Filamentous actin regulates insulin exocytosis through direct interaction with Syntaxin 4.
Glucose-induced insulin exocytosis is coupled to associations between F-actin and SNARE proteins, although the nature and function of these interactions remains unknown. Toward this end we show here that both Syntaxin 1A and Syntaxin 4 associated with F-actin in MIN6 cells and that each interaction was rapidly and transiently diminished by stimulation of cells with d-glucose. Of the two isoforms, only Syntaxin 4 was capable of interacting directly with F-actin in an in vitro sedimentation assay, conferred by the N-terminal 39-112 residues of Syntaxin 4. The 39-112 fragment was capable of selective competitive inhibitory action, disrupting endogenous F-actin-Syntaxin 4 binding in MIN6 cells. Disruption of F-actin-Syntaxin 4 binding correlated with enhanced glucose-stimulated insulin secretion, mediated by increased granule accumulation at the plasma membrane and increased Syntaxin 4 accessibility under basal conditions. However, no increase in basal level Syntaxin 4-VAMP2 association occurred with either latrunculin treatment or expression of the 39-112 fragment. Taken together, these data disclose a new underlying mechanism by which F-actin negatively regulates exocytosis via binding and blocking Syntaxin 4 accessibility, but they also reveal the existence of additional signals and/or steps required to trigger the subsequent docking and fusion steps of exocytosis.
Jewell
Jenna L
JL
Department of Biochemistry and Molecular Biology, Center for Diabetes Research, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA.
Luo
Wei
W
Oh
Eunjin
E
Wang
Zhanxiang
Z
Thurmond
Debbie C
DC
D000199:Actins / Q000378:metabolism*; D020816:Amino Acid Motifs; D000818:Animals; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D005089:Exocytosis; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D011485:Protein Binding; D020033:Protein Isoforms; D050765:Qa-SNARE Proteins / Q000378:metabolism*; D011817:Rabbits; D053778:Thiazolidines / Q000494:pharmacology
D000199:Actins; D019086:Bridged Bicyclo Compounds, Heterocyclic; D007328:Insulin; D020033:Protein Isoforms; D050765:Qa-SNARE Proteins; D053778:Thiazolidines; C037068:latrunculin B