Functional expression of frog and rainbow trout melanocortin 2 receptors using heterologous MRAP1s

Analysis of the functional expression of the melanocortin 2 receptor (MC2R) from a rather broad spectrum of vertebrates indicates that MC2R is exclusively selective for the ligand, ACTH, and the melanocortin receptor accessory protein 1 (MRAP1) is required for high affinity ACTH binding and activation of MC2R. A phylogenetic analysis of MRAP1 suggested that tetrapod sequences and bony fish sequences may represent two distinct trends in the evolution of the mrap1 gene. To test this hypothesis, a frog (Xenopus tropicalis) MC2R was expressed in CHO cells either in the presence of a tetrapod (mouse) MRAP1 or a bony fish (zebrafish) MRAP1. The response of frog MC2R to different concentrations of human ACTH(1-24) was more robust in the presence of mouse MRAP1 than in the presence of zebrafish MRAP1. Conversely, the cAMP response mediated by the rainbow trout (Oncorhynchus mykiss) MC2R was almost twofold higher and occurred at 1000-fold lower ACTH concentration in the presence of zebrafish MRAP1 than in the presence of mouse MRAP1. Collectively, these experiments raise the possibility that at least two distinct trends have emerged in the co-evolution of MC2R/MRAP1 interactions during the radiation of the vertebrates.