A CARBONIC-ANHYDRASE FROM THE NACREOUS LAYER IN OYSTER PEARLS

It is believed that the polymorphism observed in calcium carbonate cry stals, such as aragonite and calcite in mollusk shells, is controlled by organic matrix proteins secreted from the mantle epithelia. However , the fine structures of these proteins are still unknown, and to unde rstand the molecular mechanisms of mineralization process, detailed st ructural analyses of the organic matrix proteins are essential, For th is, we have carried out purification, characterization, and cDNA cloni ng of nacrein, which is a soluble organic matrix protein in the nacreo us layer of oyster pearls, Northern blot analysis showed that the nacr ein transcript was specifically expressed in mantle pallial, Analysis of the deduced amino acid sequence revealed that the protein contained two functional domains: one was a carbonic anhydrase and another was a Gly-Xaa-Asn (Xaa = Asp, Asn, or Glu) repeat domain; however, the car bonic anhydrase domain was split into two subdomains with insertion of the Gly-Xaa-Asn repeat domain between them, Our findings suggest that nacrein actually functions as a matrix protein whose repeated Gly-Xaa -Asn domain possibly binds calcium and as a carbonic anhydrase that ca talyzes the HCO3- formation, thus participating in calcium carbonate c rystal formation of the nacreous layer.