The three deletion variants were incapable of binding ALS; but of the site-specific variants, the MDGEA mutant bound ALS with 90% lower affinity (K a ‫؍‬ 2.5 ؎ 0.9 liters/nmol) than seen for rhIGF-BP-3 (K a ‫؍‬ 24.3 ؎ 5.2 liters/nmol), whereas the RGD mutation had no effect on ALS affinity (K a ‫؍‬ 21.7 ؎ 4.5 liters/nmol). The ability of IGFBP-3 to associate with the cell surface was lost in variants lacking residues 185-264 and in the 228 KGRKR 3 MDGEA mutant. We conclude that residues 228 -232 of IGFBP-3 are essential for cell association and are required for normal ALS binding affinity.

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