Component of the ESCRT-I complex, a regulator of vesicular trafficking process (PubMed:21757351, PubMed:22405001, PubMed:31203368). Binds to ubiquitinated cargo proteins and is required for the sorting of endocytic ubiquitinated cargos into multivesicular bodies (MVBs) (PubMed:21757351, PubMed:22405001). Plays a role in the proteasomal degradation of ubiquitinated cell-surface proteins, such as EGFR and BST2 (PubMed:22405001, PubMed:24284069, PubMed:31203368). {ECO:0000269|PubMed:21757351, ECO:0000269|PubMed:22405001, ECO:0000269|PubMed:24284069, ECO:0000269|PubMed:31203368}. This is the function of UBAP1 (ubiquitin associated protein 1, Ensembl gene identifier ENSG00000165006).