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). Mediates the association to the ESCRT-0 complex. Required for vacuolar targeting of temperature-sensitive plasma membrane proteins STE2 and CAN1. 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 HSE1 and VPS27. Interacts with MVB12 and SRN2. The UEV domain is required for the interaction of the complex with ubiquitin. Present with 1360 molecules/cell in log phase SD medium. Belongs to the ubiquitin-conjugating enzyme family. UEV subfamily.