Hypothesis and Theory: Revisiting Views on the Co-evolution of the Melanocortin Receptors and the Accessory Proteins, MRAP1 and MRAP2

Abstract

The evolution of the melanocortin receptors (MCRs) is closely associated with the evolution of the melanocortin-2 receptor accessory proteins (MRAPs). Recent annotation of the elephant shark genome project revealed the sequence of a putative MRAP1 ortholog. The presence of this sequence in the genome of a cartilaginous fish raises the possibility that the mrap1 and mrap2 genes in the genomes of gnathostome vertebrates were the result of the chordate 2R genome duplication event. The presence of a putative MRAP1 ortholog in a cartilaginous fish genome is perplexing. Recent studies on melanocortin-2 receptor (MC2R) in the genomes of the elephant shark and the Japanese stingray indicate that these MC2R orthologs can be functionally expressed in CHO cells without co-expression of an exogenous mrap1 cDNA. The novel ligand selectivity of these cartilaginous fish MC2R orthologs is discussed. Finally, the origin of the mc2r and mc5r genes is reevaluated. The distinctive primary sequence conservation of MC2R and MC5R is discussed in light of the physiological roles of these two MCR paralogs.

Keywords: MC2R; MC5R; MRAP1; MRAP2; evolution; melanocortin receptors.

Figure 1

Amino acid sequence alignment of MRAP1 and MRAP2 paralogs. The amino acid sequences of mouse (m) MRAP1 (NP_084120.1), zebrafish (z) MRAP1 (XP001342923.2), chicken (c) MRAP1 (XR_001470382) elephant shark (es) MRAP1 (XM_007903550.1), mouse (m) MRAP2 (XP_006511239.1), chicken (c) MRAP2 (XP_015140201), zebrafish (zf) MRAP2a (XP_001342923.4), elephant shark (es) MRAP2 (XP_007906624.1), and lamprey (lp) MRAP2 (FAA00710.1) were aligned to the sequences of mouse MRAP1 and mouse MRAP2, respectively. Note that only a partial sequence for lamprey (Petromyzon marinus) MRAP2 has been reported (1). In addition, only the partial C-terminal sequences for the MRAP2 orthologs are presented. Predicted N-linked glycosylation sites are underlined. Note that there are two potential N-linked glycosylation sites in the putative elephant shark MRAP1 amino acid sequence. The amino acids in the activation motif for mouse MRAP1 are highlighted in red. Conserved amino acid positions in the proposed activation motifs of the chicken, zebrafish, elephant shark MRAP1 orthologs are also highlighted in red. The amino acids in the reverse topology motif of mouse MRAP1 were highlighted in green. Conserved amino acid positions in the reverse topology motif of chicken, zebrafish, elephant shark, mouse, and lamprey MRAP1 and MRAP2 sequences, respectively, were also highlighted in green. Finally, the amino acids in the transmembrane domain of mouse MRAP1 are highlighted in blue, and the conserved amino acid positions in the chicken, zebrafish, elephant shark, mouse and lamprey MRAP1 and MRAP2 sequences, respectively, were also highlighted in blue.

Figure 2

Proposed evolution for the MRAP Gene Family. The evolutionary tree presented in this figure assumes that there was an ancestral mrap (MRAP) gene in the genome of ancestral agnathans. Following the 2R genome duplication event, two paralogous mrap genes emerged (MRAP1 and MRAP2) and are present in the genomes of many extant 2R chordates. A solid black box indicates that the gene has been reported in the respective taxa. A box with a dashed line border indicates a gene that is predicted, but has not been detected (i.e., lamprey MRAP1) or the organism is extinct (e.g., ancestral agnathans, ancestral in gnathostomes). A box with a dotted border indicates a taxonomic group in which MRAP1 is required for the functional expression of MC2R, but a MRAP1 sequence has not been identified in the genome of a representative from that taxonomic group.

Figure 3

Amino Acid Alignment of MC2R Orthologs. The amino acid sequences of human (h) MC2R (NP_ 001278840.1), zebrafish (z) MC2R (XP_00518229.1) stingray (s) MC2R (LC108747), elephant shark (e) MC2R (FAA704.1), stingray (s) MC4R (LC108749) were aligned, and amino acid positions in which four of the five sequences were identical are marked in red. The position of critical amino acids in the HFRW-binding site of MC4R orthologs (47) are marked with a star. The overall percent primary sequence identify (A) and the percent identity within each domain (B) are presented.

Figure 4

Amino Acid alignment of Gar MC2R, MC4R, and MC5R. The amino acid sequences of gar MC2R (ENSLOCT00000011667), gar MC4R (ENSLOCT00000022303), and gar MC5R (ENSLOCG00000018340) were aligned and positions that were identical are marked in red. (A) alignment of gar MC2R and gar MC5R; (B) alignment of gar MC4R and gar MC5R.

Figure 5

Amino acid sequence identity of MC2R orthologs. (A) The amino acid sequences of human (h) MC2R and stingray (s) MC2R were aligned. (B) The amino acid sequences of gar (g) MC2R and zebrafish (z) MC2R (XP_005158229) were aligned. The position of critical amino acids in the proposed HFRW-binding site (47) are marked with a star. Amino acid positions that are identical are marked in red.

Figure 6

Amino Acid Sequence Identity of MC5R Orthologs. (A) The amino acid sequences of human (h) MC5R (NP_ 005904.1) and stingray (s) MC5R (AY562212) are aligned. (B) The amino acid sequences of gar (g) MC5R and Takifugu rubripes (f) MC5R (AA06553.1; fugu) were aligned. The position of critical amino acids in the proposed HFRW-binding site (47) is marked with a star. Amino acid positions that are identical are marked in red.