LAUSR

The discovery of missense mutations in the signal peptide of preproinsulin that causes congenital diabetes highlights the critical role of ER translocation in insulin biosynthesis. However, the mechanisms that govern these early events in insulin biosynthesis are poorly understood. GWAS studies have shown that translocon-associated protein alpha (TRAPα) is a high risk allele for both type 2 diabetes and gestational diabetes. Yet, the pathophysiological role of TRAPα in β-cell function remains unknown. Here we show that TRAPα is the most upregulated subunit of the TRAP complexes in isolated islets exposed to high glucose, suggesting that it may play an important role in insulin biosynthesis. Crispr/cas9 mediated TRAPα deletion in pancreatic INS832/13 cells causes inefficient preproinsulin translocation across the ER membrane, resulting in an accumulation of un-translocated preproinsulin in the cells. Surprisingly, TRAPα deletion also affects proinsulin ER export, leading to a decreased insulin production in INS832/13 cells. Re-introducing an epitope-tagged TRAPα to TRAPα KO cells rescued preproinsulin translocation defect and restored insulin production in a dose-dependent manner in TRAPα KO cells infected with adenovirus encoding TRAPα. Importantly, TRAPα deletion did not significantly affect the biogenesis or secretion of α1-antitrypsin, indicating that TRAPα promotes the ER translocation and subsequent trafficking of only a subset of secreted proteins. These results establish preproinsulin as a client protein for TRAPα in ER translocation and unveil unanticipated function of TRAPα in proinsulin ER export and insulin biosynthesis, and provide key insights into the disease pathogenesis associated with TRAPα deficiency. Disclosure X. Ll: None. Funding National Natural Science Foundation of China (81570699, 81620108004, 81370895); National Institutes of Health (R01DK088856, DK111174)