The nacreous layer of molluscan shells consists of a highly organised, layered structure comprising calcium carbonate aragonite crystals, each surrounded by an organic matrix. In the Japanese pearl oyster Pinctada fucata, the Pif protein from the nacreous layer functions in aragonite binding, and plays a key role in nacre formation. Here, we investigated whether the blue mussel Mytilus galloprovincialis also has a protein with similar functions in the nacreous layer. By using a calcium carbonate-binding assay, we identified the novel protein blue mussel shell protein (BMSP) 1 that can bind calcium carbonate crystals of both aragonite and calcite. When the entire sequence of a cDNA encoding BMSP 1 was determined, it was found that BMSP is a preproprotein consisting of a signal peptide and two proteins, BMSP 12 and BMSP 1. BMSP 12 contains four von Willebrand factor A (VWA) domains and one chitin-binding domain, thus suggesting that it has a role in maintaining structure within the matrix. Immunohistochemical analysis revealed that BMSP 1 is present throughout the nacreous layer with dense localisation in the myostracum. Posttranslational modification analysis indicated that BMSP 1 is phosphorylated and glycosylated. These results suggest that there is a common molecular mechanism between P. fucata and M. galloprovincialis that underlies the nacreous layer formation.Common proteins in nacre: We have identified and characterised a protein (BMSP) from the blue mussel, Mytilus galloprovincialis. BMSP is similar to Pif, which plays a role in nacre formation in the Japanese pearl oyster. Both proteins contain a signal peptide (pink), VWA domains (blue) and a Peritrophin A-type chitin-binding domain (yellow), suggesting a common mechanism for nacre formation in these molluscs.
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