INSULIN-STIMULATED AND CONTRACTION-STIMULATED TRANSLOCATION OF GTP-BINDING PROTEINS AND GLUT4 PROTEIN IN SKELETAL-MUSCLE

Low molecular weight GTP-binding proteins and GLUT4 protein were isola ted in purified plasma membrane and low density microsome fractions fr om rat skeletal muscle. GTP-binding proteins were detected via the abi lity of these proteins to bind [P-32]GTP subsequent to Western blottin g. GLUT4 protein was detected via the anti-GLUT4 antibody F349 subsequ ent to Western blotting. The possible involvement of GTP-binding prote ins in the regulation of GLUT4 protein movement was investigated by ex amining the subcellular distribution of GTP-binding proteins and GLUT4 protein under basal conditions and following stimulation by insulin o r muscle contraction. Insulin stimulation caused a 111 +/- 34.8% incre ase in the plasma membrane content of GTP-binding proteins which was p aralleled by a 74 +/- 19.1% increase in the plasma membrane content of GLUT4 protein. The insulin-stimulated increase in plasma membrane GTP -binding proteins and GLUT4 protein occurred coincident with 27 +/- 4. 6 and 33 +/- 7.4% decreases, respectively, in the low density microsom e content of these proteins. In addition, muscle contraction significa ntly increased the plasma membrane content of GTP-binding proteins (63 +/- 18.1%) and GLUT4 protein (67 +/- 22.2%). However, with muscle con traction the concentrations of GTP-binding proteins and GLUT4 protein were not altered in low density microsome fractions. The similar patte rns with which the GTP-binding proteins and GLUT4 protein responded to stimulation by insulin and muscle contraction suggests a possible, bu t yet unidentified functional relationship between these proteins.