Abundance and subcellular distribution of GTP-binding proteins in 3T3-L1 cells before and after differentiation to the insulin-sensitive phenotype.

The abundance and the subcellular distribution of GTP-binding proteins was studied in membrane fractions (plasma membranes and low-density microsomes) from 3T3-L1 cells before and after differentiation to the insulin-sensitive phenotype. After differentiation, the abundance of α, (α subunit of GTP-binding protein Gⁱ), α (α subunit of GTP-binding protein G_o), and of a 47-kDa α, (α subunit of GTP-binding protein G_s) as detected by immunoblotting with specific antisera was reduced by 10–50% when normalized per membrane protein. In contrast, a 43-kDa α, was increased about threefold after differentiation. Furthermore, cholera-toxin-catalyzed ADP-ribosylation of both 43-kDa and 47-kDa α, was disproportionately increased ninefold and threeforld, respectively, possibly reflecting the increased production of an ADP-ribosylation factor in the differentiated cells. The small GTP-binding protein Ha-ras was reduced by 50%, whereas rab1 and other small GTP-binding proteins tentatively identified as rab-isoforms (ras-homologous gene products from brain) were increased by 100% and 70%, respectively. Since the total protein content of 3T3-L1 cells was increased by 100% and 70%, respectively. Since the total protein content of 3T3-L1 cells was increased threefold after differentiation, the observed increased of the 43-kDa α rab1 and of the other rab isoforms was eightfold, sixfold and fivefold, respectively, when normalized/cell count. With the exception of the rab isoforms, all GTP-binding proteins were predominantly, if not exclusively, located in the plasma membrane; comparable amounts of the rab isoforms were found in plasma membranes and low-density microsomes. Insulin induced the characteristic redistribution of glucose transporters GLUT4 from low-density microsomes to the plasma membranes, but failed to alter the subcellular distribution of any of the GTP-binding proteins investigated. These data suggest that the increase in the abundance of the 43-kDa α subunit and of several rab isoforms might be related to specific functions of the adipocyte-like phenotype, but that none of the investigated guanine-nucleotide-binding regulatory (G)-proteins appears to be tightly associated with the GLUT4. [ABSTRACT FROM AUTHOR]