An insulin granule biogenesis checkpoint

Abstract

Regulated secretion of insulin from pancreatic Beta cells is a pivotal process that controls glucose homeostasis and thereby energy balance. While most attention has been drawn to distal steps of insulin exocytosis, i.e. vesicle fusion with the plasma membrane, mechanisms underlying correct formation of secretory granules are still poorly elucidated. Recent work in our lab unveiled a secretory granule quality control checkpoint at the trans-Golgi-network (TGN), which ensures the biogenesis of functional transport carriers. We found two proteins, Protein Kinase D (PKD) and its novel target Arfaptin-1, at the center of this system. Arfaptin-1 is a BAR-domain containing protein and known interactor of ADP ribosylation factors (ARFs). BAR domains can prevent membrane fission through their ability to shield necks of budding vesicles from fission-inducing factors. We demonstrate that PKD phosphorylates Arfaptin-1at Serine 132, which disrupts the ability of Arfaptin-1 to inhibit the activity of ADP ribosylation factor, an important component of the vesicle scission machinery. The physiological significance of this regulatory mechanism is evidenced by loss of glucose-stimulated insulin secretion due to granule scission defects in pancreatic Beta cells expressing non-phosphorylatable Arfaptin-1. Accordingly, depletion of Arfaptin-1 leads to generation of small non-functional secretory granules due to premature fission. Hence, PKD-mediated Arfaptin-1 phosphorylation is the initiating and limiting step in secretory granule scission, which ensures that only complete and functional granules can detach from the TGN.