Specificity of tyrosine protein kinases of the structurally related receptors for insulin and insulin-like growth factor I: Tyr-containing synthetic polymers as specific inhibitors or substrates

Abstract

The receptors for insulin and insulin-like growth factor (IGF) I are structurally similar transmembrane proteins. Ligand binding to the extracellular domain of the receptor stimulates its cytoplasmic tyrosine protein kinase which phosphorylates its own β subunit as well as exogenous substrates. It is believed, from several lines of evidence, that tyrosine-specific protein kinases are mediating some or all of the actions of insulin (or IGF-I). In order to gain insights into the substrate specificity of the structurally related insulin and IGF-I receptor kinases, we have studied the action of highly purified receptors isolated from human placental membranes. Present studies using selected tyrosine-containing polymers have revealed: (i) Polymers such as (Y,A,E)_n and (Y-A-E)_n inhibit β subunit autophosphorylation and exogenous substrate phosphorylation by autophosphorylated receptors, (ii) Insulin receptor kinase is at least 10 times more sensitive to these inhibitors than IGF-I receptor kinase. (iii) (Y-A-E)_n is ^~8 times more potent an inhibitor than (Y,A,E)_n toward both receptors, (iv) While (E⁴Y¹)_n and (E⁶,A³,Y¹)_n are good substrates for both receptor kinases, the ratio of phosphate incorporation into the former to the latter is characteristically high (^~4) for the IGF-I receptor and low (^~1) for the insulin receptor. These results imply that the substrate specificity and enzymatic action of these two receptor kinases are distinct.