Recently, a distinct family of G protein-coupled receptors has been cloned that mediates the biological effects of melatonin. Of two subtypes cloned from mammals (Mel(1a) and Mel(1b)), the Mel(1a) receptor appears to mediate the circadian and reproductive effects of the hormone. We now report the cloning, characterization, and expression of the gene encoding the Mel(1a) receptor in mice. The receptor gene is composed of two exons, separated by an intron of greater than 13 kilobases. Exon 1 encodes the entire 5'-untranslated region and the coding region through the first cytoplasmic loop. Exon 2 encodes the rest of the coding region and the entire 3'-untranslated region. 5'-Rapid amplification of complementary DNA ends and ribonuclease protection analyses show that the major transcription start site is 103 nucleotides upstream of the translation start codon. Sequence analysis of 1.1 kilobases of the 5'-flanking region reveals that it does not contain TATA or CAAT boxes. The 5'-flanking region drives luciferase expression 114- fold over basal levels in a murine retinal cell line that endogenously expresses the Mel1a receptor. The mouse receptor binds 2- iodomelatonin with high affinity (K(d) = 55,6 pM) when expressed transiently in COS-7 cells. In situ hybridization studies establish that Mel(1a) receptor messenger RNA is expressed in the hypothalamic suprachiasmatic nuclei and hypophyseal pars tuberalis, presumed sites of the circadian and some of reproductive actions of melatonin, respectively. These results provide information on Mel(1a) receptor gene structure essential for designing transgenic and gene knock-out studies and analyzing the transcriptional regulation of receptor gene expression.
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