PURIFICATION AND CHARACTERIZATION OF A SHELL MATRIX PHOSPHOPROTEIN FROM THE AMERICAN OYSTER

The major protein component contained in the shell matrix of the American Oyster, Crassostrea virinica, was purified using size-exclusion and reverse phase HPLC chromatography. This protein was shown to be highly acidic by amino acid analysis which found 90% of the amino acids were comprised of aspartic acid, serine, and glycine in approximately equimolar amounts. The majority of the serine residues in this protein were present as OMICRON-phosphoserine. The remaining 10% of the amino acid residues were composed of predominantly non-polar and hydrophobic amino acids many of which appear to be located at the carboxy terminal end of the protein, preceded by polyaspartate, as revealed by carboxypeptidase digestion. The results of mild acid hydrolysis indicated the presence of domains of polyaspartic acid and polyphosphoserine. Functionally, this acidic protein was found to effectively inhibit the precipitation of calcium carbonate in vitro when compared to other less acidic protein fractions extracted from the shell, suggesting a role for this protein in the regulation of biomineralization in the oyster shell. The results of these analyses demonstrate remarkable similarities between this molluscan shell matrix protein and the mammalian phosphophoryns found in tooth dentin.