A newly characterized melanotropin in proopiomelanocortin in pituitaries of an elasmobranch, Squalus acanthias

Proopiomelanocortin (POMC) is a precursor for corticotropin (ACTH), three o r fewer molecular types of melanotropin (MSH), and beta-endorphin. This pro tein is thought to have evolved by duplication of MSH genomic segments. Her e we report that the POMC in the dogfish, an elasmobranch, contains a fourt h type of MSH in addition to classical alpha-, beta-, and gamma-MSH. POMC c DNA was amplified by PCR from double-strand cDNA prepared from dogfish pitu itary and ligated into lambda ZAP II. The POMC cDNA is composed of 1315 bp without a poly(A) rail. Northern blot analysis detected a 1.4-kb signal of dogfish POMC mRNA. An open reading frame of the POMC cDNA encodes 320 amino acids, including a signal peptide of 26 amino acids. The dogfish POMC incl udes gamma-MSH, ACTH, alpha-MSH, beta-MSH, and beta-endorphin at positions 50-61, 115-153, 115-127, 239-256, and 259-294, respectively. In addition to these classical peptides, a newly discovered MSH, which we have termed del ta-MSH, is present in dogfish POMC at position (184-195). The four dogfish MSHs can be separated into two groups based on their sequence identities: o ne pair consists of alpha-MSH and gamma-MSH, and the other consists of beta -MSH and delta-MSH, suggesting that gamma-MSH and delta-MSH may have been d uplicated evolutionarily from alpha-MSH and beta-MSH, respectively gamma-MS H might first have appeared in early gnathostomes because it is absent in t he most primitive vertebrate group, the agnathans. delta-MSH, which at this time is found only in chondrichthians, might have appeared after the diver gence of chondrichthians from a lineage leading to osteichthyans and tetrap ods. (C) 1999 Academic Press.