Proteomic characterization of oyster shell organic matrix proteins

Abstract

Oysters are economically (aquaculture) and ecologically important bivalves, with a calcareous shell that offers mechanical strength and protects them from predators. The shell composition is a blend of inorganic crystals and shell proteins that form an organic matrix which encloses and protects the soft inner tissue of the oyster. It has been found that the mechanical properties of shells of different oyster species vary. The objective of the present study was to compare the composition of shell matrix proteins of two phylogenetically related species: the Hong Kong oyster (C rassostrea hongkongensis ) and the Portuguese oyster (C rassostrea angulata ) which differ in their shell hardness and mechanical properties. Hong Kong oyster shells are comparatively stronger than Portuguese oysters. In the present study, modern proteomics and bioinformatics tools have been used to understand the nature of the Organic Matrix Proteins (OMP) and the possible mechanism involved in the variations observed in the mechanical properties of these two species of oyster shells. After visualizing proteins on SDSPAGE (1DE) and twodimensional electrophoresis (2DE) gels, the protein spots and their intensities were compared using PDQuest software and fourteen proteins of C . hongkongensis were found to be significantly different (student’s ttest; p<0.05) when compared to the C . angulata . Furthermore, shell OMP separated on 1DE gels were processed using Triple TOF5600 mass spectrometry and 42 proteins of C . hongkongensis and 37 of C . angulata identified. A Circos based comparative analysis of the shell proteins of both oyster species were prepared and compared against the shell proteome of other shell forming gastropods, molluscs and the proteome database of the Pacific oyster (C . gigas ), whose genome has recently been sequenced, to study the evolutionary conservation of OMP and their function. This pioneering comparative proteomics of the two closely related oyster species, yet with remarkably different mechanical properties, expanded our understating of the molecular mechanism behind the naturally occurring shells of Hong Kong and Portuguese oysters having different hardness and mechanical properties. From biomaterial perspectives, knowledge gained through this study will be useful in biomedical research fields to develop novel biomaterials. For example, OMP identified exclusively in the Hong Kong oysters could be a useful resource for crystallizing a much harder biomaterial.