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.

Abstract

The family of pituitary/placental glycoprotein hormones includes follicle-stimulating hormone (follitropin, FSH), luteinizing hormone (lutropin, LH), chorionic gonadotropin (choriogonadotropin, CG), and thyroid-stimulating hormone (thyrotropin, TSH). These glycoproteins are heterodimeric, with an alpha-subunit of identical primary structure and a hormone-specific beta-subunit which is believed to confer receptor specificity. Previous studies demonstrated that substitution of hFSH beta residues 88-108 in place of the carboxyl terminus of hCG beta, residues 94-145, conferred to human (h) CG an FSH receptor binding specificity. To more finely map the LH/FSH receptor binding specificity determinant, hFSH beta residues 88DSDS91, within the small hydrophilic intercysteine loop, and residues 95TVRGLG100 COOH-terminal to the loop were switched to hLH beta residues 94RRST97 and residues 101GGPKDH106, respectively. Substitution of hLH beta residues 94RRST97 in place of hFSH beta residues 88DSDS91 did not affect FSH receptor binding or activation but, remarkably, conferred LH receptor binding activity to the chimera. This chimera retained the ability to stimulate progesterone production in a Yl cell line which expresses human FSH receptor. Replacing hFSH beta residues 95TVRGLG100 with hLH beta residues 101GGPKDH106 diminished FSH receptor binding activity but did not confer LH receptor binding to the chimera. This study suggests that amino acids within hFSH beta residues 95TVRGLG100 are important for FSH binding specificity and that hLH beta residues 94RRST97 are involved in LH receptor binding specificity. Thus, although the hFSH beta and hLH beta subunits may fold similarly, the loci of receptor binding specificity are not entirely homologous.