Granuphilin molecularly docks insulin granules to the fusion machinery.
The Rab27a effector granuphilin is specifically localized on insulin granules and is involved in their exocytosis. Here we show that the number of insulin granules morphologically docked to the plasma membrane is markedly reduced in granuphilin-deficient beta cells. Surprisingly, despite the docking defect, the exocytosis of insulin granules in response to a physiological glucose stimulus is significantly augmented, which results in increased glucose tolerance in granuphilin-null mice. The enhanced secretion in mutant beta cells is correlated with a decrease in the formation of the fusion-incompetent syntaxin-1a-Munc18-1 complex, with which granuphilin normally interacts. Furthermore, in contrast to wild-type granuphilin, its mutant that is defective in binding to syntaxin-1a fails to restore granule docking or the protein level of syntaxin-1a in granuphilin-null beta cells. Thus, granuphilin not only is essential for the docking of insulin granules but simultaneously imposes a fusion constraint on them through an interaction with the syntaxin-1a fusion machinery. These findings provide a novel paradigm for the docking machinery in regulated exocytosis.
Gomi
Hiroshi
H
Laboratory of Molecular Endocrinology and Metabolism, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.
Mizutani
Shin
S
Kasai
Kazuo
K
Itohara
Shigeyoshi
S
Izumi
Tetsuro
T
D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism; D005089:Exocytosis; D018014:Gene Transfer Techniques; D005822:Genetic Vectors; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D008561:Membrane Fusion / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D008958:Models, Molecular; D010641:Phenotype; D022142:Secretory Vesicles / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
D002352:Carrier Proteins; D007328:Insulin; C122015:Sytl4 protein, mouse; D033921:Vesicular Transport Proteins