Molecular cloning of the chicken melanocortin 2 (ACTH)-receptor gene

The chicken melanocortin 2-receptor (MC2-R) gene was isolated. It is found to be a single copy gene encoding a 357 amino acid protein, sharing 65.8-68.7% identity with mammalian counterparts. The chicken MC2-R mRNA is expressed in the adrenal and spleen, suggesting that the receptor mediates both endocrine and immunoregulatory functions of ACTH in the chicken. The amino acid sequence of the chicken MC2-R is collinear with those of other subtypes of MC-R, whereas all cloned mammalian MC2-Rs contain a gap in the third intracellular loop, suggesting that mammalian MC2-R molecules have evolved by lacking a part of the domain which determines the specificity of signal transduction in G-protein coupled receptors. Interestingly, the codon usage differs dramatically between MC1-R and MC2-R in the chicken; the GC-contents at the third codon position in MC1-R and MC2-R are 94.6 and 50.6%, respectively. It may reflect selective constraints on the usage of synonymous codons.