Compartmentalization of protein traffic in insulin-sensitive cells

Insulin-sensitive cells, adipocytes and myocytes, translocate a number of intracellular proteins to the cell surface in response to insulin. Among these proteins are glucose transporters 1 and 4 (GLUT-1 and GLUT-4, respectively), receptors for insulin-like growth factor II (IGF-II)/mannose B-phosphate (Man-g-P) and transferrin, the aminopeptidase gp160, caveolin, and a few others. In the case of insulin-activated glucose transport, this translocation has been proven to be the major, if not the only regulatory mechanism of this process. It seems likely that the cell surface recruitment of the IGF-II/Man-6-P and transferrin receptors also serves the nutritional needs of cells, whereas the physiological role of the aminopeptidase gp160 remains uncertain. Analysis of the compartmentalization and trafficking pathways of translocatable proteins in fat cells identified more than one population of recycling vesicles, although all have identical sedimentation coefficients and buoyant densities in vitro. GLUT-4-containing vesicles include essentially all the intracellular GLUT-4, gp160, and the acutely recycling populations of receptors for IGF-II/Man-6-P and transferrin. Besides these proteins, which can be considered as vesicle ''cargo,'' GLUT-4-containing vesicles have other components, like secretory carrier-associated membrane proteins (SCAMP), Rab(s), and vesicle-associated membrane protein (VAMP)/cellubrevin, which are ubiquitous to secretory vesicles and granules from different tissues. GLUT-1 and caveolin are excluded from GLUT-4-containing vesicles and form different vesicular populations of unknown polypeptide composition. In skeletal muscle, two independent populations of GLUT-4-containing vesicles are found, insulin sensitive and exercise sensitive, which explains the additive effect of insulin and exercise on glucose uptake. Both vesicular populations are similar to each other and to analogous vesicles in fat cells.