A Basic Matrix Protein Songin Selectively Promotes Aragonite Aggregation and Modulates Calcium Carbonate Crystallization in Hyriopsis cumingii

Abstract In China, the pearl industry of Hyriopsis cumingii contributes approximately 80% of pearl production. Matrix proteins play crucial roles in shell and pearl biomineralization. However, the functions of basic matrix proteins in H. cumingii remain poorly understood. In this study, we identified a novel predicted secreted basic matrix protein, designated Songin, from the freshwater pearl mussel H. cumingii. Functional assays were performed using a chemically synthesized 53-amino-acid peptide corresponding to the predicted mature region of Songin. Songin transcripts showed a relative mantle-enriched expression pattern and were detected in dorsal mantle epithelial cells, suggesting a potential association with shell biomineralization. In the end point crystallization assay, Songin was associated with pronounced aggregation of mineral particles under Mg2+-containing aragonite-favoring conditions while exerting only minor effects on particle morphology in the calcite-favoring system. Raman spectroscopy supported the predominant polymorph assignments in the calcite-favoring and Mg2+-containing aragonite-favoring systems, and no obvious Songin-associated change in the predominant polymorph was detected under the tested conditions. Laser scanning confocal microscopy showed heterogeneous Cy5-associated fluorescence near localized regions of some mineral particles. Because free Cy5 and Cy5-labeled BSA controls were not included, nonspecific dye adsorption cannot be excluded, and these observations are regarded as preliminary qualitative information rather than evidence of specific crystal-surface binding. This study indicates that the synthetic mature-region Songin peptide was associated with pronounced mineral-particle aggregation under Mg2+-containing aragonite-favoring conditions in vitro, suggesting a potential role in mineral-particle assembly during shell biomineralization. These findings provide preliminary insight into the possible involvement of Songin in shell biomineralization.