A SYNTHETIC PEPTIDE CORRESPONDING TO THE GLUT4 C-TERMINAL CYTOPLASMICDOMAIN CAUSES INSULIN-LIKE GLUCOSE-TRANSPORT STIMULATION AND GLUT4 RECRUITMENT IN RAT ADIPOCYTES

In rat epididymal adipocytes, practically all of the major glucose tra nsporter isoform GLUT4 is constitutively sequestered in intracellular membranes and moves to the plasma membrane in response to insulin, whe reas about half of GLUT1, the minor isoform, is constitutively functio nal at the plasma membrane and thus less affected by insulin, Transfec tion studies using cells whose glucose transport is normally not regul ated by insulin have suggested that the C-terminal cytoplasmic domain of GLUT4 is responsible for its constitutive intracellular sequestrati on, To test if this was also the case in a classical insulin target ce ll, we introduced synthetic peptides corresponding to the C-terminal c ytoplasmic domain of GLUT4 and GLUT1 (GLUT4C and GLUT1C, respectively) into rat adipocytes and studied their effects on the glucose transpor t activity and the steady state GLUT4 and GLUT1 distribution between t he plasma membrane and intracellular membranes in host cells. GLUT4C i ntroduced into basal adipocytes caused a large (up to 4.5-fold) and do se-dependent increase in the plasma membrane GLUT4, with a proportiona l reduction in microsomal GLUT4, without affecting GLUT1 distribution, GLUT4C incorporation also caused a large (up to 3-fold) dose-dependen t stimulation of 3-O-methyl D-glucose (3OMG) flux in host cells, GLUT4 C caused little if any GLUT4 or GLUT1 redistribution and changes in 3O MG flux in insulin-stimulated adipocytes. GLUT1C, on the other hand, d id not affect GLUT1 or GLUT4 targeting and 3OMG flux in host cells, Th ese findings not only underscore the importance of the C-terminal cyto plasmic domain of GLUT4 in its constitutive intracellular sequestratio n in a classical insulin target cell but also suggest the existence of a regulatory protein in adipocytes that interacts with GLUT4 at its c ytoplasmic domain, thus participating in the constitutive intracellula r sequestration of GLUT4.