The inducible beta5i proteasome subunit contributes to proinsulin degradation in GRP94 deficient beta cells and is overexpressed in type 2 diabetes pancreatic islets.

Proinsulin is a misfolding-prone protein and its efficient breakdown is critical, when b-cells are confronted with high insulin biosynthetic demands, to prevent endoplasmic reticulum stress, a key trigger of secretory dysfunction and, if uncompensated, apoptosis. Proinsulin degradation is thought to be performed by the constitutively expressed standard proteasome, while the roles of other proteasomes are unknown. We recently demonstrated that deficiency of the proinsulin chaperone GRP94 causes impaired proinsulin-handling and defective insulin-secretion associated with a compensated endoplasmic reticulum stress-response. Taking advantage of this model of restricted folding-capacity, we investigated the role of different proteasomes in proinsulin degradation, reasoning that insulin secretory dynamics require an inducible protein degradation-system. We show that expression of only one enzymatically active proteasome subunit, namely the inducible beta5i-subunit, was increased in GRP94 CRISPR/Cas9 KO cells. Additionally, the level of beta5i containing intermediate proteasomes was significantly increased in these cells, as was beta5i-related chymotrypsin-like activity. Moreover, proinsulin levels were restored in GRP94 KO upon beta5i siRNA-mediated knock down. Finally, the fraction of beta-cells expressing beta5i subunit is increased in human islets from type 2 diabetes patients. We conclude that beta5i is an inducible proteasome subunit dedicated to the degradation of mishandled proinsulin.