Cloning and Functional Expression of the Luteinizing Hormone Receptor Complementary Deoxyribonucleic Acid from the Marmoset Monkey Testis: Absence of Sequences Encoding Exon 10 in Other Species*

Based on sequence homologies among the human, porcine, rat, and mouse genes for the LH receptor (LHR), overlapping partial fragments of LHR complementary DNAs (cDNAs) were multiplied from marmoset monkey testicular RNA using reverse transcription-PCR. Ligations of the individual cDNA fragments generated a full-length monkey LHR cDNA (2031 bp) containing the complete amino acid-coding sequence (676 amino acids). Northern hybridization analysis of monkey testicular RNA, using a complementary RNA probe corresponding to the full-length cDNA, demonstrated major transcripts of 5.5 and 1.4 kilobases and minor ones of 4.0, 2.7, and 1.9 kilobases. Sequence analysis of the monkey LHR cDNA revealed a striking feature, i.e. the absence of an 81-bp nucleotide sequence corresponding to exon 10, present in the LHR cDNAs of all other species studied to date. The monkey LHR cDNA displayed 83-94% overall sequence homology with the other mammalian LHR cDNAs. Reverse transcription-PCR with human exon 10-specific primers demonstrated the total absence of this sequence from the monkey LHR messenger RNA. Southern hybridization of monkey genomic DNA using a human exon 10 probe demonstrated its presence in the monkey gene and that it is totally spliced out from the primary transcript. COS cells transfected with the monkey LHR cDNA showed similar high affinity (Kd = 0.25 nmol/liter) of [125I]iodo-hCG binding as those transfected with human LHR cDNA (Kd = 0.20 nmol/liter). The cells expressing the recombinant monkey and human LHR displayed similar responses of extracellular cAMP and inositol trisphosphate to hCG. In conclusion, marmoset monkey LHR seems to lack the sequence corresponding to exon 10 of the LHR gene in other mammalian species. The truncation does not alter LHR function, as the monkey receptor protein bound hCG and evoked cAMP and inositol trisphosphate responses comparable to those of the human LHR containing the exon 10-encoded structure. As the sequence homologous to exon 10 is missing in the other two glycoprotein receptors, i.e. those of FSH and TSH, this extra exon is apparently inserted into the LHR messenger RNA of some species during evolution from intronic sequences by a change in alternative splicing.