HIV-1 assembles at the host cell's plasma membrane into enveloped, spherical particles that bud off to infect other cells. Garrus and colleagues now report in Cell that this budding process might use the cellular machinery that normally buds vesicles into the lumen of multivesicular bodies.

The p6 domain of the viral protein Gag contains a short motif (PTAP) known to be the docking site for cellular factors recruited to assist viral budding. In this study, a two-hybrid screen identifies Tsg101 as the cellular factor recruited by the PTAP motif. Indeed, depleting Tsg101 by small interfering RNAs blocked viral budding at a late stage.

The amino-terminal PTAP-binding domain of Tsg101 has homology with E2 ubiquitin-conjugating enzymes, and, although it is not enzymatically active, it nevertheless binds ubiquitin, as it had higher affinity to a p6–ubiquitin chimaera than to p6 alone. So ubiquitylation probably regulates the recruitment of the budding machinery.

Ubiquitylation was recently shown to participate in the formation of intralumenal vesicles in multivesicular bodies. Moreover, Tsg101 is the mammalian homologue of Vps23, which functions in the vacuolar protein sorting (vps) pathway in budding yeast. Consistent with a role of the vps machinery in viral budding, dominant-negative mutants of Vps4 — an AAA ATPase that acts in the same pathway as Tsg101 — also inhibited the budding of HIV-1 particles from the cell surface.

Although it may seem surprising at first, it makes perfect sense for HIV-1 to make use of the machinery of the vps pathway to assist its budding. Indeed, the formation of intralumenal vesicles in multivesicular bodies is the only process in the cell that is topologically equivalent to budding from the cell surface — in both cases, the membrane invaginates away from the cytoplasm.