Granuphilin Modulates the Exocytosis of Secretory Granules through Interaction with Syntaxin 1a

Abstract

The molecular mechanism for the regulated exocytosis of dense-core granules in endocrine cells remains relatively uncharacterized compared to that of synaptic vesicles in neurons. A novel set of Rab and its effector, Rab27a/granuphilin, which is localized on insulin granules in pancreatic beta cells, was recently identified. Here we demonstrate that granuphilin directly binds to syntaxin 1a on the plasma membrane, and this interaction is regulated by Rab27a. Granuphilin shows affinity to syntaxin 1a with a closed conformation but not to mutant syntaxin 1a, which adopts an open conformation constitutively. Overexpression of granuphilin significantly enhances basal insulin secretion but profoundly inhibits high K⁺-induced insulin secretion. The effect of granuphilin on insulin secretion was impaired by its mutation that disrupts the binding to either Rab27a or syntaxin 1a. Thus, granuphilin is the first regulator in the exocytotic pathway that functions by directly connecting two critical vesicle transport proteins, Rab and SNARE.

We thank M. Hosaka (Gunma University) for generous support and useful advice. We also thank K. Kasai and K. Akagawa (Kyorin University School of Medicine) for the expression vector of myc-tagged syntaxin 1a, T. C. Südhof (The University of Texas Southwestern Medical Center) for rat syntaxin 1a cDNAs, and H. Bujard (University of California Berkeley) for tetracycline-inducible expression vectors.

This work was supported by a grant in aid for scientific research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and in part by grants from the Yamanouchi Foundation for Research on Metabolic Disorders and a Japan Insulin Study Group Award to T.I.