Molecular cloning and characterization of secretory carbonic anhydrase VI in pufferfish (Takifugu rubripes)

Carbonic anhydrase VI (CA VI) has been characterized as a secretory isozyme in mammals. Our present study confirmed the occurrence of CA VI in pufferfish (Takifugu rubripes). In this study, genomic sequence information for the CA VI of pufferfish was used for molecular cloning. We cloned a 1821 bp cDNA sequence, which consisted of a complete coding sequence of 1623 bp and a deduced amino acid sequence of 540 amino acids from the open reading frame. A BLAST search indicated that this protein exhibits 53%, 79%, and 67% identity with human, tilapia, and gar CA VI, respectively. It also shows 63%-77% identity with other fish CA VI-like sequences (zebrafish, Asian arowana, salmon, and large yellow croaker). Moreover, alignment of two or more sequences revealed that the protein sequence of pufferfish CA VI has 34%-37% identity with mammalian and fish CA II sequences. An NH2-terminal signal peptide of 18 amino acids in length was predicted in the pufferfish CA VI sequence. Three potential N-linked glycosylation sites and two cysteine residues (Cys-28 and Cys-209) that are likely to form one disulfide bond were present in pufferfish CA VI. In silico and phylogenetic analyses revealed that pufferfish CA VI is an extracellular secretory protein. Active site analysis indicated that this protein is a low-activity CA isozymes due to a characteristic Val/Ile substitution at position 207. Homology modeling of puffer CA VI was performed using the crystal structure of human carbonic anhydrase XIV as a template structure, based on high similarity. Reverse transcription-polymerase chain reaction (PCR), quantitative PCR and in situ hybridization results revealed that, the pufferfish CA VI is highly expressed in liver tissue.