Role of the Protein Kinase TBK1 in Insulin-Stimulated Glucose Transport.

[摘要] Insulin stimulates glucose uptake in muscle and fat by promoting translocation of the facilitative transporter GLUT4 from intracellular compartments to the plasma membrane. While the pathways involved in GLUT4 vesicle trafficking are not completely understood, numerous studies have shown that small G proteins critically integrate signaling with the trafficking machineries in this process. Among the targets of these G proteins is the exocyst complex, which facilitates the tethering of GLUT4 vesicles to the plasma membrane. GLUT4 translocation requires both the assembly and recognition of the exocyst for targeted exocytosis, and G proteins mediate both of these processes. Exocyst assembly is controlled by activation of the Rho subfamily G protein TC10, while exocyst recognition is mediated by the G protein RalA. However, how GLUT4 vesicles dissociate from the G protein after binding is unclear, and the sequence of events that disengage GLUT4 vesicles from the individual subunits of the exocyst remain uncertain. Here I report that the protein kinase TBK1 is required for insulin-stimulated glucose transport and GLUT4 translocation in parallel with the Akt signaling pathway. Upon activation of RalA, TBK1 directly phosphorylates the exocyst subunit Exo84, a crucial step in insulin-stimulated glucose uptake. Knockdown of TBK1 blocks insulin-stimulated glucose uptake and GLUT4 translocation, and ectopic overexpression of a kinase-inactive mutant of TBK1 reduces insulin-stimulated glucose uptake in 3T3-L1 adipocytes. The phosphorylation of Exo84 on multiple sites by TBK1 reduces its affinity for RalA, and allows its release from the exocyst. Therefore, the interaction of TBK1/RalA/exocyst complex is dissociated upon Exo84 phosphorylation by TBK1 but overexpression of a kinase-inactive mutant of TBK1 blocks the dissociation of the complex, and treatment of 3T3-L1 adipocytes with specific inhibitors of TBK1 reduces the rate of complex dissociation. Introduction of mutant forms of Exo84 that prevent or mimic phosphorylation blocks insulin-stimulated GLUT4 translocation. Thus, these data indicate that TBK1 controls GLUT4 vesicle engagement and disengagement from the exocyst, suggesting that the exocyst is more than just a tethering complex for the GLUT4 vesicle, but also a ;;gatekeeper’ controlling vesicle fusion at the plasma membrane.

[发布机构] University of Michigan

[效力级别] Physiology

[关键词] Insulin-stimulated glucose transport;Physiology;Health Sciences;Molecular and Integrative Physiology