Functional analysis of acdic and fidrous proteins on biomineralization

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Aragonite and calcite are crystal polymorphs of calcium carbonate in biomineralization. Mollusk shells are composed of either one or both polymorphs of CaCO3. One olymorph is an outer prismatic layer that bears calcite. The other polymorph is an inner acreous layer bearing aragonite. These layers contain about 5% organic matrix secreted by the mantle. The organic matrix is consisted of EDTA-soluble and insoluble fractions.A.P.Wheeler et al. have suggested that the EDTA-insoluble matrix contains two distinct layers of LIM (Light Insoluble Matrix) and DIM (Dark Insoluble Matrix). LIM is made of highly anionic proteins such as EDTA-soluble proteins. In contrast to LIM, DIM is a fibrous protein rich in the glycine and alanine.A pearl is equivalent to the inner nacreous layer of the mollusk shell, which is composed of aragonite crystal and organic matrices. After decalcification of powdered pearl with 0.3 M EDTA, a EDTA-insoluble fraction was extracted with 0.3 M EDTA-8 M urea. Conchiolin15 which has a molecular weight of about 15 kDa was purified. Conchiolin 15 contains a sulfated mucopolysaccharide. It is assumed that calcium is bound to the negative charge on sulfate groups linked to the mucopolysaccharide. We cloned the cDNA codling for Conchiolin 15 and deduced its complete amino acid sequence. Conchiolin 15 is a LIM-like protein which has the same amino acid composition as EDTA-soluble proteins except that Conchiolin 15 is also rich in Tyr and Cys. Conchiolin 15 was shown to be quite different from fibrous protein rich in Ala and Gly.From these results, I suggest three possible functions of Conchiolin 15.1. Conchiolin 15, together with silk fibroin-like protein, participates in the formation of the organic matrix framework around which the calcium carbonate is deposited.2. Conchiolin 15 acts as an anchor between the soluble acidic protein and the insoluble silk fibroin-like protein.3. Conchiolin 15 functions as a template for nucleation.