Evidence for a carbonic anhydrase-related protein in the brain of rainbow trout ( Oncorhynchus mykiss)

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Carbonic anhydrase related proteins (CA-RP) are members of the alpha-CA gene family that have lost the ability to catalyze the hydration/dehydration of carbon dioxide. Nonetheless, these proteins are very highly conserved within mammals, suggesting they have an important biological function. To date, however, there has been no effort to examine the presence and role of CA-RP proteins in non-mammalian vertebrates. This study therefore examined whether CA-RPs are present in a representative teleost, the rainbow trout (Oncorhynchus mykiss). A CA-RP-like 846 bp coding region was amplified from whole body rainbow trout mRNA, and was shown to be expressed primarily in the brain. Phylogenetic analysis grouped this isozyme, and several other non-mammalian CA-RPs identified in Genbank, closely with mammalian CA-RP VIII. Analyses of the molecular structure revealed that mutations that led to the loss of catalytic activity in mammalian CA-RP VIII were also present in the active site pocket of the rainbow trout CA-RP protein, suggesting a similar lack of catalytic activity. Unlike mammalian CA-RP VIII, however, the N-terminal regions of non-mammalian CA-RPs do not contain an acidic motif. This suggests that these proteins do not function as a regulator of transcription, as has been proposed for mammals.