Miyamoto H, Miyashita T, Okushima M, Nakano S, Morita T, Matsushiro A
Faculty of Biology-Oriented Science and Technology, Kinki University, Wakayama, Japan.
Proc Natl Acad Sci U S A. 1996 Sep 3;93(18):9657-60. doi: 10.1073/pnas.93.18.9657.
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.
据信，在碳酸钙晶体中观察到的多态性，如软体动物贝壳中的文石和方解石，是由外套膜上皮分泌的有机基质蛋白控制的。然而，这些蛋白质的精细结构仍然未知，为了理解矿化过程的分子机制，对有机基质蛋白进行详细的结构分析至关重要。为此，我们对珍珠质蛋白进行了纯化、表征和cDNA克隆，珍珠质蛋白是牡蛎珍珠珍珠层中的一种可溶性有机基质蛋白。Northern印迹分析表明，珍珠质转录本在外套膜 pallial 中特异性表达。对推导的氨基酸序列的分析表明，该蛋白质包含两个功能域：一个是碳酸酐酶，另一个是Gly-Xaa-Asn（Xaa = Asp、Asn或Glu）重复域；然而，碳酸酐酶域被分成两个亚域，中间插入了Gly-Xaa-Asn重复域。我们的研究结果表明，珍珠质蛋白实际上作为一种基质蛋白发挥作用，其重复的Gly-Xaa-Asn域可能结合钙，并作为一种催化HCO3-形成的碳酸酐酶，从而参与珍珠层碳酸钙晶体的形成。
