In vitro regulation of CaCO₃ crystal polymorphism by the highly acidic molluscan shell protein Aspein

Abstract 

Biominerals, especially molluscan shells, generally contain unusually acidic proteins. These proteins are believed to function in crystal nucleation and inhibition. We previously identified an unusually acidic protein Aspein from the pearl oyster Pinctada fucata. Here we show that Aspein can control the CaCO₃ polymorph (calcite/aragonite) in vitro. While aragonite is preferentially formed in Mg²⁺-rich solutions imitating the extrapallial fluids of marine molluscs, Aspein exclusively induced calcite precipitation. Our results suggest that Aspein is involved in the specific calcite formation in the prismatic layer. Experiments using truncated Aspein demonstrated that the aspartic acid rich domain is crucial for the calcite precipitation.

Abbreviations: BCA, bicinchoninicacid, EDTA, ethylenediaminetetraacetic acid, GST, glutathione S-transferase, IPTG, isopropyl-β-d-thiogalactopyranoside, SEM, scanning electron microscope

Keywords: Acidic protein, Aspein, Biomineralization, Calcite, Crystal polymorphism, In vitro crystallization, Mollusc shell, Pearl oyster, Prismatic layer, Pinctada fucata