EXPRESSION CLONING OF A HIGH-AFFINITY MELATONIN RECEPTOR FROM XENOPUS DERMAL MELANOPHORES

Using an expression cloning strategy, a high-affinity melatonin receptor cDNA has been isolated from Xenopus laevis dermal melanophores. Transient expression of the cDNA in COS-7 cells resulted in high-affinity 2-[I-125]- iodomelatonin binding (K-d = 6.3 +/- 0.3 X 10(-11) M). In addition, six ligands exhibited a rank order of inhibition of specific 2-[I-125] iodomelatonin binding that was identical to that reported for endogenous high-affinity receptors. Functional studies of CHO cells stably expressing the receptor cDNA showed that melatonin acting through the cloned receptor inhibited forskolin-stimulated cAMP accumulation in a dose-dependent manner. Northern blot analysis showed that melatonin receptor transcripts are moderately expressed in Xenopus dermal melanophores. The cDNA encodes a protein of 420 amino acids, which contains seven hydrophobic segments. Structural analysis revealed that the receptor protein is a newly discovered member of the guanine nucleotide binding protein-coupled receptor family.