Subunit of the 11S REG-gamma (also called PA28-gamma) proteasome regulator, a donut-shaped homoheptamer which associates with the proteasome. 11S REG-gamma activates the trypsin-like catalytic subunit of the proteasome but inhibits the chymotrypsin-like and postglutamyl-preferring (PGPH) subunits. Facilitates the MDM2-TP53/p53 interaction which promotes ubiquitination- and MDM2-dependent proteasomal degradation of TP53/p53, limiting its accumulation and resulting in inhibited apoptosis after DNA damage. May also be involved in cell cycle regulation.