Molecular cloning of lungfish proopiomelanocortin cDNA.

To investigate the evolution of proopiomelanocortin (POMC) from fish to tetrapods, nucleotide sequence of POMC cDNA from a lobe-finned fish, the African lungfish, was determined. POMC cDNA was prepared from lungfish pituitary glands. The POMC cDNA is composed of 1114 bp, excluding a poly-A tail, and encodes 255 amino acids (aa) including a signal peptide of 25 aa. The lungfish POMC contains the segment corresponding to gamma-melanotropin (MSH), corticotropin, alpha-MSH, beta-MSH, and beta-endorphin at positions (50-61), (108-146), (108-120), (178-194), and (197-230), respectively. The lungfish POMC shows greater sequence identity on average with amphibian (62%), ancient ray-finned fishes including acipenseriformes and semionotiformes (62%), and mammalian POMC (52%) than with teleostean (49%), elasmobranch (46%), and agnathan POMC (31%). Thus, the overall structural feature of lungfish POMC is close to the tetrapod POMCs which contain gamma-MSH and the ancient ray-finned fishes POMCs containing gamma-MSH-like sequence. However, amino acid sequence of lungfish beta-endorphin exhibits properties which are specifically observed in the ray-finned fishes and the elasmobranchs.