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:24284069, PubMed:22405001, PubMed:31203368). Component of an ESCRT-I complex (endosomal sorting complex required for transport I) which consists of TSG101, VPS28, VPS37A and UBAP1 in a 1:1:1:1 stoichiometry (PubMed:21757351, PubMed:24284069, PubMed:22405001, PubMed:31203368) (Probable). Interacts with PTPN23 (PubMed:21757351, PubMed:27839950). Interacts (via UBA domains) with ubiquitinated proteins (PubMed:22405001, PubMed:30929741). Predominantly cytosolic (PubMed:21757351). Recruited to endosomes as part of the ESCRT-I complex (PubMed:21757351). Ubiquitous. Highly expressed in heart, brain, placenta, lung, liver, skeletal muscle and pancreas. The UMA domain mediates association with the ESCRT-I complex. The disease is caused by variants affecting the gene represented in this entry. According to a report, can also be a component of ESCRT-I complexes containing VPS37B, VPS37C or VPS37D (PubMed:22405001). However, another publication showed that UBAP1 has specificity for complexes containing VPS37A and not VPS37 paralogs (PubMed:24284069). Truncated N-terminus.