The Disulfide Bonds in the C-terminal Domains of the Human Insulin Receptor Ectodomain

Abstract The human insulin receptor is a homodimer consisting of two monomers linked by disulfide bonds. Each monomer comprises an α-chain that is entirely extracellular and a β-chain that spans the cell membrane. The α-chain has a total of 37 cysteine residues, most of which form intrachain disulfide bonds, whereas the β-chain contains 10 cysteine residues, four of which are in the extracellular region. There are two classes of disulfide bonds in the insulin receptor, those that can be reduced under mild reducing conditions to give α-β monomers (class I) and those that require stronger reducing conditions (class II). The number of class I disulfides is small and includes the α-α dimer bond Cys524. In this report we describe the use of cyanogen bromide and protease digestion of the exon 11 plus form of the receptor ectodomain to identify disulfide linkages between the β-chain residues Cys798 and Cys807 and between the α-chain Cys647 and the β-chain Cys872. The latter bond is the sole α-β link in the molecule and implies a side-by-side alignment of the two fibronectin III domains of the receptor. Also presented is evidence for additional α-α dimer bond(s) involving at least one of the cysteine residues of the triplet at positions 682, 683, and 685. Evidence is also presented to show that Cys884 exists as a buried thiol in the soluble ectodomain.