A novel extracellular EF-hand protein involved in the shell formation of pearl oyster

Mollusk shell formation is a complicated and highly controlled calcium metabolism process. Previous studies revealed that several EF-hand calcium-binding proteins actively participate in the regulation of shell mineralization. In this study, we cloned a full-length cDNA encoding a novel extracellular EF-hand calcium-binding protein (named EFCBP) from the pearl oyster, Pinctada fucata, according to the EF-hand motifs of calmodulin. Although it shares high similarity with the calmodulin family in its EF-hand signatures, EFCBP just has two EF-hand motifs and belongs to a new separate group from the other EF-hand proteins according to a phylogenetic analysis. EFCBP is specifically expressed in shell mineralization-related tissues, viz. the mantle, the gill, and the hemocytes. Moreover, its expression responds quickly only to the shell damage, but not to the damage of other tissues and the infection of the lipopolysaccharides from Escherichia coli. These results suggest that EFCBP might be an important regulator of shell formation. This finding may help better understand the functions of EF-hand proteins on the regulation of mollusk shell formation.