RECEPTOR-BINDING AND FUNCTIONAL-PROPERTIES OF CHIMERIC HUMAN FOLLITROPIN PREPARED BY AN EXCHANGE BETWEEN A SMALL HYDROPHILIC INTERCYSTEINE LOOP OF HUMAN FOLLITROPIN AND HUMAN LUTROPIN

The family of pituitary/placental glycoprotein hormones includes folli cle-stimulating hormone (follitropin, FSH), luteinizing hormone (lutro pin, LH), chorionic gonadotropin (choriogonadotropin, CG), and thyroid -stimulating hormone (thyrotropin, TSH). These glycoproteins are heter odimeric, with an cu-subunit of identical primary structure and a horm one-specific beta-subunit which is believed to confer receptor specifi city. Previous studies demonstrated that substitution of hFSH beta res idues 88-108 in place of the carboxyl terminus of hCG beta, residues 9 4-145, conferred to human (h) CG an FSH receptor binding specificity. To more finely map the LH/FSH receptor binding specificity determinant , hFSH beta residues (DSDS91)-D-88, within the small hydrophilic inter cysteine loop, and residues (95)TVRGLG(100) COOH-terminal to the loop were switched to hLH beta residues (94)RRST(97) and residues (101)GGPK DH(106), respectively. Substitution of hLH beta residues (94)RRST(97) in place of hFSH beta residues (DSDS91)-D-88 did not affect FSH recept or binding or activation but, remarkably, conferred LH receptor bindin g activity to the chimera. This chimera retained the ability to stimul ate progesterone production in a Y1 cell line which expresses human FS H receptor. Replacing hFSH beta residues (95)TVRGLG(100) with hLH beta residues (101)GGPKDH(106) diminished FSH receptor binding activity bu t did not confer LH receptor binding to the chimera. This study sugges ts that amino acids within hFSH beta residues (95)TVRGLG(100) are impo rtant for FSH binding specificity and that hLH beta residues (94)RRST( 97) are involved in LH receptor binding specificity. Thus, although th e hFSH beta and hLH beta subunits may fold similarly, the loci of rece ptor binding specificity are not entirely homologous.