 2 L and Bac-2 D (a) or Bac-1 L (⌬28-175)2 L and Bac-2 D (b).The cells were harvested at 72 h p.i. and processed.The CLPs putatively contained within the cell lysates were allowed to form discrete bands, visible by eye, on a 1.25to 1.35-g/cm 3 CsCl equilibrium gradient and then photographed.Arrows, numbers of discrete bands visible in each gradient.The upper parts of the tubes have been cropped from these images but contained no visible bands.FIG. 3. Protein compositions of purified CLPs.CLPs were purified from the lysate of S. frugiperda 21 cells infected with either Bac-1 L 2 L (lane 2) or Bac-1 L 2 L and Bac-2 D (lane 3).(a) The CLPs were analyzed by SDS-PAGE and Coomassie staining to estimate their protein contents.Reovirus T1L core particles (lane 1) were included on the gel to mark the positions of the core proteins as well as to provide a standard for the relative intensities of the and 2 bands.(b) The same samples, including core particles, were analyzed in the same respective lanes of another gel by SDS-PAGE and immunoblotting as described for Fig. 1b.

FIG. 6 .
FIG. 6.Schematic diagram of 1 deletion mutants.Deletions in the T1L L3 gene were constructed in order to express versions of the full-length T1L 1 protein (1,275 amino acids, 142 kDa) with deletions.The missing region in each deletion mutant is indicated by the horizontal narrow line.As a result of these deletions, the different versions of 1 differ in expected molecular mass as indicated at the right.Numbers at the top and bottom are residue numbers.Boxes I to VI, positions of the six putative helicase-like motifs (7, 48); diamonds, positions of the zinc finger motifs (6, 27, 50).

FIG. 10 .
FIG.10.Views of 1 and 2 from the reovirus core crystal structure.Approximate locations of the threefold (OE), twofold ( ), and fivefold (ଙ) axes are marked.(a) Inside-to-outside view of a portion of the Tϭ1 1 shell with accompanying 2 subunits, centered on a threefold axis of the core (50) (see Fig.5for whole-particle surface views that aid in orientation).Three 1.3 subunits are shown, mostly in pink, orange, or yellow (backbone format).One 1.5 subunit is also shown, mostly in violet (backbone format).The 2.3 and 2.5 subunits that associate with the tops of these 1 subunits are shown in light gray (space-filling format).Amino acids 1 to 259 of each 1 subunit (maximum extent of N-terminal sequences that were removed from the deletion mutants in this study) are shown in cyan or blue for emphasis.Amino acids 1 to 240 were not visualized in the core crystal structure of the 1.5 subunits (50) and are thus not shown in this diagram.In the one 1.5 subunit pictured, amino acids 241 to 259 are shown in blue as well as in thicker backbone format for greater emphasis.The 251-to-259 region of this subunit adopts an extended conformation and is seen diving into the main body of 1 (arrowhead) beneath the 2.5 subunit.Amino acids 1 to 259 of one 1.3 subunit (the one for which amino acids 260 to 1275 are shown in orange) are shown in thicker backbone format for greater emphasis of that subunit.Amino acids 13 to 39 and 168 to 180 were not visualized in the core crystal structure of the 1.3 subunits (50), but their proposed locations (50) are shown for the emphasized 1.3 subunit as straight lines between the flanking visualized regions.In all three 1.3 subunits pictured, amino acids 1 to 230 are shown in cyan and amino acids 231 to 259 are shown in blue.The 251-to-259 regions of these subunits also adopt extended conformations and can be seen diving into the main body of 1 (arrowhead for the emphasized subunit) beneath the 2.3 subunit.The zinc ion in the zinc finger of each 1.3 subunit is shown as a black ball, which is larger and labeled for the emphasized subunit.(b) The violet 1.5 subunit from panel a was enlarged and rotated as indicated such that the view of this subunit is now that from outside the core.Amino acids 241 to 259 are shown in blue and in thicker backbone format as in panel a.The arrowhead points to approximately the same position in the 1.5 subunit as in panel a.The ␣-helix composed of amino acids 457 to 473, which overlies the 251-to-259 region, is shown in light and dark green.Residues 463, 464, 467, 470, and 471, which make extensive contacts with the base of the overlying 2.5 subunit, are shown in dark green.Only the outline of the 2.5 subunit is shown to allow clear views of the underlying 1.5 subunit.