Mechanistic Studies of HIV Budding

Abstract

The HIV Gag protein coordinates viral trafficking, membrane binding, assembly, cofactor packaging, budding, and maturation. Late in the infectious cycle, Gag assembles on plasma membranes and forms enveloped particles that bud through the membrane. Efficient HIV budding depends on the actions of at least two cellular proteins that bind directly to conserved elements within the C-terminal p6 region of Gag: TSG101 and ALIX. Both of these proteins normally function as part of a multi-cellular pathway termed the, ESCRT pathway (Endosomal Sorting Complex Required for Transport). In the cell, the ESCRT pathway helps to sort ubiquitylated protein cargos into vesicles that bud into late endosomal multivesicular bodies (MVB), and also helps mediate the final step of cytokinesis (called abscission). Thus, HIV and many other enveloped RNA viruses have evolved to usurp the cellular ESCRT pathway and utilize its intrinsic membrane remodeling activities to bud from cells. Recent studies have suggested that late-acting ESCRT pathway factors, including subunits of the ESCRT-III complex and the AAA ATPase, VPS4, may constrict the neck of the budding vesicle and/or mediate membrane fission. I will review evidence suggesting that ESCRT-III subunits can assemble into “rings” that surround the necks of budding particles, and then describe our structural, biophysical, and biochemical studies that indicate how: 1) ESCRT-III proteins change conformation as they are deposited from the cytoplasm onto the membrane, 2) ESCRT-III proteins bind and recruit VPS4 ATPases, and 3) VPS4 complexes assemble and act on their ESCRT-III substrates. These studies, together complementary studies from other laboratories, are providing a framework for understanding the mechanics of HIV budding.