LABILE DISULFIDE BONDS IN HUMAN PLACENTAL INSULIN-RECEPTOR

The disulfide crosslinking pattern of human placental insulin receptor was investigated using selective reduction with tributylphosphine followed by alkylation with N-[3H]ethylmaleimide. Insulin receptor contains a single sulfhydryl group in each β subunit whose alkylation with N-[3H]ethylmaleimide inhibits receptor autophosphorylation. Alkylation is partially inhibited by ATP or the nonhydrolyzable substrate analog adenosine 5'-[β,γ-imido]triphosphate when the nucleotides are added as Mn2+ complexes. Neither insulin nor 6 M guanidinium chloride renders additional sulfhydryl groups accessible to alkylation. When the receptor is reduced under drastic conditions with tributylphosphine in guanidinium chloride, 32 of the 37 sulfhydryl groups in the receptor's α subunit can be alkylated with N-[3H]ethylmaleimide. Surprisingly only three of the 10 cysteines in the β subunit become titratable under identical conditions. By using highly selective reducing conditions, we were able to determine quantitatively the maximum number of disulfide bridges that link the two αβ halves to form the tetrameric structure and those that couple the α to the β subunits. Liberation of two sulfhydryl groups in the α and one in the β subunit resulted in formation of αβ dimers. Free β subunit was formed when an additional disulfide bond was reduced. It is remarkable that the tetrameric structure of this highly complex receptor molecule, which contains a large number of cysteine residues, is maintained by such a small number of disulfide bonds. Three models of the arrangement of the labile disulfide bonds, consistent with these findings, are proposed.