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). Appears to be involved in cargo sorting and release of the ESCRT-I complex from the MVBs. Component of the ESCRT-I complex (endosomal sorting complex required for transport I) which consists of STP22, VPS28, SRN2 and MVB12 in a 1:1:1:1 stoichiometry. Interacts with STP22 and SRN2. Present with 752 molecules/cell in log phase SD medium.