Role of Pif97 in Nacre Biomineralization: In Vitro Characterization of Recombinant Pif97 as a Framework Protein for the Association of Organic–Inorganic Layers in Nacre

Abstract

Nacre is the inner layer of the mollusc shell and provides exceptional toughness via its highly organized organic–inorganic composite structure. Pif is an organic matrix protein from the nacreous layer of the pearl oyster Pinctada fucata and exhibits regulatory behavior in nacre formation. Here, we investigated features of Pif97, the N-terminal of Pif, using a recombinant form of Pif97 produced in Escherichia coli . We observed that recombinant Pif97 was able to efficiently form a complex with calcium ions. Additionally, recombinant Pif97 showed both in vitro growth inhibition of thermodynamically stable calcite, stabilization of amorphous calcium carbonate, and exclusive binding affinity to metastable aragonite and chitin. These results imply the participation of Pif97 in the calcification of nacre including the association of the inorganic phase and polysaccharide template. We propose that recombinant Pif97 has inherent characteristics of the native form, which are significant for interrelating with organic matrix and inorganic calcium carbonate during nacre biomineralization.