A novel bifunctional protein PNU7 in CaCO3 polymorph formation: Vaterite stabilization and surface energy minimization

CaCO3, which occurs in three crystalline anhydrous polymorphs named calcite, aragonite, and vaterite, is always found in mineralized skeletons or growing shells of many marine organisms. However, understanding how these organisms achieve this control has been a significant challenge in biomineralization. In this work, we proposed a novel vaterite stabilizer acidic matrix protein PNU7 that existed in both prism and nacre of Pinctada fucata, and identified its functional domain DDDDDDHDDVEETED. Our experiments reveal that PNU7 triggers a stable large vaterite formation with Mg2+ deficiency even under low Ca2+. Increasing PNU7 in the calcium carbonate crystallization system with Mg2+ leads to a significant shrinking in crystal size and rising in nucleation quantity. Moreover, it converts an atomically rough dome-like shape to a smooth sphere on unsiliconized glass. These effects rescind after removing the asp-rich region at the C-terminus. We also find that decreasing pnu7 mRNA in vivo leads to nacreous inner surface growth substantially lessened. Thus, PNU7 may not only supply vaterite in shell formation but also involve the nacreous regulation via surface energy minimization.

Automated summary

This article investigates a novel vaterite stabilizer acidic matrix protein PNU7, which is found in both the prism and nacre of Pinctada fucata. The study shows that PNU7 can trigger the formation of stable vaterite even with a deficiency in Mg2+ and low Ca2+ levels. Moreover, increasing PNU7 in the calcium carbonate crystallization system leads to a decrease in crystal size and an increase in nucleation quantity. The results suggest that PNU7 not only plays a role in shell formation but also regulates the formation of nacre through surface energy minimization.