Association of AP1 adaptor complexes with GLUT4 vesicles.

Nycodenz gradients have been used to examine the in vitro effects of GTP-(gamma)-S on adaptor complex association with GLUT4 vesicles. On addition of GTP-(gamma)-S, GLUT4 fractionates as a heavier population of vesicles, which we suggest is due to a budding or coating reaction. Under these conditions there is an increase in co-sedimentation of GLUT4 with AP1, but not with AP3. Western blotting of proteins associated with isolated GLUT4 vesicles shows the presence of high levels of AP1 and some AP3 but very little AP2 adaptor complexes. Cell free, in vitro association of the AP1 complex with GLUT4 vesicles is increased approximately 4-fold by the addition of GTP-(gamma)-S and an ATP regenerating system. Following GTP-(gamma)-S treatment in vitro, ARF is also recruited to GLUT4 vesicles, and the temperature dependence of ARF recruitment closely parallels that of AP1. The recruitment of both AP1 and ARF are partially blocked by brefeldin A. These data demonstrate that the coating of GLUT4 vesicles can be studied in isolated cell-free fractions. Furthermore, at least two distinct adaptor complexes can associate with the GLUT4 vesicles and it is likely that these adaptors are involved in mediating distinct intracellular sorting events at the level of TGN and endosomes.