A synthetic peptide corresponding to amino acids 9–30 of the extracellular domain of the follitropin (FSH) receptor specifically binds FSH

Abstract

As deduced on the basis of cloning experiments, the putative extracellular domain of pituitary glycoprotein hormone (lutropin (LH), thyrotropin (TSH), and FSH) receptors (rec) is sufficiently large to suggest involvement in hormone binding. Comparison of the amino acid sequences of the extracellular domains of the glycoprotein hormone receptors indicates that the FSH receptor has a peptide sequence in the external domain close to the amino terminus (residues 9–30) which has no sequence homology to receptors for LH or TSH. To examine whether this region is involved in FSH-receptor interaction, we studied the hormone-binding properties of a corresponding synthetic peptide in several systems. (1) Binding of 125I-hFSH to receptor-containing bovine testis membranes was inhibited by preincubation with FSH rec-(9–30) peptide amide in a concentration-dependent manner. (2) 125I-labeled rec-(9–30) peptide amide bound to ovine, bovine, or human FSH preparations, and the binding was inhibited by solubilized bovine FSH receptor. 125I-labeled rec-(9–30) peptide amide, however, did not bind to LH or TSH. (3) 125I-hFSH bound to unlabeled rec-(9–30) peptide amide, and the binding was inhibited by excess unlabeled FSH, but not by LH or TSH. (4) Scatchard analysis indicated that the FSH rec-(9–30) peptide amide contained a single class of FSH binding sites with a K a = 1.1 × 10 6 M −1. (5) The binding of 125I-labeled rec-(9–30) peptide amide to hFSH, bFSH or oFSH was effectively inhibited by rabbit polyclonal antibodies raised against rec-(9–30) peptide amide but not by preimmune rabbit serum. In conclusion, our results represent the first demonstration that the region corresponding to residues 9–30 in the extracellular domain of the follitropin receptor specifically binds to FSH.