title: Docking is not a prerequisite but a temporal constraint for fusion of secretory granules.
abstract: We examined secretory granule dynamics using total internal reflection fluorescence microscopy in normal pancreatic beta cells and their mutants devoid of Rab27a and/or its effector, granuphilin, which play critical roles in the docking and recruitment of insulin granules to the plasma membrane. In the early phase of glucose stimulation in wild-type cells, we observed marked fusion of granules recruited from a relatively distant area, in parallel with that from granules located underneath the plasma membrane. Furthermore, despite a lack of granules directly attached to the plasma membrane, both spontaneous and evoked fusion was increased in granuphilin-null cells. In addition to these granuphilin-null phenotypes, Rab27a/granuphilin doubly deficient cells showed the decreases in granules located next to the docked area and in fusion from granules near the plasma membrane in the early phase of glucose-stimulated secretion, similar to Rab27a-mutated cells. Thus, the two proteins play nonoverlapping roles in insulin exocytosis: granuphilin acts on the granules underneath the plasma membrane, whereas Rab27a acts on those in a more distal area. These findings demonstrate that, in contrast to our conventional understanding, stable attachment of secretory granules to the plasma membrane is not prerequisite but temporally inhibitory for both spontaneous and evoked fusion.
authors.affiliation: Department of Molecular Medicine, Institute for Molecular and Cellular Regulation, Gunma University, 3-39-15, Showa-machi, Maebashi 371-8512, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Kasai
authors.forename: Takuji
authors.initials: T
authors.lastname: Fujita
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Gomi
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Izumi
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D005089:Exocytosis; D005947:Glucose / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D050417:Insulin-Secreting Cells / Q000166:cytology; D051379:Mice; D008809:Mice, Inbred C3H; D018345:Mice, Knockout; D010641:Phenotype; D011485:Protein Binding; D022142:Secretory Vesicles / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000378:metabolism; D020691:rab GTP-Binding Proteins / Q000378:metabolism; D000075745:rab27 GTP-Binding Proteins
chemical_list: D007328:Insulin; C122015:Sytl4 protein, mouse; D033921:Vesicular Transport Proteins; D000075745:rab27 GTP-Binding Proteins; C512962:Rab27a protein, mouse; D020691:rab GTP-Binding Proteins; D005947:Glucose