Characterization of the intracellular GLUT-4 compartment.
Insulin stimulates glucose transport in muscle and adipose tissue by triggering the translocation of the glucose transporter GLUT-4 from intracellular vesicles to the cell surface. In the present study we have attempted to characterize the intracellular GLUT-4 compartment using vesicle immunoadsorption. Silver staining of this fraction indicates that this compartment contains numerous polypeptides that exhibit a marked change in mobility upon treatment with reducing agents. The polypeptide composition of GLUT-4-containing vesicles isolated from a variety of insulin-sensitive cell types, including heart, adipose tissue, skeletal muscle and 3T3-L1 adipocytes, is similar. In addition, the polypeptide composition of the GLUT-4 compartment isolated from CHO cells transfected with GLUT-4 resembles that observed in insulin-sensitive cells. Two major proteins in this vesicle fraction isolated from all cell types are the transferrin receptor (TfR) and the mannose 6-phosphate/IGF II receptor (MPR). Furthermore, vesicles immunoadsorbed from adipocytes, with antibodies specific for GLUT-4 and the TfR, also show conservation in their overall polypeptide composition. Protein micro sequencing of a major 80 kDa polypeptide enriched in the GLUT-4 compartment isolated from skeletal muscle revealed this protein to be rat transferrin. These data indicate that there is a close relationship between the intracellular GLUT-4 compartment and the endosomal system. Future studies will be required to determine if it is possible to isolate subcompartments within this system to determine if GLUT-4 is targeted to a specialized secretory compartment in insulin-sensitive cells or simply a subdomain within recycling endosomes.
Hanpeter
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Department of Cell Biology and Physiology, Washington University School of Medicine, St Louis, MO, USA.
James
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D016475:3T3 Cells; D017667:Adipocytes / Q000166:cytology; D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D004591:Electrophoresis, Polyacrylamide Gel; D011992:Endosomes / Q000187:drug effects / Q000378:metabolism; D051275:Glucose Transporter Type 4; D006056:Golgi Apparatus / Q000187:drug effects / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology; D051379:Mice; D008861:Microsomes / Q000737:chemistry; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000737:chemistry* / Q000187:drug effects; D009124:Muscle Proteins; D010455:Peptides / Q000737:chemistry; D014168:Transferrin / Q000737:chemistry
D051275:Glucose Transporter Type 4; D007328:Insulin; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D010455:Peptides; C495693:Slc2a4 protein, mouse; D014168:Transferrin