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Behrouzi, Reza

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Behrouzi

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Reza

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Behrouzi, Reza
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    Recruitment dynamics of ESCRT-III and Vps4 to endosomes and implications for reverse membrane budding
    (eLife Sciences Publications, Ltd, 2017) Adell, Manuel Alonso Y; Migliano, Simona M; Upadhyayula, Srigokul; Bykov, Yury S; Sprenger, Simon; Pakdel, Mehrshad; Vogel, Georg F; Jih, Gloria; Skillern, Wesley; Behrouzi, Reza; Babst, Markus; Schmidt, Oliver; Hess, Michael W; Briggs, John AG; Kirchhausen, Tomas; Teis, David
    The ESCRT machinery mediates reverse membrane scission. By quantitative fluorescence lattice light-sheet microscopy, we have shown that ESCRT-III subunits polymerize rapidly on yeast endosomes, together with the recruitment of at least two Vps4 hexamers. During their 3–45 s lifetimes, the ESCRT-III assemblies accumulated 75–200 Snf7 and 15–50 Vps24 molecules. Productive budding events required at least two additional Vps4 hexamers. Membrane budding was associated with continuous, stochastic exchange of Vps4 and ESCRT-III components, rather than steady growth of fixed assemblies, and depended on Vps4 ATPase activity. An all-or-none step led to final release of ESCRT-III and Vps4. Tomographic electron microscopy demonstrated that acute disruption of Vps4 recruitment stalled membrane budding. We propose a model in which multiple Vps4 hexamers (four or more) draw together several ESCRT-III filaments. This process induces cargo crowding and inward membrane buckling, followed by constriction of the nascent bud neck and ultimately ILV generation by vesicle fission.
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    Heterochromatin assembly by interrupted Sir3 bridges across neighboring nucleosomes
    (eLife Sciences Publications, Ltd, 2016) Behrouzi, Reza; Lu, Chenning; Currie, Mark; Jih, Gloria; Iglesias, Nahid; Moazed, Danesh
    Heterochromatin is a conserved feature of eukaryotic chromosomes with central roles in regulation of gene expression and maintenance of genome stability. Heterochromatin formation involves spreading of chromatin-modifying factors away from initiation points over large DNA domains by poorly understood mechanisms. In Saccharomyces cerevisiae, heterochromatin formation requires the SIR complex, which contains subunits with histone-modifying, histone-binding, and self-association activities. Here, we analyze binding of the Sir proteins to reconstituted mono-, di-, tri-, and tetra-nucleosomal chromatin templates and show that key Sir-Sir interactions bridge only sites on different nucleosomes but not sites on the same nucleosome, and are therefore 'interrupted' with respect to sites on the same nucleosome. We observe maximal binding affinity and cooperativity to unmodified di-nucleosomes and propose that nucleosome pairs bearing unmodified histone H4-lysine16 and H3-lysine79 form the fundamental units of Sir chromatin binding and that cooperative binding requiring two appropriately modified nucleosomes mediates selective Sir recruitment and spreading. DOI: http://dx.doi.org/10.7554/eLife.17556.001