Hiroshi Miyamoto1, Tomoyuki Miyashita1,2, Minoru OKUSHIMA1,3, Shigeru Nakano1,3, Morita Toshiro1,3, Aizo Matsushiro1,2
1Faculty of Biology-Oriented Science and Technology, Kinki University, Wakayama, Japan. [email protected] 2Osaka University, Yamadaoka, Suita, Osaka 565, Japan
3National Institutes of Health, Bethesda, MD, May 27, 1996
Tóm tắt
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.
