Stimulation of tyrosine-specific phosphorylation in vitro by insulin-like growth factor I

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Abstract
Several mitogens elicit tyrosine-specific protein kinase activities1–7. Although the physiological significance of this is unclear, the generality of these reactions implies that this may be an inherent feature of growth factor–growth factor receptor interactions. The observed mitogenic properties of the polypeptide insulin-like growth factor I (IGF-I)8,9 indicated that it might also stimulate such activity. We report here that IGF-I stimulates a tyrosine-specific protein kinase in a time- and dose-dependent fashion. The close correspondence between an approximate 50% effective dose (ED50) of phosphorylation and an approximate Kd for IGF-I binding leads us to conclude that a high-affinity IGF-I receptor, not the structurally similar insulin receptor10, is the mediator of IGF-I-stimulated kinase activity. Immunoprecipitation indicates that both the β-subunit of the IGF-I receptor and the β-subunit of the insulin receptor are targets for the IGF-I-stimulated protein kinase.