The protein, Pif, secreted by pearl oysters contributes to the strength and shape of the shell due to key amino acids that guide mineral crystal growth.

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Behind every hard, mineral crystalline material in nature, there's a bevy of soft proteins guiding its formation and contributing to its strength and resilience. The hard calcium carbonate shells of oysters rely on a set of proteins that perform several key functions. They trigger dissolved calcium carbonate to crystallize out of solution, direct crystal growth into an ordered arrangement, and attach themselves to the crystals and each other to form a strongly bonded, properly shaped material. Although these proteins are made up of hundreds of amino acid building blocks, several amino acids appear to play major roles. Acidic amino acids, particularly aspartate, play a major role in crystal formation and binding protein and crystals together. Sulfur-containing amino acids, particularly cysteine, play a major role in binding proteins to each other to add strength and hold its shape.

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"Suzuki et al. identify two novel proteins, Pif80 and Pif97, as key players in controlling the crystal structure of calcium carbonate (CaCO3) in the pearl-forming oyster Pinctada fucata. The work provides the most complete data set to date for the role of any known protein involved in mollusk biomineralization...Both proteins remain associated with each other in vivo via disulfide bonds in an assembly—the Pif complex—that also includes N16/pearlin, a previously identified nacre protein. The Pif complex appears to be primed for a role in nacre formation: N16/pearlin can induce aragonite formation in vitro, Pif80 binds to aragonite, and Pif97 contains a chitin-binding domain that allows for attachment of the entire complex to the chitin-containing organic framework. Indeed, when adsorbed to a chitin-coated glass slide, the Pif complex induced the formation of aragonite crystals with uniform orientation of the crystal c axis, akin to natural nacre...Previously, the protein Starmaker has been shown to be essential for the formation of the layered aragonite structure of otolith biominerals in zebrafish. Both this protein and Pif80 and Pif97 are rich in the amino acid aspartate. Indeed, aspartate-rich proteins appear to be a common tool of biomineral-forming organisms, irrespective of the chemistry of the mineral phase. They are involved in the formation of calcium phosphate biominerals of bone and teeth and the amorphous silica cell walls of diatoms." (Kroger 2009:1351-1352).

"The mollusk shell is a hard tissue consisting of calcium carbonate crystals and an organic matrix. The nacre of the shell is characterized by a stacked compartment structure with a uniformly oriented c axis of aragonite crystals in each compartment. Using a calcium carbonate–binding assay, we identified an acidic matrix protein, Pif, in the pearl oyster Pinctada fucata that specifically binds to aragonite crystals...Both Pif 80 and Pif 97 are acidic with calculated isoelectric point (pI) values of 4.99 and 4.65, respectively. Pif 97 consists of 525 amino acid residues and has two conserved domains, a von Willebrand type A (VWA) for protein-protein interaction domain and a chitin-binding domain similar to that of Peritrophin A. Pif 97 contains a high proportion of charged amino acid residues, Asp (14.9%), Glu (6.5%), Lys (11.1%), and Arg (5.0%), and many Cys residues (23 residues) that might form disulfide bridges to confer a rigid three-dimensional conformation. In contrast, Pif 80 consists of 460 amino acid residues and has no conserved domains. Pif 80 has more charged amino acid residues [Asp (28.5%), Glu (4.1%), Lys (18.7%), and Arg (10.9%)] than does Pif 97. Pif 80 has 17 repeats of a four-amino-acid motif, Asp-Asp-Arg (Lys)–Lys (Arg), scattered throughout its sequence and a cluster of acidic amino acid residues (Asp2-Glu-Asp7) near the center of the molecule. The high ratio of Asp in Pif 80 may play a role in aragonite-binding, considering that acidic amino acid residues are associated with the regulation of crystal polymorph and that poly-(Asp) has been shown to induce aragonite formation." (Suzuki et al. 2009:1388).

Journal article
An Acidic Matrix Protein, Pif, Is a Key Macromolecule for Nacre FormationScienceAugust 14, 2009
M. Suzuki, K. Saruwatari, T. Kogure, Y. Yamamoto, T. Nishimura, T. Kato, H. Nagasawa

Journal article
The molecular basis of nacre formation

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Black-lip Pearl ShellPinctadaSpecies

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