title: Granuphilin exclusively mediates functional granule docking to the plasma membrane.
abstract: In regulated exocytosis, it is generally assumed that vesicles must stably "dock" at the plasma membrane before they are primed to become fusion-competent. However, recent biophysical analyses in living cells that visualize fluorescent secretory granules have revealed that exocytic behaviors are not necessarily uniform: some granules beneath the plasma membrane are resistant to Ca(2+)-triggered release, while others are accelerated to fuse without a pause for stable docking. These findings suggest that stable docking is unnecessary, and can even be inhibitory or nonfunctional, for fusion. Consistently, pancreatic β cells deficient in the Rab27 effector, granuphilin, lack insulin granules directly attached to the plasma membrane in electron micrographs but nevertheless exhibit augmented exocytosis. Here we directly compare the exocytic behaviors between granuphilin-positive and -negative insulin granules. Although granuphilin makes granules immobile and fusion-reluctant beneath the plasma membrane, those granuphilin-positive, docked granules release a portion of granuphilin upon fusion, and fuse at a frequency and time course similar to those of granuphilin-negative undocked granules. Furthermore, granuphilin forms a 180-nm cluster at the site of each docked granule, along with granuphilin-interacting Rab27a and Munc18-1 clusters. These findings indicate that granuphilin is an exclusive component of the functional and fusion-inhibitory docking machinery of secretory granules.
authors.affiliation: Laboratory of Molecular Endocrinology and Metabolism, Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.
authors.forename: Kouichi
authors.initials: K
authors.lastname: Mizuno
authors.affiliation: Laboratory of Molecular Endocrinology and Metabolism, Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.
authors.forename: Takuji
authors.initials: T
authors.lastname: Fujita
authors.affiliation: Laboratory of Molecular Endocrinology and Metabolism, Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Gomi
authors.affiliation: Laboratory of Molecular Endocrinology and Metabolism, Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, Gunma 371-8512, Japan.|Research Program for Signal Transduction, Division of Endocrinology, Metabolism and Signal Research, Gunma University Initiative for Advanced Research, Maebashi, Gunma 371-8512, Japan.
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Izumi
mesh_terms: D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D003594:Cytoplasmic Granules / Q000378:metabolism; D005089:Exocytosis / Q000502:physiology*; D006801:Humans; D007328:Insulin / Q000235:genetics / Q000378:metabolism; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D007340:Insulinoma / Q000473:pathology; D008561:Membrane Fusion; D062105:Molecular Docking Simulation; D010190:Pancreatic Neoplasms / Q000473:pathology; D011498:Protein Precursors / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D022142:Secretory Vesicles / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000502:physiology*
chemical_list: D007328:Insulin; D011498:Protein Precursors; D011993:Recombinant Fusion Proteins; C512779:SYTL4 protein, human; D033921:Vesicular Transport Proteins; C013737:preproinsulin