Based on limited published evidence, PSME3IP1 (also known as NIP30) promotes association of the proteasome activator PSME3 with the 20S proteasome while inhibiting PSME3-mediated substrate degradation by regulating substrate diffusion into the catalytic chamber 1. It also inhibits PSME3-COIL interaction and suppresses PSME3 accumulation in Cajal bodies while positively regulating Cajal body number. PSME3IP1 functions as a negative regulator of proteasomal protein catabolic processes and protein binding, localizing to the nucleoplasm and nucleus with roles in mRNA splicing.