Binds and presumably selects ubiquitin-conjugates for destruction. The 26S proteasome is composed of a core protease, known as the 20S proteasome, capped at one or both ends by the 19S regulatory complex (RC). The RC is composed of at least 18 different subunits in two subcomplexes, the base and the lid, which form the portions proximal and distal to the 20S proteolytic core, respectively (By similarity). Interacts with Ubc4. Belongs to the proteasome subunit S5A family. Extended N-terminus.