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

CG, LH, 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 59-87 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 alpha gene was contransfected with the wild-type human LH beta or FSH beta gene into Chinese hamster ovary (CHO) cells or GH3 cells, and assembly was assessed by continuous labeling with [35S]methionine/cysteine, immunoprecipitation with anti-alpha or-beta serum, and SDS-PAGE. Our data revealed that disruption of either disulfide bond 7-31 or 59-87 in the alpha-subunit markedly reduced the dimer formation with LH beta-subunit in both CHO and GH3 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.