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Kinase for both inflammatory and insulin responses
After a meal, insulin released from the pancreas triggers glucose uptake by cells in part by promoting the translocation of the glucose transporter GLUT4 from intracellular vesicles to the plasma membrane. GLUT4 translocation requires a protein complex called the exocyst, which tethers GLUT4-containing vesicles to the plasma membrane. Uhm et al. found that the kinase TBK1, which mediates inflammatory responses, also phosphorylated exocyst subunits. These phosphorylation events were required for the fusion of GLUT4-containing vesicles with the plasma membrane. Insulin increased glucose uptake in adipocytes from wild-type mice but not in those from TBK1-deficient mice. The authors speculate that the involvement of TBK1 in insulin responses may be a means to counter the catabolic effects of inflammation.
Insulin stimulates glucose uptake through the translocation of the glucose transporter GLUT4 to the plasma membrane. The exocyst complex tethers GLUT4-containing vesicles to the plasma membrane, a process that requires the binding of the G protein (heterotrimeric guanine nucleotide–binding protein) RalA to the exocyst complex. We report that upon activation of RalA, the protein kinase TBK1 phosphorylated the exocyst subunit Exo84. Knockdown of TBK1 blocked insulin-stimulated glucose uptake and GLUT4 translocation; knockout of TBK1 in adipocytes blocked insulin-stimulated glucose uptake; and ectopic overexpression of a kinase-inactive mutant of TBK1 reduced insulin-stimulated glucose uptake in 3T3-L1 adipocytes. The phosphorylation of Exo84 by TBK1 reduced its affinity for RalA and enabled its release from the exocyst. Overexpression of a kinase-inactive mutant of TBK1 blocked the dissociation of the TBK1/RalA/exocyst complex, and treatment of 3T3-L1 adipocytes with specific inhibitors of TBK1 reduced the rate of complex dissociation. Introduction of phosphorylation-mimicking or nonphosphorylatable mutant forms of Exo84 blocked insulin-stimulated GLUT4 translocation. Thus, these data indicate that TBK1 controls GLUT4 vesicle engagement and disengagement from the exocyst, suggesting that exocyst components not only constitute a tethering complex for the GLUT4 vesicle but also act as “gatekeepers” controlling vesicle fusion at the plasma membrane.