Oxidative Assembly of the Outer Membrane Lipopolysaccharide Translocon LptD/E and Progress towards Its X-Ray Crystal Structure

Abstract

Lipopolysaccharide (LPS) is the glycolipid that comprises the outer leaflet of the Gram-negative outer membrane (OM). Because it is essential in nearly all Gram-negative species, and because it is responsible for making these bacteria impervious to many types of antibiotics, LPS biogenesis has become an important area of research. While its biosynthesis at the cytoplasmic face of the inner membrane (IM) is well studied, the process by which it is removed from the IM, transported across the aqueous periplasmic compartment, and specifically inserted into the outer leaflet of the OM is only beginning to be understood. This transport process is mediated by the essential seven-protein LPS transport (Lpt) complex, LptA/B/C/D/E/F/G. The OM portion of the exporter, LptD/E, is a unique plug-and-barrel protein complex in which LptE, a lipoprotein, sits inside of LptD, a β-barrel integral membrane protein. LptD is of particular interest, as it is the target of an antibiotic in Pseudomonas aeruginosa. Part I of this thesis investigates how the cell forms the two non-consecutive disulfide bonds that connect LptD's C-terminal β-barrel to its N-terminal soluble domain. These disulfides, one of which is almost universally conserved among Gram-negatives, are essential for cell viability. Here, we show that an intermediate oxidation state with non-native disulfide bonds accumulates in the absence of LptE and in strains defective in either LptE or LptD. We then demonstrate that this observed intermediate is on-pathway and part of the native LptD oxidative folding pathway. Using a defective mutant of DsbA, the protein that introduces disulfide bonds into LptD, we are able to identify additional intermediates in the LptD oxidative folding pathway. We ultimately demonstrate that the disulfide rearrangement that activates the LptD/E complex occurs following an exceptionally slow β-barrel assembly step and is dependent on the presence of LptE. Part II describes work towards obtaining X-ray crystal structures of the LptD N-terminal domain and LptD/E complex. Expression construct and purification optimization enabled the production of stable LptD/E in quantities that make crystallography feasible. Numerous precipitants, detergents, and additives were screened, ultimately resulting in protein crystals that diffract to a resolution of 3.85 Å.