Alv Protein Plays Opposite Roles in the Transition of Amorphous Calcium Carbonate to Calcite and Aragonite during Shell Formation

Amorphous calcium carbonate (ACC) is an important precursor in biominerals such as shells, coral, foraminiferans, and urchin spine. However, the mechanism underlying the transition from ACC to stable biosynthetic crystals is still poorly understood. Herein, we identified a matrix protein referred to as Alv in Pinctada fucata, which has dramatically opposite functions during the different transition processes from ACC to stable crystals-calcite and aragonite in shell formation. The functions of Alv were studied by RNA interference, binding of recombinant Alv (rAlv) to chitin, calcite and aragonite assay, ACC transition, in vitro crystallization, calcium carbonate precipitation, and near-UV CD spectra. We found that rAlv could promote nucleation during ACC crystallization, stimulate the transition from ACC to calcite, but suppress transition from ACC to aragonite. It is concluded that Alv is involved in the transition of ACC, and plays a crucial role in the formation of shells. As far as we know, Alv is one of the few reported matrix proteins which play opposite roles in the transition of ACC to calcite and aragonite both in vivo and in vitro. This study could further enhance our understanding of the important regulatory role of biomacromolecules in biomineralization.