Site of docking and fusion of insulin secretory granules in live MIN6 beta cells analyzed by TAT-conjugated anti-syntaxin 1 antibody and total internal reflection fluorescence microscopy.
To determine the site of insulin exocytosis in the pancreatic beta cell plasma membrane, we analyzed the interaction between the docking/fusion of green fluorescent protein-tagged insulin granules and syntaxin 1 labeled by TAT-conjugated Cy3-labeled antibody (Ab) using total internal reflection fluorescence microscopy (TIRFM). Monoclonal Ab against syntaxin 1 was labeled with Cy3 then conjugated with the protein transduction domain of HIV-1 TAT. TAT-conjugated Cy3-labeled anti-syntaxin 1 Ab was transduced rapidly into the subplasmalemmal region in live MIN6 beta cells, which enabled us to observe the spatial organization and distribution of endogenous syntaxin 1. TIRFM imaging revealed that syntaxin 1 is distributed in numerous separate clusters in the intact plasma membrane, where insulin secretory granules were docked preferentially to the sites of syntaxin 1 clusters, colocalizing with synaptosomal-associated protein of 25 kDa (SNAP-25) clusters. TIRFM imaging analysis of the motion of single insulin granules demonstrated that the fusion of insulin secretory granules stimulated by 50 mm KCl occurred exclusively at the sites of the syntaxin 1 clusters. Cholesterol depletion by methyl-beta-cyclodextrin treatment, in which the syntaxin 1 clusters were disintegrated, decreased the number of docked insulin granules, and, eventually the number of fusion events was significantly reduced. Our results indicate that 1) insulin exocytosis occurs at the site of syntaxin 1 clusters; 2) syntaxin 1 clusters are essential for the docking and fusion of insulin granules in MIN6 beta cells; and 3) the sites of syntaxin 1 clusters are distinct from flotillin-1 lipid rafts.
Ohara-Imaizumi
Mica
M
Department of Biochemistry (II), Kyorin University School of Medicine, Shinkawa 6-20-2, Mitaka, Tokyo 181-8611, Japan.
Nishiwaki
Chiyono
C
Kikuta
Toshiteru
T
Kumakura
Konosuke
K
Nakamichi
Yoko
Y
Nagamatsu
Shinya
S
D000818:Animals; D000911:Antibodies, Monoclonal; D000954:Antigens, Surface / Q000032:analysis / Q000276:immunology*; D002232:Carbocyanines; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry / Q000648:ultrastructure; D005089:Exocytosis; D019793:Fluorescein; D005456:Fluorescent Dyes; D015696:Gene Products, tat / Q000276:immunology*; D049452:Green Fluorescent Proteins; D015497:HIV-1 / Q000737:chemistry; D007328:Insulin / Q000235:genetics / Q000378:metabolism*; D000078790:Insulin Secretion; D007425:Intracellular Membranes / Q000648:ultrastructure; D007515:Islets of Langerhans / Q000648:ultrastructure*; D008164:Luminescent Proteins / Q000235:genetics; D008561:Membrane Fusion; D008565:Membrane Proteins / Q000032:analysis; D051379:Mice; D008856:Microscopy, Fluorescence / Q000379:methods*; D009419:Nerve Tissue Proteins / Q000032:analysis / Q000276:immunology*; D011993:Recombinant Fusion Proteins; D022142:Secretory Vesicles / Q000648:ultrastructure*; D050825:Synaptosomal-Associated Protein 25; D050827:Syntaxin 1; D014162:Transfection; D054322:tat Gene Products, Human Immunodeficiency Virus
D000911:Antibodies, Monoclonal; D000954:Antigens, Surface; D002232:Carbocyanines; D005456:Fluorescent Dyes; D015696:Gene Products, tat; D007328:Insulin; D008164:Luminescent Proteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D011993:Recombinant Fusion Proteins; C494209:Snap25 protein, mouse; C494214:Stx1a protein, mouse; D050825:Synaptosomal-Associated Protein 25; D050827:Syntaxin 1; C089187:cyanine dye 3; D054322:tat Gene Products, Human Immunodeficiency Virus; D049452:Green Fluorescent Proteins; D019793:Fluorescein