Figure 10.

Two-step model for insulin-regulated traffic in adipocytes. The first retention step occurs at the TR-containing endosomes. GLUT4 and IRAP are sorted from the rapid TR recycling pathway because they are sequestered in specialized transport vesicles that bud slowly from endosomes. We propose that this step is common to differentiated and undifferentiated cells, and therefore not adipocyte specific. In adipocytes these vesicles do not fuse directly with the plasma membrane but there is an additional insulin-regulated retention step. GLUT4 and IRAP in this postendosomal pool constitute the adipocyte-specific compartment. This specialized post endosomal compartment may be a stable fusion/fission competent compartment or it may be a collection of tethered vesicles. The available data do not distinguish between these two possibilities. Proper insulin-induced redistribution of GLUT4 and IRAP to the cell surface involves the recruitment of GLUT4/IRAP from both endosomes and the adipocyte-specific compartment.