Characterization of two molluscan crystal-modulating biomineralization proteins and identification of putative mineral binding domains.

Abstract

Ethylenediamine-tetraacetic acid extracted water-soluble matrix proteins in molluscan shells secreted from the mantle epithelia are believed to control crystal nucleation, morphology, orientation, and phase of the deposited mineral. Previously, atomic force microscopy demonstrated that abalone nacre proteins bind to growing step edges and to specific crystallographic faces of calcite, suggesting that inhibition of calcite growth may be one of the molecular processes required for growth of the less thermodynamically stable aragonite phase. Previous experiments were done with protein mixtures. To elucidate the role of single proteins, we have characterized two proteins isolated from the aragonitic component of nacre of the red abalone, Haliotis rufescens. These proteins, purified by hydrophobic interaction chromatography, are designated AP7 and AP24 (aragonitic protein of molecular weight 7 kDa and 24 kDa, respectively). Degenerate oligonucleotide primers corresponding to N-terminal and internal peptide sequences were used to amplify cDNA clones by a polymerase chain reaction from a mantle cDNA library; the deduced primary amino acid sequences are presented. Preliminary crystal growth experiments demonstrate that protein fractions enriched in AP7 and AP24 produced CaCO(3) crystals with morphology distinct from crystals grown in the presence of the total mixture of soluble aragonite-specific proteins. Peptides corresponding to the first 30 residues of the N-terminal sequences of both AP7 and AP24 were generated. The synthetic peptides frustrate the progression of step edges of a growing calcite surface, indicating that sequence features within the N-termini of AP7 and AP24 include domains that interact with CaCO(3). CD analyses demonstrate that the N-terminal peptide sequences do not possess significant percentages of alpha-helix or beta-strand secondary structure in solution. Instead, in both the presence and absence of Ca(II), the peptides retain unfolded conformations that may facilitate protein-mineral interaction.