Class E VPS protein implicated in concentration and sorting of cargo proteins of the multivesicular body (MVB) for incorporation into intralumenal vesicles. The lumenal sequestrated membrane proteins will be targeted into the vacuole after fusion of the endosome with the vacuole. Probably acts as a peripherally associated component of the ESCRT-III complex, which appears to be critical for late steps in MVB sorting, such as membrane invagination and final cargo sorting and recruits late-acting components of the sorting machinery. The MVB pathway requires the sequential function of ESCRT-O, -I,-II and -III complex assemblies. Regulates the membrane association of VPS4. Can stimulate VPS4 ATPase activity directly or via VTA1.