COMPARISON OF GLUCOSE-TRANSPORTER-CONTAINING VESICLES FROM RAT FAT AND MUSCLE TISSUES - EVIDENCE FOR A UNIQUE ENDOSOMAL COMPARTMENT

Insulin-sensitive tissues (fat and muscle) express a specific isoform of glucose-transporter protein, GLUT4, which normally resides in intra cellular vesicular structures and is translocated to the cell surface in response to insulin. Here we provide a biochemical comparison of GL UT4-containing structures from fat and muscle cells. We demonstrate th at, in spite of totally different protocols for cell homogenization an d fractionation used for adipocytes as compared with skeletal-muscle t issue, GLUT4-containing vesicles from both sources have identical buoy ant densities, sedimentation coefficients, and a very similar, if not identical, protein composition. The individual proteins first identifi ed in GLUT4-containing vesicles from adipocytes (GTV3/SCAMPs proteins and aminopeptidase gp160) are also present in the analogous vesicles f rom muscle. Intracellular microsomes from rat adipocytes also contain GLUT1, a ubiquitously expressed glucose-transporter isoform. GLUT1 has not been detected in intracellular vesicular pool(s) from skeletal-mu scle cells, probably because of its low abundance there. GLUT1 in adip ocytes is excluded from GLUT4-containing vesicles, but is found in mem brane structures which are indistinguishable from the former by all me thods tested and demonstrate the same type of regulation by insulin. T hat is, the GLUT1- and GLUT4-containing vesicles have identical densit ies and sedimentation coefficients in sucrose gradients, and transloca te to the cell surface in response to hormonal exposure. Also, we desc ribe a simple procedure for the purification of native glucose-transpo rter vesicles from rat adipocytes. Overall, our data suggest the exist ence of a unique endosomal compartment in fat and muscle cells which i s functionally and compositionally different from other microsomal ves icles and which is responsible for insulin-sensitive glucose transport in these tissues.