A carbonic anhydrase from the nacreous layer in oyster pearls.

TLDR

Polymorphism of calcium carbonate crystals in mollusk shells is thought to be controlled by organic matrix proteins secreted by the mantle. The study aims to elucidate the fine structure of these matrix proteins to understand mineralization mechanisms. The authors purified, characterized, and cloned the cDNA of nacrein, a soluble matrix protein from oyster pearl nacre. Northern blotting revealed nacrein expression restricted to the mantle pallial, and sequence analysis showed it contains a split carbonic anhydrase domain flanked by a Gly‑Xaa‑Asn repeat, indicating it acts both as a calcium‑binding matrix protein and a carbonic anhydrase in nacre formation.

Abstract

It is believed that the polymorphism observed in calcium carbonate crystals, 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 understand the molecular mechanisms of mineralization process, detailed structural analyses of the organic matrix proteins are essential. For this, we have carried out purification, characterization, and cDNA cloning of nacrein, which is a soluble organic matrix protein in the nacreous layer of oyster pearls. Northern blot analysis showed that the nacrein 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 carbonic 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 catalyzes the HCO3- formation, thus participating in calcium carbonate crystal formation of the nacreous layer.