Mechanisms of receptor-mediated transmembrane communication

Our experiments with the hIR protein have been designed to address a very general question of transmembrane receptor structure and function: What are the roles and interactions of the various deduced structural domains of such molecules in the initiation of the response of cells to extracellular signals? All of the evidence to date supports the previous hypothesis based on biochemical data that the IR requires ligand-activated TPK functions to initiate the insulin response by cells (for review, see Kahn 1985). Thus, mutations that compromise hIR TPK activity (site-directed point mutations or deletions) result in a concomitant decrease in at least one aspect of insulin action (glucose uptake; Ellis et al. 1986a). Other studies utilizing microinjection of antibodies to inhibit the receptor kinase have extended this conclusion to include a critical role for the receptor kinase in insulin's ability to stimulate ribosomal protein S6 phosphorylation in CHO cells, glycogen synthetase in hepatoma cells, glucose uptake in adipocytes (Morgan and Roth 1987), and frog oocyte maturation (Morgan et al. 1986). Second, analyses of cell lines that express experimentally truncated hIR TPKs demonstrate that, when membrane-anchored, this TPK domain is in fact capable of autonomous hormone-independent IR function: Such cells exhibit a constitutively elevated, insulin-independent uptake of 2-deoxyglucose (Ellis et al. 1987). Finally, by substitution of a homologous TPK for that of hIR, we find that although such a hybrid is capable of insulin-dependent transmembrane signaling (phosphorylation of the hybrid beta-subunit on tyrosine residues), the hybrid IR.ros molecule does not function as an IR in such cells: It mediates neither short-term (uptake of 2-deoxyglucose) nor long-term (incorporation of [3H]thymidine) effects of insulin (L. Ellis et al., in prep.). Together, these results suggest that (1) the hIR TPK domain conveys a substrate specificity for the insulin response and (2) that a functional hIR extracellular domain alone is not sufficient for generation of the insulin response (e.g., ligand-induced aggregation, or simple delivery of insulin into the cell). With the linking of the extracellular and cytoplasmic domains of the hIR molecule has evolved a cellular mechanism for the control of hIR TPK activity; the result is that cells which express the IR are now insulin responsive, and the physiological responses associated with the hormone are ligand-activated. Thus, the uncontrolled state of autonomous TPK activity, with the associated constitutive physiological response (e.g., as exhibited by the spBam hIR mutant), is circumvented.(ABSTRACT TRUNCATED AT 400 WORDS)