Demonstration of putative subunit(s) of a novel insulin receptor-associated 66 kDa GTP-binding protein, G(IR)

A 66 kDa GTP-binding protein, G(ir), and insulin receptor (IR) were copurified from human placental membrane by DEAE-Sephacel and Wheat Germ Agglutinin (WGA)-Sepharose affinity chromatography. The WGA-fraction containing IR and G(ir) (IR-G(ir)-fraction) was phosphorylated (95 kDa IR-β and 66 kDa G(ir)) by IR-tyrosine kinase using [³²P]ATP or photolabeled with [³²P]8-azido-GTP (mainly 66 kDa), and was cross-linked with a bifunctional reagent, bis-[sulfosuccinimidyl] suberate (Bs³). The G(ir) cross-linked with putative subunit(s) to form a 110 kDa complex. Phosphorylation as well as 8-azido-GTP binding to G(ir) was not affected by cross-linking indicating that like other G-Proteins, G(ir) may also have subunits and that cross-linking of G(ir) with its putative subunits(s) does not block GTP-binding and phosphorylation sites.