The structure-function relationship of MSI7, a matrix protein from pearl oyster Pinctada fucata

We previously identified a matrix protein, MSI7, from pearl oyster Pinctada fucata. According to the struc-tural analysis, the DGD site in the N-terminal of MSI7 is crucial for its role in the shell formation. In this study, we expressed a series of recombinant MSI7 pro-teins, including the wild-type and several mutants directed at the DGD site, using an Escherichia coli expression system to reveal the structure–function relationship of MSI7. Furthermore, in vitro crystalliza-tion, crystallization speed assay, and circular dichroism spectrometry were carried out. Results indicated that wild-type MSI7 could induce the nucleation of arago-nite and inhibit the crystallization of calcite. However, none of the mutants could induce the nucleation of ara-gonite, but all of them could inhibit the crystallization of calcite to some extent. And all the proteins acceler-ated the crystallization process. Taken together, the results indicated that MSI7 could contribute to arago-nite crystallization by inducing the nucleation of arago-nite and inhibiting the crystallization of calcite, which agrees with our prediction about its role in the nacr-eous layer formation of the shell. The DGD site was critical for the induction of the nucleation of aragonite