Required for the sorting and concentration of proteins resulting in the entry of these proteins into the invaginating vesicles of the multivesicular body (MVB). Acts a 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 recruitment of late-acting components of the sorting machinery. The MVB pathway requires the sequential function of ESCRT-O, -I,-II and -III complex assemblies (By similarity). Core component of the ESCRT-III complex (endosomal sorting required for transport complex III). ESCRT-III appears to be sequentially assembled as a flat lattice on the endosome membrane (By similarity). Leads to multivesicular bodies sorting defects. Belongs to the SNF7 family.