Acts as a chaperone during the assembly of the 26S proteasome, specifically of the PA700/19S regulatory complex (RC). In the initial step of the base subcomplex assembly is part of an intermediate PSMD10:PSMC4:PSMC5:PAAF1 module which probably assembles with a PSMD5:PSMC2:PSMC1:PSMD2 module. Independently of the proteasome, regulates EGF-induced AKT activation through inhibition of the RHOA/ROCK/PTEN pathway, leading to prolonged AKT activation. Plays an important role in RAS-induced tumorigenesis. Acts as an proto-oncoprotein by being involved in negative regulation of tumor suppressors RB1 and p53/TP53. Overexpression is leading to phosphorylation of RB1 and proteasomal degradation of RB1. Regulates CDK4-mediated phosphorylation of RB1 by competing with CDKN2A for binding with CDK4. Facilitates binding of MDM2 to p53/TP53 and the mono- and polyubiquitination of p53/TP53 by MDM2 suggesting a function in targeting the TP53:MDM2 complex to the 26S proteasome. Involved in p53-independent apoptosis. Involved in regulation of NF-kappa-B by retaining it in the cytoplasm. Binds to the NF-kappa-B component RELA and accelerates its XPO1/CRM1-mediated nuclear export. Part of transient complex containing PSMD10, PSMC4, PSMC5 and PAAF1 formed during the assembly of the 26S proteasome. Stays associated throughout the assembly of the PA700/19S RC and is released upon association with the 20S core. Interacts with PSMC4. Interacts with RB1. Interacts with CDK4. Interacts with MDM2. Interacts with RELA. Associates with a CDK4:CCND2 serine/threonine kinase complex. Interacts with ARHGDIA and increases the interaction between ARHGDIA and RHOA, hence promotes ARHGDIA inactivation of RHOA and ROCK. Tends to be up-regulated in cancer cells with RAS mutations, including lung cancers and adenocarconimas (at protein level). Was initially identified as a genuine component of the 26S proteasome.