Deficient endoplasmic reticulum translocon‐associated protein complex limits the biosynthesis of proinsulin and insulin

The conserved endoplasmic reticulum (ER) membrane protein TRAPα (translocon-associated protein, also known as signal sequence receptor 1, SSR1) has been reported to play a critical but unclear role in insulin biosynthesis.TRAPα/SSR1 is one component of a four-protein complex including TRAPβ/SSR2, TRAPγ/SSR3, and TRAPδ/SSR4.The TRAP complex topologically has a small exposure on the cytosolic side of the ER via its TRAPγ/SSR3 subunit, whereas TRAPβ/SSR2 and TRAPδ/SSR4 function along with TRAPα/SSR1 largely on the luminal side of the ER membrane.Here, we have examined pancreatic β-cells with deficient expression of either TRAPβ/SSR2 or TRAPδ/SSR4, which does not perturb mRNA expression levels of other TRAP subunits, or insulin mRNA.However, deficient protein expression of TRAPβ/SSR2, and to a lesser degree, TRAPδ/SSR4, diminishes the protein levels of other TRAP subunits, concomitant with deficient steady-state levels of proinsulin and insulin.Deficient TRAPβ/SSR2 or TRAPδ/SSR4 is not associated with any apparent defect of exocytotic mechanism but rather by a decreased abundance of the proinsulin and insulin that accompanies glucose-stimulated secretion.Amino acid pulse-labeling directly establishes that much of the steady state deficiency of intracellular proinsulin can be accounted for by diminished proinsulin biosynthesis; observed in a pulse-labeling as short as 5 min.The proinsulin and insulin levels in TRAPβ/SSR2 or TRAPδ/SSR4 null mutant β-cells are notably recovered upon re-expression of the missing TRAP subunit, accompanying a rebound of proinsulin biosynthesis.Remarkably, overexpression of TRAPα/SSR1 can also suppress defects in β-cells with diminished expression of TRAPβ/SSR2, strongly suggesting that TRAPβ/SSR2 is needed to support TRAPα/SSR1 function.