Characterization of purified insulin receptor subunits

Three insulin receptor subunits prepared from the purified receptor were isolated and characterized. Peptide mapping of the isolated subunits revealed that the Mr = 125,000 subunit (alpha) is distinct from the Mr = 90,000 subunit (beta) whereas the Mr = 50,000 subunit (beta 1) shows considerable structural homology to beta, indicating that the alpha and beta subunits are components of the intact insulin receptor. From two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the absence and presence of dithiothreitol, the purified insulin receptor was shown to be composed of heterogeneous disulfide-linked complexes of (alpha 2, 2 beta), (alpha 2, beta, beta 1), (alpha 2, 2 beta 1), (alpha 2), (alpha beta), and (alpha beta 1). The largest disulfide-linked complex (alpha 2, 2 beta) appears to be the minimum unit of the intact insulin receptor whereas the other complexes appear to be generated from (alpha 2, 2 beta) by proteolytic degradation and/or reduction. These studies provide conclusive evidence that the alpha 2 beta 2 complex is the basic structural unit of insulin receptor, as previously proposed from affinity cross-linking experiments using crude membranes by Czech's group (Czech, M. P., Massague, J., and Pilch, P. F. (1981) Trends Biochem. Sci. 6, 222-225). The biochemical approach described here should allow us to further elucidate the mechanism of insulin action.