Layer-by-Layer Proteomic Analysis of Mytilus galloprovincialis Shell.

Bivalve shell is a biomineralized tissue with various layers/microstructures and excellent mechanical properties. Shell matrix proteins (SMPs) pervade and envelop the mineral crystals and play essential roles in biomineralization. Despite that Mytilus is an economically important bivalve, only few proteomic studies have been performed for the shell, and current knowledge of the SMP set responsible for different shell layers of Mytilus remains largely patchy. In this study, we observed that Mytilus galloprovincialis shell contained three layers, including nacre, fibrous prism, and myostracum that is involved in shell-muscle attachment. A parallel proteomic analysis was performed for these three layers. By combining LC-MS/MS analysis with Mytilus EST database interrogations, a whole set of 113 proteins was identified, and the distribution of these proteins in different shell layers followed a mosaic pattern. For each layer, about a half of identified proteins are unique and the others are shared by two or all of three layers. This is the first description of the protein set exclusive to nacre, myostracum, and fibrous prism in Mytilus shell. Moreover, most of identified proteins in the present study are novel SMPs, which greatly extended biomineralization-related protein data of Mytilus. These results are useful, on one hand, for understanding the roles of SMPs in the deposition of different shell layers. On the other hand, the identified protein set of myostracum provides candidates for further exploring the mechanism of adductor muscle-shell attachment.

In-depth proteomic analysis of nacre, prism, and myostracum of Mytilus shell.

Mytilus is an economically important bivalve and its shell is a biomineralized tissue with various microstructures/layers. In the present study, the shell of marine mussel, Mytilus coruscus, was analyzed and three shell layers with different morphologies and polymorphs were observed, which includes nacre, fibrous prism, and myostracum strongly attached by adductor muscles to the interior of the shell surface. In order to understand whether these different shell layers contain different shell matrix proteins (SMPs), the transcriptome sequencing of M. coruscus mantle and a parallel proteomic analysis of SMPs in the three shell layers were performed. A combination of LC-MS/MS analysis with the mantle transcriptome dataset search resulted in the identification of a total of 63 proteins from M. coruscus shell. From this protein set, fifteen, fourteen, and eight proteins were found to be unique to nacre, fibrous prism, and myostracum layers, respectively. In addition, many novel shell proteins were also identified. The data in this study could be used as a background to explore the roles of SMPs in the deposition of different shell layers (nacre vs. fibrous prism vs. myostracum), the different polymorphisms of calcium carbonate (aragonite vs. calcite); and further, the identified proteins from the myostracum could provide candidates for studying the mechanism of adductor muscle-shell attachment. BIOLOGICAL SIGNIFICANCE: In this paper, we characterized for the first time the protein set from different shell layers in Mytilus. Shell matrix proteins are the major component that controls different aspects of the shell formation process and thus a source of bioactive molecules that would offer interesting perspectives in biomaterials and biomedical fields. Our data can be used as a resource for further exploring the roles of shell matrix proteins in the deposition of different shell layers (nacre vs. fibrous prism vs. myostracum) or different polymorphisms of calcium carbonate (aragonite vs. calcite), and the identified protein set of myostracum provided candidates for studying the mechanism of adductor muscle-shell attachment.