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Negative cooperativity in the insulin-like growth factor-I receptor and a chimeric IGF-I/insulin receptor
C T Christoffersen1, K E Bornfeldt, C M Rotella
1Hagedorn Research Institute, Gentofte, Denmark.
Abstract:
Insulin and insulin-like growth factor-I (IGF-I) share a spectrum of metabolic and growth-promoting effects, mediated through homologous receptors that belong to the tyrosine kinase family. The dissociation rate 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 negative cooperativity like the insulin receptor, the concentration dependence of the dissociation kinetics is distinct from that of native human insulin by not being bell-shaped, but monophasic like that of insulin analogues mutated at the hexamer-forming surface; it is changed to an insulin-type curve by substitution of IGF-I receptor's sequence including residues 382-565 with the homologous insulin receptor domain. The data suggest that like insulin, IGF-I has a bivalent binding mode and crosslinks two distinct areas of the two alpha subunits that are close, but distinct from the equivalent insulin receptor binding sites.
Insights
Insulin-like growth factor-I (IGF-I) receptor exhibits negative cooperativity, but its dissociation kinetics differ from the insulin receptor. Modifying the IGF-I receptor can alter its binding kinetics to resemble insulin.
Area of Science:
- Biochemistry
- Molecular Biology
- Endocrinology
Background:
- Insulin and IGF-I share metabolic and growth effects via tyrosine kinase receptors.
- Insulin receptor exhibits negative cooperativity affecting insulin dissociation kinetics.
- IGF-I receptor interaction kinetics were previously less understood.
Purpose of the Study:
- To investigate the dissociation kinetics of IGF-I from its receptor.
- To compare IGF-I receptor kinetics with insulin receptor kinetics.
- To explore the structural basis for differences in receptor binding.
Main Methods:
- Studied IGF-I receptor negative cooperativity and dissociation kinetics.
- Compared IGF-I receptor kinetics to insulin and insulin analogues.
- Utilized receptor domain substitution to alter IGF-I receptor binding characteristics.
Main Results:
- IGF-I receptor shows negative cooperativity, similar to the insulin receptor.
- IGF-I receptor dissociation kinetics are monophasic, unlike the bell-shaped curve of insulin.
- Substitution of specific IGF-I receptor residues with insulin receptor domains shifted kinetics towards an insulin-type curve.
- IGF-I likely binds bivalently, crosslinking distinct alpha subunit areas.
Conclusions:
- IGF-I and insulin receptors share negative cooperativity but differ in dissociation kinetics.
- Specific regions of the IGF-I receptor determine its unique binding kinetics.
- Structural differences in binding sites contribute to distinct kinetic profiles of IGF-I and insulin signaling.