Disulfide bonds 7-31 and 59-87 of the alpha-subunit play a different role in assembly of human chorionic gonadotropin and lutropin

CG, LR, FSH. and TSH are a family of heterodimeric glycoprotein hormones that contain a common alpha-subunit but differ in their hormone-specific beta-subunits. Both subunits have five and six disulfide bonds, respectively, which consists of cystine knot structure. We previously eliminated the disulfide bonds 7-31 and 59-57 in alpha-subunit without significantly affecting assembly with human CG beta-subunit. To study the role of these disulfide bonds in dimerization with other beta-subunits, the wild-type or mutated a gene was cotransfected with the wild-type human LH beta or FSH beta gene into Chinese hamster ovary (CHO) cells or GH(3) cells, and assembly was assessed by continuous labeling with [S-35]methionine/cysteine, immunoprecipitation with anti-alpha or -beta serum, and SDS-PAGE. Our data revealed that disruption of either disulfide bond 7-31 or 59-57 in the alpha-subunit markedly reduced the dimer formation viith LH beta-subunit in both CHO and GH(3) cells, whereas it did not significantly affect the assembly of FSH. This suggests that the regions in the alpha-subunit recognized by beta-subunits for assembly are different among gonadotropins.