ZFOR2, a new opioid receptor-like gene from the teleost zebrafish (Danio rerio)

A new opioid receptor-like (ZFOR2) has been cloned and characterized in an anamniote vertebrate, the teleost zebrafish (Danio rerio). ZFOR2 encodes a 384-amino-acid protein with seven potential transmembrane domains, and its predicted amino acid sequence presents an overall 74% degree of identity to mammalian mu opioid receptors. Its inclusion in a dendrogram generated from the alignment of the opioid receptor's protein sequences, confirms its classification as a mu opioid receptor. Divergences in sequence are greater in the regions corresponding to extracellular loops, suggesting possible differences in ligand selectivity with respect to the classical mu opioid receptors. The genomic structure of ZFOR2 is also highly conserved throughout the phylogenetic scale, supporting the origin of opioid receptors early in evolution. Nevertheless, ZFOR2 lacks the fourth exon found in human and rodent mu opioid receptors, that is known to be involved in desensibilization and internalization processes.