One of the two trout proopiomelanocortin messenger RNAs potentially encodes new peptides

Abstract

POMC is the precursor for a number of biologically active peptides such as ACTH, α-MSH, β-MSH, and β-endorphin. It is well known that some of these peptides, especially β-endorphin, are involved in the regulation of reproductive functions in mammals. In order to investigate the possible role of POMC-derived peptides in the control of fish reproduction, we have cloned and sequenced two different trout POMC cDNAs called POMC A and POMC B. These cDNAs exhibited limited sequence homology (44%). The deduced amino acid sequences also showed weak similarity (43%), despite the high conservation of some peptide sequences (α-MSH, β-MSH, and β-endorphin). The POMC A coding sequence exhibited an unusual length, generating the longest endorphin ever sequenced. The long carboxy-terminal part of the β-endorphin A contained three potential dibasic cleavage sites, allowing the occurrence of three new peptides: EQWGREEGEE, ALGE, and YHFQG. Using in situ hybridization, we found that the two POMC genes were expressed in the same pituitary cells. POMC A mRNA was the only one detectable in the hypothalamus of sexually inactive fish, whereas the two POMC genes were expressed in the hypothalamus of sexually active fish. These results indicate that two functional POMC genes are present in the rainbow trout. In POMC neurons, the expression of the POMC B gene is likely to be under the control of sexual steroids.