Mutational analysis of the carboxy-terminal phosphorylation site of GLUT-4 in 3T3-L1 adipocytes

The carboxy terminus of GLUT-4 contains a functional internalization motif (Leu-489Leu-490) that helps maintain its intracellular distribution in basal adipocytes. This motif is flanked by the major phosphorylation site in this protein (Ser-488), which may play a role in regulating GLUT-4 trafficking in adipocytes. In the present study, the targeting of GLUT-4 in which Ser-488 has been mutated to alanine (SAG) has been examined in stably transfected 3T3-L1 adipocytes. The trafficking of SAG was not significantly different from that of GLUT-4 in several respects. First, in the absence of insulin, the distribution of SAG was similar to GLUT-4 in that it was largely excluded from the cell surface and was enriched in small intracellular vesicles. Second, SAG exhibited insulin-dependent movement to the plasma membrane (4- to 5-fold) comparable to GLUT-4 (4- to 5-fold). Finally, okadaic acid, which has previously been shown to stimulate both GLUT-4 translocation and its phosphorylation at Ser-488, also stimulated the movement of SAG to the cell surface similarly to GLUT-4. Using immunoelectron microscopy, we have shown that GLUT-4 is localized to intracellular vesicles containing the Golgi-derived gamma-adaptin subunit of AP-1 and that this localization is enhanced when Ser-488 is mutated to alanine. We conclude that the carboxy-terminal phosphorylation site in GLUT-4 (Ser-488) may play a role in intracellular sorting at the trans-Golgi network but does not play a major role in the regulated movement of GLUT-4 to the plasma membrane in 3T3-L1 adipocytes.