NEGATIVE COOPERATIVITY IN THE INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR AND A CHIMERIC IGF INSULIN RECEPTOR/

Insulin and insulin-like growth factor-I (IGF-I) share a spectrum of m etabolic and growth-promoting effects, mediated through homologous rec eptors that belong to the tyrosine kinase family. The dissociation rat e of insulin from its receptor is affected by negative cooperativity, i.e. accelerates with increased receptor occupancy. The dose-response curve for the acceleration of tracer dissociation by unlabeled insulin has a distinct bell-shaped curve, with a progressive slowing down at insulin concentrations greater than 100 nM. The kinetics of the IGF-I interaction with its receptor has not been studied in such detail. In the present work, we report that while the IGF-I receptor exhibits neg ative cooperativity like the insulin receptor, the concentration depen dence of the dissociation kinetics is distinct from that of native hum an insulin by not being bell-shaped, but monophasic like that of insul in analogues mutated at the hexamer-forming surface; it is changed to an insulin-type curve by substitution of IGF-I receptor's sequence inc luding residues 382-565 with the homologous insulin receptor domain. T he data suggest that like insulin, IGF-I has a bivalent binding mode a nd crosslinks two distinct areas of the two alpha subunits that are cl ose, but distinct from the equivalent insulin receptor binding sites.