INTRAMOLECULAR SUBUNIT INTERACTIONS BETWEEN INSULIN AND INSULIN-LIKE GROWTH FACTOR-I ALPHA-BETA-HALF-RECEPTORS INDUCED BY LIGAND AND MN/MGATP BINDING

We have previously demonstrated that isolated insulin and IGF-1 alphabeta half-receptors can be reconstituted into a functional alpha2beta2 hybrid receptor complex [Treadway et al. (1989) J. Biol. Chem. 264, 21450-21453]. In the present study, we have examined this assembly process by determining the effect of ligand occupancy and Mn/MgATP binding on the dimerization of mutant and wild-type insulin and IGF-1 alphabeta half-receptors. IGF-1 or Mn/MgAMPPCP binding to wild-type IGF-1 alphabeta half-receptors resulted in the specific assembly of the alphabeta half-receptors into an alpha2beta2 heterotetrameric IGF-1 holoreceptor complex. Similarly, insulin binding to the kinase-deficient mutant (A/K1018) insulin alphabeta half-receptor also resulted in the specific assembly into an alpha2beta2 holoreceptor complex. In contrast, Mn/MgAMPPCP treatment of A/K1018 mutant insulin alphabeta half-receptors did not induce heterotetramer assembly, consistent with the inability of this mutant receptor to bind ATP. The ability of the insulin alphabeta receptors to assemble with the IGF-I alphabeta half-receptors was used to examine the intermolecular subunit interactions responsible for dimerization. In the presence of Mn/MgAMPPCP, the wild-type insulin and wild-type IGF-1 alphabeta half-receptors were observed to assemble into an insulin/IGF-1 alpha2beta2 hybrid receptor complex. Similarly, a combination of insulin and IGF-1 induced hybrid receptor formation between wild-type IGF-1 and A/K1018 mutant insulin alphabeta half-receptors. In contrast, mixing insulin-occupied A/K1018 mutant insulin alphabeta half-receptors with Mn/MgAMPPCP-occupied wild-type IGF-1 alpha,beta half-receptors did not result in hybrid receptor formation. Thus, these data demonstrate that ligand binding to the alpha subunit and Mn/MgATP binding to the beta subunit do not result in complementary conformational changes necessary for alphabeta half-receptor dimerization.