Promotes the association of the proteasome activator complex subunit PSME3 with the 20S proteasome and regulates its activity. Inhibits PSME3-mediated degradation of some proteasome substrates, probably by affecting their diffusion rate into the catalytic chamber of the proteasome. Also inhibits the interaction of PSME3 with COIL, inhibits accumulation of PSME3 in Cajal bodies and positively regulates the number of Cajal bodies in the nucleus. Interacts (via C-terminus) with both free and 20S proteasome-bound forms of the proteasome activator complex subunit PSME3/PA28G; the interaction is direct. Expressed in skeletal muscle. Shows relatively constant expression in both proliferating myoblasts and in differentiated myotubes, when assayed in C2C12 cell line (at protein level). Up-regulated in response to denervation-induced skeletal muscle atrophy. Induced by MYOD1. Phosphorylation by CK2 stabilizes the interaction with PSME3.