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 mechanism of transduction of an IGFI signal by the IGF I receptor. Insight into this mechanism will require an understanding of the structure and interaction of the functionally-defined domains of the receptor. To accomplish this, both in vitro and in vivo studies are proposed. High affinity IGF I receptor purified from human placenta by IGF I affinity chromatography will be used to characterize covalent modifications of the IGF I receptor which affect its function in vitro. Reassociation studies of dimeric IGF I receptor forms, isolated after the selective reduction of disulfide bonds linking the two alpha/beta dimers of the IGF I receptor tetramer, will be performed to provide conclusive evidence that alpha/beta dimers interact to modulate receptor function. The functional consequences of IGF I receptor autophosphorylation in vitro will be determined, and the sites at which autophosphorylation occurs in vitro and in vivo will be compared. The formation of functional domains during the post- translational processing of the IGF I receptor in HepG2 cells will be defined in vivo. The importance of glycosylation per se for formation of active IGF I receptor will be examined. Finally, defects in IGF I action will be investigated in cultured fibroblasts from children with short stature. IGF binding protein production and the regulation thereof will be elucidated in the cultured fibroblasts from a patient with IGF I resistance which we have reported, and other defects in IGF binding proteins or in IGF receptors in cultured fibroblasts which demonstrate altered biological activity in response to IGF I will be characterized, as they are 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 the pathogenesis of growth failure in children.