Abstract
The relationship between the structure of the insulin receptor and its kinase activity was studied on the purified receptor treated with different concentrations of dithiothreitol. An enhanced autophosphorylation of the beta subunit (Mr, 90,000) was observed on NaDodSO4/PAGE under reducing conditions when the receptor was treated with 0.1-0.75 mM dithiothreitol in the presence of 1 microM insulin. Since we have previously observed (unpublished data) that incubation of the purified receptor with 1 mM dithiothreitol completely reduced an intact form of the receptor, alpha 2 beta 2, to free alpha subunit (Mr, 125,000) and beta subunit, the population of disulfide-linked complexes of the receptor after the dithiothreitol treatment was analyzed by NaDodSO4/PAGE under nonreducing conditions. The same receptor preparations were assayed for tyrosine kinase activity by using an exogenous substrate. Treatment of the receptor with dithiothreitol significantly enhanced both basal and insulin-dependent kinase activity. The kinase activity was enhanced 12- to 37-fold at concentrations of 0.5-0.75 mM dithiothreitol in the presence of 1 microM insulin. The amount of alpha 2 beta 2, alpha beta, and beta forms in each dithiothreitol-treated receptor preparation was quantified and compared with its kinase activity. These studies clearly indicate a correlation between the appearance of an alpha beta form and an increase in kinase activity. Therefore, we conclude that the alpha beta form of the insulin receptor exhibits much higher kinase activity than the intact receptor in the alpha 2 beta 2 form.