First remodel, then recycle

M ultivesicular bodies (MVBs) are specialized endosomes that promote the degradation of membrane proteins by delivering them to lysosomes. This process is regulated by a series of endosomal sorting complexes required for transport (ESCRTs) (1). The ESCRT-0,-I, and-II complexes sequester ubiquitinated membrane proteins and recruit the ESCRT-III complex to drive the invagination and release of cargo-laden vesicles into the interior of the MVB. The AAA-ATPase Vps4 then binds and disassembles the ESCRT-III complex so that its subunits can be recycled for further rounds of intraluminal vesicle (ILV) formation (2). Adell et al. reveal that Vps4 also participates in an earlier step of the pathway, helping ESCRT-III proteins to constrict the neck of nascent ILVs (3). The ESCRT-III complex consists of four subunits, Vps20, Snf7, Vps24, and Vps2, which, when recruited by a component of the ESCRT-II complex, assemble into ring-or spiral-shaped fi laments on the surface of MVBs (4, 5). In vitro, ESCRT-III assembly is suffi cient to induce the formation of vesi-cles in the lumen of artifi cial liposomes (6). But, says David Teis from the Biocenter, Innsbruck Medical University in Austria, these in vitro vesicles are larger and more irregular than the ILVs formed in MVBs, suggesting that additional proteins play a role in vivo. " We wondered whether the only function of Vps4 is to recycle ESCRT-III or whether, in vivo, it somehow helps to shape ILVs. " Every ESCRT-III subunit carries a so-called MIM domain, capable of binding to the MIT domain of Vps4. To investigate how Vps4 interacts with the ESCRT-III complex in vivo, Teis and colleagues, led by Manuel Alonso Y Adell, systematically replaced the endogenous ESCRT-III proteins of budding yeast with versions carrying mutated MIM domains. " The MIM domains of two subunits—Snf7 and Vps2— were critical for recruiting Vps4 and stabilizing its association with the ESCRT-III complex, " Teis explains. The Vps4–ESCRT-III interaction was especially reduced in yeast expressing mutant versions of both Snf7 and Vps2. Accordingly , ESCRT-III disassembly was dramatically slowed in these cells. " Vps4 still binds a little bit, but the disassembly process is very ineffi cient, " Teis says. The sorting and degradation of membrane proteins was strongly perturbed in double mutant yeast, a similar phenotype to that of Vps4 knockout cells, which can't recycle the ESCRT-III complex and therefore fail to transport membrane proteins correctly. But the sorting defects of yeast expressing mutant Snf7 and Vps2 …