A complex of Rab13 with MICAL-L2 and α-actinin-4 is essential for insulin-dependent GLUT4 exocytosis

Rab13 is necessary for insulin-regulated GLUT4-vesicle exocytosis in muscle. Biochemical and imaging analyses provide evidence that activated Rab13 engages a scaffold protein MICAL-L2 to form a complex with Rab13 and α-actinin-4. Through GLUT4 interaction with α-actinin-4, GLUT4 vesicles are recruited to the muscle plasma membrane.

Insulin promotes glucose uptake into skeletal muscle through recruitment of glucose transporter 4 (GLUT4) to the plasma membrane. Rab GTPases are molecular switches mobilizing intracellular vesicles, and Rab13 is necessary for insulin-regulated GLUT4–vesicle exocytic translocation in muscle cells. We show that Rab13 engages the scaffold protein MICAL-L2 in this process. RNA interference–mediated knockdown of MICAL-L2 or truncated MICAL-L2 (MICAL-L2-CT) impaired insulin-stimulated GLUT4 translocation. Insulin increased Rab13 binding to MICAL-L2, assessed by pull down and colocalization under confocal fluorescence and structured illumination microscopies. Association was also visualized at the cell periphery using TIRF microscopy. Insulin further increased binding of MICAL-L2 to α-actinin-4 (ACTN4), a protein involved in GLUT4 translocation. Rab13, MICAL-L2, and ACTN4 formed an insulin-dependent complex assessed by pull down and confocal fluorescence imaging. Of note, GLUT4 associated with the complex in response to insulin, requiring the ACTN4-binding domain in MICAL-L2. This was demonstrated by pull down with distinct fragments of MICAL-L2 and confocal and structured illumination microscopies. Finally, expression of MICAL-L2-CT abrogated the insulin-dependent colocalization of Rab13 with ACTN4 or Rab13 with GLUT4. Our findings suggest that MICAL-L2 is an effector of insulin-activated Rab13, which links to GLUT4 through ACTN4, localizing GLUT4 vesicles at the muscle cell periphery to enable their fusion with the membrane.