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Cumulative Mutagenesis of the Basic Residues in the 201–218 Region of Insulin-Like Growth Factor (IGF)-Binding Protein-5 Results in Progressive Loss of Both IGF-I Binding and Inhibition of IGF-I Biological Action

Hannah Research Institute (G.J.A., E.T., M.S., J.H.S., K.P., R.A.C., I.F.G., J.B., D.J.F.), Ayr KA6 5HL, Scotland, United Kingdom; and Institute of Biomedical and Life Sciences, University of Glasgow (S.M.K.), Glasgow G12 8QQ, Scotland, United Kingdom

Address all correspondence and requests for reprints to: Dr. Gordon J. Allan, Hannah Research Institute, Ayr KA6 5HL, Scotland, United Kingdom. E-mail: allang{at}hri.sari.ac.uk.

We have reported previously that mutation of two conserved nonbasic amino acids (G203 and Q209) within the highly basic 201–218 region in the C-terminal domain of IGF-binding protein-5 (IGFBP-5) decreases binding to IGFs. This study reveals that cumulative mutagenesis of the 10 basic residues in this region, to create the C-Term series of mutants, ultimately results in a 15-fold decrease in the affinity for IGF-I and a major loss in heparin binding. We examined the ability of mutants to inhibit IGF-mediated survival of MCF-7 cells and were able to demonstrate that this depended not only upon the affinity for IGF-I, but also the kinetics of this interaction, because IGFBP-5 mutants with similar affinity constants (K_D) values, but with different association (K_a) and dissociation (K_d) rate values, had markedly different inhibitory properties. In contrast, the affinity for IGF-I provided no predictive value in terms of the ability of these mutants to enhance IGF action when bound to the substratum. Instead, these C-Term mutants appeared to enhance the actions of IGF-I by a combination of increased dissociation of IGF-IGFBP complexes from the substratum, together with dissociation of IGF-I from IGFBP-5 bound to the substratum. These effects of the IGFBPs were dependent upon binding to IGF-I, because a non-IGF binding mutant (N-Term) was unable to inhibit or enhance the actions of IGF-I. These results emphasize the importance of the kinetics of association/dissociation in determining the enhancing or inhibiting effects of IGFBP-5 and demonstrate the ability to generate an IGFBP-5 mutant with exclusively IGF-enhancing activity.

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