Organization of proenkephalin in amphibians: cloning of a proenkephalin cDNA from the brain of the anuran amphibian, Spea multiplicatus

Cloning of a proenkephalin cDNA from the pelobatid anuran amphibian, Spea m ultiplicatus, provides additional evidence that Leu-enkephalin, although pr esent in the brain of anuran amphibians, is not encoded by the proenkephali n gene. The S. multiplicatus proenkephalin cDNA is 1375 nucleotides in leng th, and the open reading frame contains the sequences of seven opioid seque nces. There are five copies of the Met-enkephalin sequence, as well as an o ctapeptide opioid sequence (YGGFMRNY) and a heptapeptide opioid sequence (Y GGFMRF). In the proenkephalin sequence of S. multiplicatus the penultimate opioid is a Met-enkephalin sequence rather than the Leu-enkephalin present in mammalian sequences. The same order of opioid sequences also is observed for the proenkephalin sequence of the pipid anuran amphibian, Xenopus laev is. Hence, from a phylogenetic standpoint the organization of tetrapod proe nkephalin has been remarkably conserved. What remains to be resolved is whe ther the Leu-enkephalin sequence found in mammalian proenkephalin is an anc estral trait or a derived trait for the tetrapods. Unlike the proenkephalin precursor of X. laevis, all of the opioid sequences in the S. multiplicatu s proenkephalin cDNA are Ranked by paired-basic amino acid proteolytic clea vage sites. in this regard the proenkephalin sequence for S, multiplicatus is more similar to mammalian proenkephalins than the proenkephalin sequence of X. laevis. However, a comparison of the proenkephalin sequences in huma n, X. laevis, and S. multiplicatus revealed several conserved features in t he evolution of the tetrapod proenkephalin gene. By contrast, a comparison of tetrapod proenkephalin sequences with the partial sequence of a sturgeon proenkephalin cDNA indicates that the position occupied by the penultimate opioid sequence in vertebrate proenkephalins may be a highly variable locu s in this gene. (C) 2000 Elsevier Science Inc. All rights reserved.