Component of the ESCRT-I complex, a regulator of vesicular trafficking process. Required for normal endocytic and biosynthetic traffic to the yeast vacuole. 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. Self-associates. Interacts with VPS27; the interaction mediates the association with the ESCRT-0 complex. Interacts with VPS20; the interaction mediates the association with the ESCRT-III complex. Present with 1420 molecules/cell in log phase SD medium. Belongs to the VPS28 family.