Abstract
BackgroundThe formation of the molluscan shell is regulated to a large extent by a matrix of extracellular macromolecules that are secreted by the shell forming tissue, the mantle. This so called "calcifying matrix" is a complex mixture of proteins and glycoproteins that is assembled and occluded within the mineral phase during the calcification process. While the importance of the calcifying matrix to shell formation has long been appreciated, most of its protein components remain uncharacterised.ResultsRecent expressed sequence tag (EST) investigations of the mantle tissue from the tropical abalone (Haliotis asinina) provide an opportunity to further characterise the proteins in the shell by a proteomic approach. In this study, we have identified a total of 14 proteins from distinct calcified layers of the shell. Only two of these proteins have been previously characterised from abalone shells. Among the novel proteins are several glutamine- and methionine-rich motifs and hydrophobic glycine-, alanine- and acidic aspartate-rich domains. In addition, two of the new proteins contained Kunitz-like and WAP (whey acidic protein) protease inhibitor domains.ConclusionThis is one of the first comprehensive proteomic study of a molluscan shell, and should provide a platform for further characterization of matrix protein functions and interactions.
Highlights
The formation of the molluscan shell is regulated to a large extent by a matrix of extracellular macromolecules that are secreted by the shell forming tissue, the mantle
We identified Has-Sometsuke which is thought to be involved in pigmenting the outer periostracum of H. asinina [8,9] but apparently plays other shell forming roles, while HasCL10contig2 was identified from a mantle expressed sequence tag (EST) study but was not characterised in anyway [10]
Nacre consists of the columnar superimposition of 0.5 μm thick aragonitic tablets, embedded within a peripheral thin organic matrix (Figure 1D-F)
Summary
The formation of the molluscan shell is regulated to a large extent by a matrix of extracellular macromolecules that are secreted by the shell forming tissue, the mantle. The wide morphological diversity of shell-bearing molluscs (bivalves, gastropods, cephalopods, monoplacophorans and scaphopods) extends to a tremendous diversity of shell micro-textures Despite this diversity, molluscan shells are produced by an evolutionarily homologous structure known as the mantle. The polymorph of CaCO3 (primarily aragonite or calcite), along with all other nano-scale features of the biomineral, are thought to be determined and regulated by an extracellular ‘cellfree’ matrix that is secreted by the mantle. This matrix is incorporated into and surrounds nascent CaCO3 crystals during shell growth. Even though it constitutes only a small part of the total shell weight (1-5%), this matrix is clearly essential for initiating biomineral formation
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