Component of the ESCRT-II complex, which is required for multivesicular body (MVB) formation and sorting of endosomal cargo proteins into MVBs (PubMed:12194858, PubMed:15469844, PubMed:15329733). The MVB pathway mediates delivery of transmembrane proteins into the lumen of the lysosome for degradation (PubMed:12194858). The ESCRT-II complex is probably involved in the recruitment of the ESCRT-III complex (PubMed:12194858). Involved in the trafficking of the plasma membrane ATPase (PubMed:12194858). Its ability to bind ubiquitin plays a central role in endosomal sorting of ubiquitinated cargo proteins by the ESCRT complexes (PubMed:15029239).