STRUCTURE DEFINITION OF IGF I RECEPTOR FUNCTION DOMAINS

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Insulin-like growth factor I (IGF I) plays a key role in mediating the growth effects of pituitary growth hormone. In responsive cells, IGF I interacts at the plasma membrane with the IGF I receptor, a heterotetrameric glycoprotein composed of two disulfide-linked alpha beta dimers. IGF I binding causes the transmembrane activation of the intrinsic catalytic activity of the IGF I receptor, but the mechanism by which this occurs is not yet well understood. Similarly, the biochemical messages generated by this interaction have not been elucidated, although it is widely assumed that they must include tyrosine phosphorylation of cytoplasmic protein substrates. In this application, we propose to study the structure and interaction of the functionally-defined domains of the IGF I receptor. To accomplish this, six specific aims are proposed. First, a rigorous analysis of the relationship of IGF I receptor autophosphorylation to activation of kinase activity in the presence and absence of IGF I will be performed. Second, the cysteinyl residues contributing to the disulfide bond(s) linking the two alpha beta dimers of the IGF I receptor tetramer will be identified. Third, the binding domain(s) for IGF I, IGF II, and insulin in the purified IGF I receptor will be compared. Following affinity cross-linking and cyanogen bromide treatment of the purified IGF I receptor, peptides comprising the IGF I, IGF II, and insulin binding domains in the IGF I receptor will be isolated and sequenced. Anti-peptide antibodies directed against surface-exposed portions of the IGF I receptor will also be characterized. Fourth, the sites of IGF I receptor autophosphorylation in vitro and in vivo will be identified. Fifth, the functional consequences of phosphorylation of the IGF I receptor by protein kinase C and the sites at which this modification occurs in vitro and in vivo will be determined. Finally, endogenous substrates of IGF I receptor tyrosine kinase activity in cells will be investigated. In these studies, phenylarsine oxide, a trivalent arsenical which has been reported to enhance the detection of endogenous substrates phosphorylated in insulin- and IGF I-stimulated cells, will be employed. The ability of phenylarsine oxide to inhibit IGF I-stimulated biological activities will be examined, and phosphoproteins that accumulate in the presence of IGF I and phenylarsine oxide will be identified. It is anticipated that these studies will lead to a better understanding of signal transduction by the IGF I receptor and its role in human growth.