Insights from the Shell Proteome: Biomineralization to Adaptation

Abstract

Bivalves have evolved a range of complex shell forming mechanisms that are reflected by their incredible diversity in shell mineralogy and microstructures. A suite of proteins exported to the shell matrix space plays a significant role in controlling these features, in addition to underpinning some of the physical properties of the shell itself. Although, there is a general consensus that a minimum basic protein tool kit is required for shell construction, to date, this remains undefined. In this study, the shell matrix proteins (SMPs) of four highly divergent bivalves (The Pacific oyster, Crassostrea gigas; the blue mussel, Mytilus edulis; the clam, Mya truncata, and the king scallop, Pecten maximus) were analyzed in an identical fashion using proteomics pipeline. This enabled us to identify the critical elements of a “basic tool kit” for calcification processes, which were conserved across the taxa irrespective of the shell morphology and arrangement of the crystal surfaces. In addition, protein domains controlling the crystal layers specific to aragonite and calcite were also identified. Intriguingly, a significant number of the identified SMPs contained domains related to immune functions. These were often are unique to each species implying their involvement not only in immunity, but also environmental adaptation. This suggests that the SMPs are selectively exported in a complex mix to endow the shell with both mechanical protection and biochemical defense.

Identification of conserved proteins from diverse shell matrix proteome in Crassostrea gigas: characterization of genetic bases regulating shell formation

“…The mineralogy and shell microstructure of these species may account for that differences. The C. gigas shell is purely calcite: the outer shell layers are calcite prisms, the middle shell layers are chalky calcite and the inner shell layer are foliated calcite18. In contrast, M. truncata shell is composed only of aragonite.…”

“…Collectively, comparison of the shell proteomes for sequence similarity gives some clues about the relationships between evolution and sequence conservation. However, it has been suggested that the similarity in SMPs depends not only on the evolutionary distance but is also influenced by mineralogy of shell, parallel evolution, adaptation to the environment etc18.…”

“…In C. gigas , a SMPE consisting of 53 proteins was recently published18. The SMPE was constructed from peptide fragments in shell, which can be mapped to the genome.…”

“…Recently, high-throughput transcriptomics and proteomics analyses have been successively used to characterize shell matrix proteomes (SMPEs) in Haliotis asinina 8, Pinctada margaritifera 12, Pinctada maxima 12, Crassostrea gigas 13, Lottia gigantea 14, Cepaea nemoralis 6, Mytilus coruscus 15, Pinctada. fucata 10, Magellania venosa 2, Mytilus galloprovincialis 16, Mya truncata 17, and C. gigas, Mytilus edulis , and Pecten maximus 18. However, only two global SMPEs comparisons have been performed in C. nemoralis and M. venosa , revealing low similarities.…”

“…A previous study of SMPE consisting of 259 SMPs (SMPE-Cg1) suggested that the matrix model may not apply to C. gigas because the absence of silk-like proteins13. Recently, a SMPE of C. gigas consisting of 53 SMPs (SMPE-Cg2) was published18, providing new opportunities for comprehensive comparison and characterization of SMPs in marine molluscan.…”