p24 family type 1 transmembrane proteins are required for insulin biosynthesis and secretion in pancreatic β-cells

The p24 protein family have multiple functions in protein transport in the early secretory pathway. In this study we examined the role of p24 proteins in insulin transport. Several members were detected in insulinoma cell lines and rat islets and expression levels positively correlated with insulin abundance, particularly for p24δ1 and p24β1. Knocking down p24δ1 in insulinoma cell lines, which also resulted in the concomitant knock-down of other family members, impaired glucose-stimulated insulin secretion, decreased total cellular insulin content and reduced proinsulin biosynthesis. There was no effect on overall protein biosynthesis or ER stress. These results suggest that p24δ1 and possibly other p24 family proteins are required for normal insulin biosynthesis and subsequent secretion in pancreatic β-cells.