ALTERNATIVE SPLICING CONVERTS THE G-PROTEIN COUPLED FOLLITROPIN RECEPTOR GENE INTO A GROWTH-FACTOR TYPE-I RECEPTOR - IMPLICATIONS FOR PLEIOTROPIC ACTIONS OF THE HORMONE

Pituitary follitropin (FSH) has pleiotropic actions on gonads, but it is not certain if all these events are mediated by a single receptor. A single gene for the FSH receptor undergoes extensive alternate splic ing generating multiple transcripts, and several of these have been cl oned and characterized from the sheep testis. In this study we have in vestigated the expression in HEK (human embryonic kidney) 293 cells of a cloned cDNA encoding the first eight exons of the FSH receptor alon g with a carboxy-terminal extension that contributed a hypothetical si ngle transmembrane domain. This cDNA, which lacked the conventional se ven transmembrane motif of the full-length 695 residue wild-type recep tor protein, was also efficiently expressed on the cell surface and ex hibited high affinity and specificity for FSH binding. LH did not comp ete for FSH binding indicating that these structures contained all the motifs necessary for specific hormone recognition. Following hormone binding and affinity crosslinking the deduced Mr of the expressed rece ptor was compatible with dimer formation. The expression of these alte red FSH receptors on the cell surface was confirmed by immunohistochem istry, which revealed punctate labeling in a pattern comparable to tha t shown by cells transfected by wild-type receptor cDNA. Addition of F SH stimulated H-3-thymidine incorporation in trasfected cells in a bip hasic manner. By performing RT-PCR we could show that similar altered receptor transcripts were present in both the ovary and testis, Our re sults reveal for the first time that the seven transmembrane structure of FSH-receptor is not absolutely necessary for cell surface expressi on and hormone binding provided other compensating motifs are present in the protein structure for membrane insertion. Some of these feature s are typical of growth factor receptors. Our investigations also demo nstrate that alternate splicing of the FSH receptor gene provides a me chanism for creating receptor diversity and suggest that multiple rece ptors could be involved in regulation of hormone action. (C) 1997 Wile y-Liss, Inc.