Acts as a regulatory subunit of the 26 proteasome which is involved in the ATP-dependent degradation of ubiquitinated proteins. Is not a genuine component of the 26S proteasome, but an auxiliary factor that interacts with the proteasomal ATPase of 19S regulatory particle (RP). Acts as a chaperone which regulates the highly structured assembly of the 19S regulatory particle. Involved in the substrate specificity of the 26S proteasome and is especially involved in the degradation of ubiquitinated GCN4. May contribute to the stability of the 26S proteasome in some stress conditions. Associates with the 19S proteasome regulatory particle (RP). Interacts directly with RPT5 and RPT6. Present with 2210 molecules/cell in log phase SD medium. Belongs to the WD repeat PAAF1/RPN14 family.