Component of the ESCRT-II complex, which is required for multivesicular body (MVB) formation and sorting of endosomal cargo proteins into MVBs. The MVB pathway mediates delivery of transmembrane proteins into the lumen of the lysosome for degradation. The ESCRT-II complex is probably involved in the recruitment of the ESCRT-III complex. Its ability to bind ubiquitin plays a central role in endosomal sorting of ubiquitinated cargo proteins by the ESCRT complexes. Required for vacuolar sorting of carboxypeptidase S (CPS)