MOLLUSCAN SHELL MATRIX PHOSPHOPROTEINS - CORRELATION OF DEGREE OF PHOSPHORYLATION TO SHELL MINERAL MICROSTRUCTURE AND TO INVITRO REGULATION OF MINERALIZATION

The potential role of phosphoproteins in the regulation of carbonate biomineralization is examined by comparing the EDTA-soluble matrix (SM) of bivalve shells from representatives of eight superfamilies having various aragonitic and calcitic microstructures. In each case the SM is separated into two fractions by gel permeation chromatography. The lower molecular weight fractions (M(r) range 15,000-500,000) were assayed for protein and phosphate content, and their ability to regulate calcitic CaCO3 formation was determined from their performance as inhibitors in a pH-stat crystal growth assay. The SM fraction from two species with foliated calcitic shells is 12.9-15.7% phosphate by weight while SM fractions isolated from shells or shell layers of six species with calcitic prismatic or various aragonitic microstructures all contain significantly lower phosphate levels (0-3.3% by weight). The more highly phosphorylated SM fractions are significantly better inhibitors of calcitic crystal growth, requiring approximately 30% of the concentration on a weight basis as the low phosphate fractions to effect a 50% reduction in growth rate. Enzymatic dephosphorylation of the high phosphate SM fractions results in reduced inhibition capabilities that are approximately equal to those of the low phosphate SM fractions. Finally, neither dephosphorylated nor naturally low phosphate SM fractions were as effective as phosphorylated foliated matrix in maintaining inhibition in the pH-stat assay as crystals continue to grow. In conclusion, it appears that degree of SM phosphorylation in part may be a factor in the regulation of carbonate crystal growth in vivo and thus explain its variability among the various shell microstructures.