Component of the ESCRT-I complex, a regulator of vesicular trafficking process. Binds to ubiquitinated cargo proteins and is required for the sorting of endocytic ubiquitinated cargos into multivesicular bodies (MVBs). Plays a role in the proteasomal degradation of ubiquitinated cell-surface proteins, such as EGFR and BST2. 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. Interacts with PTPN23. Interacts (via UBA domains) with ubiquitinated proteins. Predominantly cytosolic. Recruited to endosomes as part of the ESCRT-I complex. The UMA domain mediates association with the ESCRT-I complex.