Exophilin4/Slp2-a targets glucagon granules to the plasma membrane through unique Ca2+-inhibitory phospholipid-binding activity of the C2A domain.
Rab27a and Rab27b have recently been recognized to play versatile roles in regulating the exocytosis of secretory granules and lysosome-related organelles by using multiple effector proteins. However, the precise roles of these effector proteins in particular cell types largely remain uncharacterized, except for those in pancreatic beta cells and in melanocytes. Here, we showed that one of the Rab27a/b effectors, exophilin4/Slp2-a, is specifically expressed in pancreatic alpha cells, in contrast to another effector, granuphilin, in beta cells. Like granuphilin toward insulin granules, exophilin4 promotes the targeting of glucagon granules to the plasma membrane. Although the interaction of granuphilin with syntaxin-1a is critical for the targeting activity, exophilin4 does this primarily through the affinity of its C2A domain toward the plasma membrane phospholipids phosphatidylserine and phosphatidylinositol-4,5-bisphosphate. Notably, the binding activity to phosphatidylserine is inhibited by a physiological range of the Ca(2+) concentration attained after secretagogue stimulation, which presents a striking contrast to the Ca(2+)-stimulatory activity of the C2A domain of synaptotagmin I. Analyses of the mutant suggested that this novel Ca(2+)-inhibitory phospholipid-binding activity not only mediates docking but also modulates the subsequent fusion of the secretory granules.
Yu
Miao
M
Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan.
Kasai
Kazuo
K
Nagashima
Kazuaki
K
Torii
Seiji
S
Yokota-Hashimoto
Hiromi
H
Okamoto
Koichi
K
Takeuchi
Toshiyuki
T
Gomi
Hiroshi
H
Izumi
Tetsuro
T
D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002413:Cations, Divalent / Q000378:metabolism / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D005089:Exocytosis; D005934:Glucagon / Q000378:metabolism*; D050416:Glucagon-Secreting Cells / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D050417:Insulin-Secreting Cells / Q000378:metabolism; D008565:Membrane Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism; D010718:Phosphatidylserines / Q000378:metabolism; D010743:Phospholipids; D017434:Protein Structure, Tertiary; D022142:Secretory Vesicles / Q000737:chemistry / Q000378:metabolism*; D014018:Tissue Distribution; D033921:Vesicular Transport Proteins / Q000378:metabolism; D020691:rab GTP-Binding Proteins / Q000378:metabolism; D000075745:rab27 GTP-Binding Proteins
D002413:Cations, Divalent; D008565:Membrane Proteins; D019269:Phosphatidylinositol 4,5-Diphosphate; D010718:Phosphatidylserines; D010743:Phospholipids; C486043:SYTL2 protein, human; C512779:SYTL4 protein, human; D033921:Vesicular Transport Proteins; D000075745:rab27 GTP-Binding Proteins; D005934:Glucagon; C066694:RAB27A protein, human; C512960:Rab27B protein, human; D020691:rab GTP-Binding Proteins; D002118:Calcium