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 (By similarity). 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 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 (By similarity). 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 (By similarity). Interacts with ARHGDIA and increases the interaction between ARHGDIA and RHOA, hence promotes ARHGDIA inactivation of RHOA and ROCK (By similarity). Up-regulated by activated HRAS.