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)