Identification of Duplicated Fourth α2-Adrenergic Receptor Subtype by Cloning and Mapping of Five Receptor Genes in Zebrafish

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Abstract
The α2-adrenergic receptors (α2-ARs) belong to the large family of rhodopsinlike G-protein–coupled receptors that share a common structure of seven transmembrane (TM) α-helices. The aims of this study were (1) to determine the number of α2-AR genes in a teleost fish, the zebrafish (Danio rerio), (2) to study the gene duplication events that generated the α2-AR subtypes, and (3) to study changes in receptor structure that have occurred since the divergence of the mammalian and fish lineages. Here, we report the cloning and chromosomal mapping of fish orthologs for all three mammalian α2-ARs. In addition, we identified a fourth α2-AR subtype with two duplicates in zebrafish. Chromosomal mapping showed that the zebrafish α2-AR genes are located within conserved chromosomal segments, consistent with the origin of the four α2-AR subtypes by two rounds of chromosome or block duplication before the divergence of the ray fin fish and tetrapod lineages. Thus, the fourth subtype has apparently been present in the common ancestor of vertebrates but has been deleted or is yet to be identified in mammals. The overall percentage identity between the fish and mammalian orthologs is 53% to 67%, and in the TM regions 80% to 87%. These values are clearly lower than what is observed between mammalian orthologs. Still, all of the residues thought to be important for α2-adrenergic ligand binding are conserved across species and subtypes, and even the most divergent regions of the fish receptors show clear “molecular fingerprints” typical for orthologs of a given subtype.