Cloning of a proopiomelanocortin cDNA from the pituitary of the Australian lungfish, Neoceratodus forsteri: Analyzing trends in the organization of this prohormone precursor

The polypeptide hormone precursor, proopiomelanocortin (POMC), was cloned and sequenced from the pituitary of the Australian lungfish, Neoceratodus forsteri, the only surviving species of the oldest extant lineage of lungfish. The Australian lungfish POMC cDNA had an open reading frame that coded for a 255-amino acid precursor. A comparison of POMC sequences from the Australian lungfish and the African lungfish indicated that the deduced amino acid sequences for ACTH, β-MSH, and β-endorphin were over 90% identical. Furthermore, within the open reading frames of the two lungfish POMCs, there was 84% identity at the nucleotide level. Although a γ-MSH-like region was detected in the Australian lungfish POMC cDNA, this sequence contained mutations that have been detected in the γ,-MSH sequences of some ray-finned fish and are not found in the γ-MSH sequence of the African lungfish or those of tetrapods. In addition, the sequence of β- endorphin in the two species of lungfish has amino acid motifs that are found in the β-endorphin sequences of cartilaginous fish and ray-finned fish but not in tetrapods. However, maximum parsimony analysis of the entire POMC open reading indicated that the lungfish POMC sequences form a clade with two amphibian POMC sequences rather than with POMC sequences from ray-finned fish. This result is consistent with the accepted view that the sarcopterygians (lungfishes and tetrapods) are a monophyletic assemblage. Analysis of rates of divergence for various POMC sequences indicate that point mutations are accumulating in the lungfish POMC sequences at a slower rate than in either amphibian or mammalian POMC sequences. The phylogenetic implications of these observations are discussed.