Dissection of GLUT4 recycling pathway into exocytosis and endocytosis in rat adipocytes. Evidence that GTP-binding proteins are involved in both processes.

The effects of guanine nucleotides on either exocytosis or endocytosis of GLUT4 were examined in electrically permeabilized rat adipocytes by using Dk-(62-85), a major histocompatibility complex class I-derived peptide. Reversal of glucose transport activity that had been stimulated with insulin was completely blocked by Dk-(62-85). Likewise, endocytosis of the trypsin-cleaved 35-kDa fragment of GLUT4 was almost completely inhibited by the peptide. Insulin-stimulated glucose transport activity was enhanced about 50% by Dk-(62-85), whereas the basal transport activity was stimulated only slightly. Although guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) augmented glucose transport to the same extent as insulin in the absence of the peptide, glucose transport stimulated by GTP gamma S was only 60% of the insulin effect in the presence of the peptide; the effect of insulin was markedly enhanced by Dk-(62-85), whereas GTP gamma S-induced glucose transport was not affected, suggesting that GTP gamma S has an effect similar to that of the peptide. In fact, endocytosis of the 35-kDa fragment of GLUT4 was markedly inhibited by GTP gamma S. Additionally, GLUT4 endocytosis was accelerated by GTP but was inhibited by guanosine 5'-O-(2-thiodiphosphate). These results indicate that GTP gamma S induces translocation of GLUT4 by both stimulating exocytosis and inhibiting endocytosis. With respect to the dependence on GTP hydrolysis, distinct types of GTP-binding proteins are involved in exocytosis and endocytosis of GLUT4.