Structural requirements for signal transduction of the insulin receptor

Structural requirements for signal processing by human placental insulin receptors have been examined. Insulin binding has been found to change the physico-chemical properties of (alpha beta)2 receptors solubilized with Triton X-100, indicating a marked alteration of the form, i.e. size and shape, of the molecular complex. (a) The Stokes radius decreases from about 9.5 nm to 7.9 nm, as determined by PAGE with Triton X-100 in the buffer (Triton X-100/PAGE), and from 9.1 nm to 8.7 nm, as assessed by gel filtration. (b) The sedimentation coefficient s20,w rises from 10.1 S to 11.4 S. Upon dissociation of the receptor-hormone complex, the alterations are reversed. After autophosphorylation of hormone-bound (alpha beta)2-insulin receptors, phosphate incorporation was found for 7.9-nm receptor forms when receptor-insulin complexes were crosslinked with disuccinimide suberate prior to Triton X-100/PAGE. However, phosphate incorporation was demonstrated for the 9.5-nm receptor forms when receptor-insulin complexes were not prevented from dissociation. This strongly indicates that the (alpha beta)2 receptor is autophosphorylated after assuming its 7.9-nm form upon insulin binding. Moreover, the insulin-dependent structural alterations are not affected by autophosphorylation. In contrast to (alpha beta)2 receptors, the diffusion and the sedimentation behaviour of alpha beta receptors, which carry a dormant tyrosine kinase even in the hormone-laden state, has been found to be insensitive to insulin binding. Different molecular properties of alpha beta and (alpha beta)2 receptors have also been detected by hormone binding studies. Insulin binding to (alpha beta)2 and alpha beta receptors differs markedly with respect to pH, ionic strength, and temperature. This might indicate that the structure of the hormone binding domain of alpha beta receptor changes on association into the (alpha beta)2 species. Alternatively, distinct hormone-induced conformational alterations at the molecular level of alpha beta and (alpha beta)2 receptor species may lead to the different binding properties. Our data demonstrate that the (alpha beta)2-insulin receptor undergoes extended conformational alterations upon insulin binding. This capacity for structural changes coincides with the hormone-inducable enhancement of tyrosine autophosphorylation of the 7.9-nm insulin-bound receptor form. In contrast, alpha beta receptors appear to be locked in an inactive nonconvertable state. Thus, interaction between two alpha beta receptor units is required to allow extended conformational alterations, which are assumed to be the triggering event for augmented auto-phosphorylation.