THE UNIQUE EXON-10 OF THE HUMAN LUTEINIZING-HORMONE RECEPTOR IS NECESSARY FOR EXPRESSION OF THE RECEPTOR PROTEIN AT THE PLASMA-MEMBRANE IN THE HUMAN LUTEINIZING-HORMONE RECEPTOR, BUT DELETERIOUS WHEN INSERTED INTO THE HUMAN FOLLICLE-STIMULATING-HORMONE RECEPTOR

The LH receptor (LHR) is a member of the family of G protein-coupled s even-times plasma membrane transversing receptors. Its gene consists o f 11 exons, the last one encoding the transmembrane and intracellular domains of the receptor. The FSHR, and its gene, resemble structurally those of the LHR, with the exception that the sequences corresponding to exon 10 in LHR are missing in FSHR, which is thus encoded by a tot al of ten exons. Our recent studies on the marmoset monkey testis LHR cDNA indicated that an 81 bp nucleotide sequence, encoding the complet e exon 10 of the LHR gene in other mammalian species, is absent in thi s species without affecting the LHR function. To study further the rol e of the exon 10 encoded sequences of the LHR in the gonadotropin rece ptor function, a deletion of exon 10 from the human LHR (hLH Delta exo n10R), and a chimeric hFSHR with exon 10 from hLKR inserted (hFSHLHexo n10R), were constructed in expression vectors. The results presented h ere demonstrate that 293 cells transfected with the hLH Delta exon10R display a decrease in the proportion of the receptor binding at the ce ll surface, compared with cells transfected with wild-type hLHR. Howev er, the cells expressing hLH Delta exon10R showed similar high affinit y binding of [I-125]iodo-hCG as those transfected with wild-type hLHR, in either intact cells or their detergent extracts. In addition. cell s expressing the hLH Delta exon10R and wild-type hLHR displayed simila r dose-response of cAMP production to hCG stimulation. Cells transfect ed with chimeric hFSHLHexon10R showed barely detectable [I-125]iodo-FS H binding in intact cells compared with those transfected with wild-ty pe hFSHR. The FSH binding detected in cellular detergent extracts disp layed 5-fold lower binding activity than wild-type receptors, in spite of similar level of immunoreactive FSHR protein expression in the tra nsfected cells. The hFSHLHexon10R had a modest 5-fold lower binding af finity for FSH as compared with wild-type hFSHR. In conclusion, the pr esent study indicates that the sequences encoding exon 10 of the hLHR are essential for the LHR expression at the plasma membrane, but delet erious for function if inserted into the hFSHR. (C) 1998 Elsevier Scie nce Ireland Ltd. All rights reserved.