Acts as a regulatory subunit of the 26S proteasome which is involved in the ATP-dependent degradation of ubiquitinated proteins. May help to control the degradation of one or more factors that repress cytokinin signaling. Plays an important role for balancing cell expansion with cell proliferation rates during shoot development. Component of the 19S regulatory particle (RP/PA700) lid 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. Interacts with PUB22 and PUB23. Binds to the translation initiation factors TIF3E1 (PubMed:19704582). Interacts with UCH1 and UCH2 (PubMed:22951400). Ubiquitous with highest expression in flowers. By cytokinins. Ubiquitinated by PUB22 and PUB23. Decreased rate of leaf formation, reduced root elongation, delayed skotomorphogenesis and altered growth responses to exogenous cytokinins. Increased sensitivity to heat shock and increased tolerance to oxidative stress. Decreased 26S proteasome accumulation. In flowers and cotyledons, epidermal cells were larger than those in the wild type. Belongs to the proteasome subunit S14 family.