A follicle-stimulating hormone receptor ecto-domain epitope that is a target for receptor immunoneutralization yet does not affect ligand contact andactivation

The follicle-stimulating hormone receptor (FSHR) large extracellular domain suggests that interaction of ligand with receptor is likely to be complex. Residues 265-296 of the FSHR are part of a sequence primarily nonhomologou s with other glycoprotein hormone receptors, A reasonable hypothesis is tha t th is sequence of the FSHR plays a role in binding FSH, Flow cytometry st udies of this region revealed that antibody X179 against peptide R265-S296 binds to human FSHR expressed by CHO cells and can be competed against by p reincubating the cells with hFSH, These results suggested that the region c orresponding to residues 265-296 in the extracellular domain of the FSHR is involved in binding to hormone. To test this hypothesis 10 scanning alanin e mutants of rFSHR at the 265-296 epitope were generated, and the binding c haracteristics of these mutants were studied. Their affinity constants for I-125-hFSH did not deviate greatly from that of wild-type FSHR, in which so me mutants exhibited an approximately two- to threefold reduction in K-a co mpared to wild-type receptor, and no mutation abolished signal transduction . These results lead to rejection of the hypothesis that this region contai ns residues critical for conveying hormone specificity and receptor-depende nt hormone action.