A novel carbonic anhydrase from the giant clam Tridacna gigas contains two carbonic anhydrase domains

This report describes the presence of a unique dual domain carbonic anhydrase (CA) in the giant clam, Tridacna gigas. CA plays an important role in the movement of inorganic carbon (C-i) from the surrounding seawater to the symbiotic algae that are found within the clam's tissue. One of these isoforms is a glycoprotein which is significantly larger (70 kDa) than any previously reported from animals (generally between 28 and 52 kDa). This alpha-family CA contains two complete carbonic anhydrase domains within the one protein, accounting for its large size; dual domain CAs have previously only been reported from two algal species. The protein contains a leader sequence, an N-terminal CA domain and a C-terminal CA domain. The two CA domains have relatively little identity at the amino acid level (29%). The genomic sequence spans in excess of 17 kb and contains at least 12 introns and 13 exons. A number of these introns are in positions that are only found in the membrane attached/secreted CAs. This fact, along with phylogenetic analysis, suggests that this protein represents the second example of a membrane attached invertebrate CA and it contains a dual domain structure unique amongst all animal CAs characterized to date.