A novel matrix protein PNU5 facilitates the transformation from amorphous calcium carbonate to calcite and aragonite.

For both nacre formation and biomineralization in mollusks, understanding the molecular mechanism is imperative. Biomineralization, especially shell formation, is dedicatedly regulated by multiple matrix proteins. However, ACC conversion to stable crystals still lacks positive factors. In this research, we found a novel matrix protein named PNU5 in Pinctada fucata that plays a regulatory role in both prismatic layer and nacreous layer formation. Functional studies in vivo and in vitro have shown that it might be involved in shell formation in a positive manner. RT-qPCR analysis showed that pnu5 was highly expressed in mantle pallial and participated in shell repairing and regeneration. RNAi-mediated repression of pnu5 could affect the normal structure of prismatic layer and nacreous layer. The recombinant protein rPNU5 significantly enhanced the precipitation rate of CaCO3 both in the calcite and aragonite crystallization systems, as well as altering the morphology of the crystals. Based on ACC transition experiments, the recombinant protein rPNU5 facilitated amorphous calcium carbonate (ACC) transformation into stable calcite or aragonite. This study could provide us with a better understanding of how positive regulatory mechanisms contribute to biomineralization.