The high resolution crystal structure of an N-terminal fragment of the IGF-I receptor, has been reported. While this fragment is itself devoid of ligand binding activity, mutational analysis has indicated that its N terminus (L1, amino acids 1-150) and the C terminus of its cysteine-rich domain (amino acids 190 -300) contain ligand binding determinants. Mutational analysis also suggests that amino acids 692-702 from the C terminus of the ␣ subunit are critical for ligand binding. A fusion protein, formed from these fragments, binds IGF-I with an affinity similar to that of the whole extracellular domain, suggesting that these are the minimal structural elements of the IGF-I binding site. To further characterize the binding site, we have performed structure directed and alanine-scanning mutagenesis of L1, the cysteinerich domain and amino acids 692-702. Alanine mutants of residues in these regions were transiently expressed as secreted recombinant receptors and their affinity was determined. In L1 alanine mutants of The insulin-like growth factors I and II are essential for normal fetal and post-natal growth (1). They were originally identified as circulating polypeptides with potent mitogenic activity, which mediated many of the actions of growth hormone, and were later shown to be structurally homologous to proinsulin. It is now apparent that these growth factors are produced by many cell types and have paracrine and autocrine as well as endocrine functions. Targeted disruption of the gene for IGF-I 1 in transgenic mice results in both embryonic and post-natal growth retardation (2). In contrast, the effects of disruption of the IGF-II gene are confined to growth retardation during the embryonic period (2). In addition to being mitogens, it is now evident that these peptides play a crucial role in cell survival (3) and contribute to transformation and the maintenance of the malignant phenotype in many tumor systems (4). However, despite extensive study, the signal transduction mechanisms underlying the biological effects of these peptides remain to be elucidated.The mitogenic effects of these growth factors appear to be mediated by receptors belonging to the insulin receptor subclass of receptor tyrosine kinases (for review see Ref. (5)). The type 1 IGF receptor binds both peptides with high affinity; the affinity for IGF-I being greater than that for IGF-II. Transgenic experiments indicate that the growth-promoting effects of both peptides can be mediated by this receptor (2, 6). Such studies also point to the role of a second receptor in mediating the mitogenic effects of IGF-II (2, 6), and recent in vitro studies indicate that this is the A isoform of the insulin receptor (7); this receptor binds IGF-II with high affinity and can mediate the growth-promoting effects of the peptide (8).The receptors in this family are dimeric protein-tyrosine kinases with significant homology (5). In higher vertebrates there are three known members, the insulin receptor (9, 10), the type 1 IGF receptor (11), and the orphan in...

The human insulin receptor is expressed as two isoforms that are generated by alternate splicing of its mRNA; the B isoform has 12 additional amino acids

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