Insulin-stimulated GLUT4 translocation is central to glucose homeostasis. Functional assays to distinguish individual steps in the GLUT4 translocation process are lacking, thus limiting progress towards elucidation of the underlying molecular mechanism. Prof. Tao Xu's group developed a robust method that relies on dynamic tracking of single GLUT4 vesicles in real time, for dissecting and systematically analyzing the docking, priming, and fusion steps of GLUT4 vesicles with the cell surface in living adipocytes. Using this method they showed that the preparation of GLUT4 vesicles for

fusion competence after docking at the surface is a key step regulated by insulin, while the docking step itself is regulated by PI3K and its downstream effector Rab GAP AS160. Their data showed that Akt-dependent phosphorylation of AS160 is not the major regulated step in GLUT4 trafficking, implicating alternative Akt substrates or alternative signaling pathways downstream of vesicle docking at the cell surface as the major regulatory node.