Identification and Molecular Characterization of Hc-Upsalin, a Novel Matrix Protein Involved in Nacreous-Layer Biomineralization in Hyriopsis cumingii

Biomacromolecules, including matrix proteins, are important in shell-mineralization processes, such as inducing crystal nucleation, regulating crystal polymorphism and morphology. In this study, we identified the novel matrix protein hc-upsalin from the freshwater pearl mussel Hyriopsis cumingii, associated with its significantly high similarity to the known matrix protein upsalin in the freshwater mussel Unio pictorum. The full-length cDNA of hc-upsalin encodes 119 amino acids, including a potential signal peptide of 17 residues. The mature protein is characterized by high proportions of Gly (13.7%), Cys (11.8%), and Pro (11.8%), with a molecular weight of 11.4kDa and a theoretical pI of 6.93. Hc-upsalin is specifically expressed in muscular tissues mostly in the mantle, with positive signals detected in dorsal epithelial cells of the mantle by in situ hybridization, and decreased expression of hc-upsalin leaded to random accumulation of tablet, suggesting its involvement in shell nacreous-layer biomineralization. Furthermore, hc-upsalin gene-expression patterns during pearl biomineralization indicated a potential role in the process of pearl nacreous-layer calcification.