The 26S proteasome is involved in the ATP-dependent degradation of ubiquitinated proteins. The regulatory (or ATPase) complex confers ATP dependency and substrate specificity to the 26S complex (Probable). Interacts with transit peptides of proteins targeted to the chloroplast, and may be involved in the degradation of unimported plastid protein precursors (PubMed:24846764). Plays a essential role in the gametophyte development (PubMed:19223514). Involved in tolerance to zinc deficiency, possibly through alleviation of oxidative stresses or processing of poly-ubiquitinated proteins (PubMed:21389614). Component of the 19S regulatory particle (RP/PA700) base subcomplex of the 26S proteasome. The 26S proteasome is composed of a core protease (CP), known as the 20S proteasome, capped at one or both ends by the 19S regulatory particle (RP/PA700). The RP/PA700 complex is composed of at least 17 different subunits in two subcomplexes, the base and the lid, which form the portions proximal and distal to the 20S proteolytic core, respectively. Ubiquitous. Displays a severe male gametophyte development defects in cv. Wassilewskija but not in cv. Columbia due to the complementation by RPT5B. Belongs to the AAA ATPase family.