IDENTIFICATION OF REGIONS OF THE FOLLITROPIN (FSH) BETA-SUBUNIT THAT INTERACT WITH THE N-TERMINUS REGION (RESIDUES-9-30) OF THE FSH RECEPTOR

We have recently identified a region, N-terminus residues 9-30, in the extracellular domain of the follicle-stimulating hormone (FSH) receptor capable of binding FSH, but not luteinizing hormone (LH) or thyroid-stimulating hormone (FSH) (Dattatreyamurty and Reichert (1992) Mol. Cell. Endocrinol. 87, 9-17). The objectives of the present study were to examine the interaction between a synthetic peptide corresponding to this receptor sequence and the beta-subunit of FSH, and to identify which FSH-beta regions are involved in the interaction. FSH-beta subunit and synthetic FSH-beta peptides 1-15, 71-85 and 101-111 effectively bound I-125-labeled FSH rec-(9-30) peptide, and binding was inhibited by excess unlabeled FSH receptors. Scatchard analysis indicated that the synthetic FSH-beta peptides had affinities for FSH rec-(9-30) peptide in the order of 10(6) M-1 (K(a)), with the sum of individual peptide affinities (K(a) = 1.21 x 10(7) M-1) closely approximating that of the intact beta-subunit (1.02 x 10(7) M-1). Polyclonal antibodies raised against FSH rec-(9-30) peptide completely inhibited the binding of I-125-labeled receptor peptide to hFSH, hFSH-beta, and hFSH-beta peptides 1-15, 71-85 and 101-111. Our results indicate that recognition of FSH-beta by N-terminus region (9-30) of the FSH receptor involves contact with residues in three discontinuous binding regions on FSH-beta. The latter finding suggests that these three discontinuous sequence regions of FSH-beta may be closely oriented on the hormone surface to form a contiguous region in the three-dimensional structure required for recognition by the N-terminus region (residues 9-30) of the FSH receptor.