Insights from shell proteome : Biomineralization control and environmental adaptation in bivalves

Biomineral formation is widely found in organisms ranging from unicellular foraminifera to biggest animal that exist in biosphere. The range and complexity of biominerals formed by the organisms are mediated by biological control. Bivalves are one among several organisms that form wide array of complex composite calcium carbonate biomineral structures via organic-inorganic interactions. Shell matrix protein (SMP), the important constituent of the organic components plays a significant role in CaCO3 nucleation, crystal growth and in generating different crystal polymorphs.

In this study, the SMPs from four commercially important and divergent bivalve species crassostrea gigas (pacific oyster), Mya truncata (soft shell clam), Mytilus edulis (blue mussel) and Pecten maximus (king scallop) were extracted and analyzed using standardized extraction protocol and proteomic pipeline. This enabled us to identify critical elements of basic biomineralization tool kit for calcification process irrespective of their shell morphology, mineralogy and microstructure.  In addition, it enabled the identification of SMPs that are specific to calcite and aragonite mineralogies. The significant numbers of SMPs found species-specific were hypothesized as adaptation to their modus vivendi. In fact, the latter proteins possess immunity-related functions and fit in to specific pathway, phenoloxidase, suggesting their role in defense against pathogen.

The comparative study of shell proteome of mussels living in full marine condition, North Sea and the low saline Baltic Sea using label free semi-quantitative proteomic approach showed the modulation of the SMPs that constitute the basic biomineralization tool kit. Higher modulation of chitin related proteins and non-modulated proteins such as carbonic anhydrase, EGF and fibronectin domain containing proteins points out the impaired scaffold and mineral nucleation in Baltic mussel. The modulation of immunity related proteins denote the influence of biotic components in the shell formation. These findings implicate that the successful adaptation of mussels to their environment is possible with the modulation of SMPs and, which results in shell phenotypes with distinct mechanical and biochemical defense properties.

These investigations show the functional diversity of SMPs and their roles beyond shell formation in the bivalves. In addition, the identification of immunity related proteins and phosphoproteins reveal the complexity of shell formation process and questions about their role during shell formation and their possible association in the biomineralizing matrix proteins. These findings put forth the idea that shell is dynamic, endowed with both mechanical and biochemical protection.

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