Plays an important role in the control of proteasome function. Inhibits the hydrolysis of protein and peptide substrates by the 20S proteasome. Enhances 26S proteasome function by promoting its assembly through the interaction with the assembly chaperones PSMD9 and PMSD5. Functions together with ntc to control non-apoptotic caspase activation during sperm individualization. In testis, is required for proper protein degradation and germline cell cycle progression. Interacts with ntc. Interacts with Tnks (via ANK repeats). Interacts with 20S proteasome subunits Prosalpha1, Prosalpha3, Prosalpha4 and Prosalpha6, but not with Prosalpha2 and Prosalpha5. The interaction with Prosalpha4 is reduced by PI31 ADP-ribosylation. Interacts with the 19S assembly chaperones PSMD5 and PSMD9; these interactions are increased by PI31 ADP-ribosylation. In the spermatid, localizes to the actin-based individualization complex. In testis, expressed in germline stem cells and primary spermatocytes (at protein level). Expressed in adults. The Hydrophobic-Tyrosine-X (HbYX) motif is involved in proteasome activation. ADP-ribosylation by Tnks increases its affinity for assembly chaperones PSMD9 and PMSD5, sequestering them away from the proteasome Rpt base subunits and promoting in this way 26S proteasome assembly. Also interferes with its inhibitory effect on 20S proteasome. Larval lethal. Mutant larvae develop melanotic tumors. Belongs to the proteasome inhibitor PI31 family.