The mature insulin receptor already contains two extracellular chains and two membrane-spanning chains, joined through protein disulfide bonds, before insulin binds. Insulin induces rearrangement of this assembly. Partner-mediated phosphorylation of the inhibitory activation loop permits increased protein kinase activity. Ligand-induced creation of a previously absent dimer is therefore an unsuitable universal explanation.[ref]
Cell signaling by receptor-tyrosine kinases by Mark A Lemmon and Joseph Schlessinger. “Mechanisms of Receptor Activation” and “TKD autoinhibition by the activation loop: insulin and FGF receptors”, Figure 2E
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