Relationship between Insulin Receptor Subunit Association and Protein Kinase Activation: Insulin-Dependent Covalent and Mn/MgATP-Dependent Noncovalent Association of αβ Heterodimeric Insulin Receptors into an α₂β₂ Heterotetrameric State

The purified human placenta α₂β₂ heterotetrameric insulin receptor was reduced and dissociated into a functional αβ heterodimeric complex by a combination of alkaline pH and dithiothreitol treatment. In the presence of Mn/MgATP, insulin binding to the isolated αβ Heterodimeric insulin receptor was found to induce the formation of a covalent disulfide-linked α₂β₂ heterotetrameric complex. In the absence of insulin, a noncovalent association of the αβ heterodimeric insulin receptor complex into an α₂β₂ heterotetrameric state required the continuous presence of both a divalent metal ion (Mn or Mg) and an adenine nucleotide (ATP, ADP, or AMPPCP). Thus, Mn/MgATP binding and not insulin receptor auto-phosphorylation was responsible for the noncovalent association into the α₂β₂ heterotetrameric state. However, the divalent metal ions or NaATP separately was ineffective in inducing the noncovalent association between the αβ heterodimers. The specific sulfhydryl agent iodoacetamide (IAN) was observed to inhibit the insulin-dependent covalent association of the αβ heterodimers without affecting the Mn/MgATP-induced noncovalent association into the α₂β₂ heterotetrameric state. Insulin treatment of the isolated αβ Heterodimeric complex in the presence of IAN demonstrated that the Mn/MgATP-induced noncovalent association into the α₂β₂ Heterotetrameric state was sufficient for insulin stimulation of both β-subunit autophosphorylation and exogenous substrate protein kinase activity. These data indicate that although interaction between the individual insulin receptor αβ heterodimers is necessary for insulin stimulation of protein kinase activity it does not require covalent disulfide bond formation.