Phosphorylation of the exocyst protein Exo84 by TBK1 promotes insulin-stimulated GLUT4 trafficking.
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.
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.|Department of Internal Medicine, University of Michigan, Ann Arbor, MI 48109, USA.|Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI 48109, USA.
Maeran
http://orcid.org/0000-0003-3660-022X
M
Uhm
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.
Merlijn
M
Bazuine
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.|Institute for Diabetes and Metabolic Health, Departments of Medicine and Pharmacology, University of California, San Diego, La Jolla, CA 92093, USA.
Peng
http://orcid.org/0000-0002-4924-0086
P
Zhao
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.
Shian-Huey
SH
Chiang
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.
Tingting
T
Xiong
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.
Sheelarani
S
Karunanithi
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA.
Louise
http://orcid.org/0000-0001-5501-4774
L
Chang
Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA. asaltiel@ucsd.edu.|Department of Internal Medicine, University of Michigan, Ann Arbor, MI 48109, USA.|Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI 48109, USA.|Institute for Diabetes and Metabolic Health, Departments of Medicine and Pharmacology, University of California, San Diego, La Jolla, CA 92093, USA.
Alan R
http://orcid.org/0000-0002-9726-9828
AR
Saltiel
D041721:3T3-L1 Cells; D017667:Adipocytes / Q000187:drug effects* / Q000378:metabolism; D000818:Animals; D051275:Glucose Transporter Type 4 / Q000235:genetics / Q000378:metabolism*; D007004:Hypoglycemic Agents / Q000494:pharmacology; D015151:Immunoblotting; D007328:Insulin / Q000494:pharmacology*; D051379:Mice; D009154:Mutation; D010766:Phosphorylation; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000187:drug effects; D034622:RNA Interference; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*; D020725:ral GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
C000625047:Exo84 protein, mouse; D051275:Glucose Transporter Type 4; D007004:Hypoglycemic Agents; D007328:Insulin; D033921:Vesicular Transport Proteins; C498588:Tbk1 protein, mouse; D017346:Protein Serine-Threonine Kinases; C118120:Rala protein, mouse; D020725:ral GTP-Binding Proteins