title: Regulation of insulin-stimulated GLUT4 translocation by Munc18c in 3T3L1 adipocytes.
abstract: Insulin stimulates glucose transporter (GLUT) 4 vesicle translocation from intracellular storage sites to the plasma membrane in 3T3L1 adipocytes through a VAMP2- and syntaxin 4-dependent mechanism. We have observed that Munc18c, a mammalian homolog of the yeast syntaxin-binding protein n-Sec1p, competed for the binding of VAMP2 to syntaxin 4. Consistent with an inhibitory function for Munc18c, expression of Munc18c, but not the related Munc18b isoform, prevented the insulin stimulation of GLUT4 and IRAP/vp165 translocation to the plasma membrane without any significant effect on GLUT1 trafficking. As expected, overexpressed Munc18c was found to co-immunoprecipitate with syntaxin 4 in the basal state. However, these complexes were found to dissociate upon insulin stimulation. Furthermore, endogenous Munc18c was predominantly localized to the plasma membrane and its distribution was not altered by insulin stimulation. Although expression of enhanced green fluorescent protein-Munc18c primarily resulted in a dispersed cytosolic distribution, co-expression with syntaxin 4 resulted in increased localization to the plasma membrane. Together, these data suggest that Munc18c inhibits the docking/fusion of GLUT4-containing vesicles by blocking the binding of VAMP2 to syntaxin 4. Insulin relieves this inhibition by inducing the dissociation of Munc18c from syntaxin 4 and by sequestering Munc18c to an alternative plasma membrane binding site.
authors.affiliation: Department of Physiology and Biophysics, University of Iowa, Iowa City, Iowa 52242, USA.
authors.forename: D C
authors.initials: DC
authors.lastname: Thurmond
authors.forename: B P
authors.initials: BP
authors.lastname: Ceresa
authors.forename: S
authors.initials: S
authors.lastname: Okada
authors.forename: J S
authors.initials: JS
authors.lastname: Elmendorf
authors.forename: K
authors.initials: K
authors.lastname: Coker
authors.forename: J E
authors.initials: JE
authors.lastname: Pessin
mesh_terms: D016475:3T3 Cells; D017667:Adipocytes / Q000187:drug effects* / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D007328:Insulin / Q000494:pharmacology*; D007329:Insulin Antagonists / Q000494:pharmacology; D053590:Interleukin 1 Receptor Antagonist Protein; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D051938:Munc18 Proteins; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D011506:Proteins / Q000378:metabolism*; D050765:Qa-SNARE Proteins; D050766:Qb-SNARE Proteins; D050767:Qc-SNARE Proteins; D050683:R-SNARE Proteins; D050600:SNARE Proteins; D012795:Sialoglycoproteins / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism; D033921:Vesicular Transport Proteins
chemical_list: D002352:Carrier Proteins; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; C507763:IL1RN protein, human; C507764:Il1rn protein, mouse; D007328:Insulin; D007329:Insulin Antagonists; D053590:Interleukin 1 Receptor Antagonist Protein; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D051938:Munc18 Proteins; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D011506:Proteins; D050765:Qa-SNARE Proteins; D050766:Qb-SNARE Proteins; D050767:Qc-SNARE Proteins; D050683:R-SNARE Proteins; C100493:SNAP23 protein, human; D050600:SNARE Proteins; D012795:Sialoglycoproteins; C495678:Slc2a1 protein, mouse; C495693:Slc2a4 protein, mouse; C107017:Snap23 protein, mouse; C497941:Stxbp2 protein, mouse; C497943:Stxbp3 protein, mouse; D033921:Vesicular Transport Proteins