cDNA cloning of proopiomelanocortin (POMC) and mass spectrometric identification of POMC-derived peptides from snake and alligator pituitaries

Proopiomelanocortin (POMC) is the precursor of melanocyte-stimulating hormone (MSH) and β-endorphin, and is suggested to have evolved by the insertion and deletion of ancestral MSH segments. Here, the primary structure of POMC was determined with cDNA cloning of brown tree snakes of Squamata and American alligators of Crocodylia to show an overview of the molecular evolution of POMC in reptiles. Snake and alligator POMCs are composed of α-, β-, and γ-MSH segments and a single β-END segment as in other tetrapods; however, the γ-MSH segment in snake POMC has a mutation in the essential sequence from His-Phe-Arg-Trp to His-(d)-(d)-Arg, in which (d) means deletion. It is conceivable that the ancestry of snake γ-MSH had weak functional constraint and lacked biological significance during evolution. Phylogenetic analyses using the neighbor-joining method show that snake prePOMC is most diverged, and alligator prePOMC is most conserved in reptilian POMCs while it shows the highest sequence identity with ostrich prePOMC. These relationships are comparable to those observed in mitochondrial DNA. On the other hand, analyses of the pituitary with mass spectrometry revealed several peptides by post-translational processing as predicted by the locations of processing sites consisting of basic amino acid residues in snake and alligator POMCs. Remarkably, the monobasic site at the N-terminal side of the snake β-MSH is suggested to act as a processing site. Thus, the study shows the divergence of snake POMC such as the critical mutation of γ-MSH and high conservation of hormone organization of alligator POMC.