Shell Matrix Protein N38 of Pinctada fucata, Inducing Vaterite Formation, Extends the DING Protein to the Mollusca World.

In the animal kingdom, DING proteins were only found in Chordata and Aschelminthes. At present study, a potential DING protein, matrix protein N38, was isolated and purified from the shell of Pinctada fucata. Tandem mass spectrometry analysis revealed that 14 peptide segments matched between N38 and human phosphate-binding protein (HPBP). HPBP belongs to the DING protein family and has a "DINGGG-" sequence, which is considered a "signature" of HPBP. In this study, the mass spectrometry analysis results showed that N38 had a "DIDGGG-" sequence; this structure is a mutation from the "DINGGG-" structure, which is a distinctive feature of the DING protein family. The role of N38 during calcium carbonate formation was explored through the in vitro crystallization experiment. The results of scanning electron microscopy and Raman spectrum analysis indicated that N38 induced vaterite formation. These findings revealed that N38 might regulate and participate in the precise control of the crystal growth of the shell, providing new clues for biomineralization mechanisms in P. fucata and DING protein family studies. In addition, this study helped extend the research of DING protein to the Mollusca world.