| Popis: |
In metazoans, the secretion of skeletal calcium carbonate tissues is regulated by a complex array of extracellular macromolecules, namely proteins and polysaccharides, and sometimes lipids that collectively constitute the skeletal matrix. Because of its ability to ‘sculpt’ the biominerals and to organize spatially crystallites in well-defined microstructures, this matrix has been the focus of several studies. In the last few years, by using proteomics alone or by merging transcriptomics and proteomics, there has been a wealth of information obtained on the protein moieties of the skeletal matrix, in different model and non-model organisms: corals, molluscs, brachiopods, sea urchins. These data allow now establishing preliminary comparisons between the ‘molecular kits’ of different species, from phylum to phylum, and within one given phylum. The obtained data show a remarkable diversity of the proteins involved in mineral deposition, in term of primary structures and of putative functions deduced from in silico sequence analysis. This diversity invites to refine the existing molecular models of mineral deposition and to integrate the role of the calcifying epithelium/calcifying cells facing the front of mineral growth. As illustrated by molluscan nacre on the one hand, and by examples taken from other phyla on the other hand, it appears that ‘similar’ microstructures may emerge from very different protein ‘toolkits’ ; this finding puts into question past attempts to use the matrix for taxonomic purposes. It reveals the astonishing plasticity of the ‘calcifying matrix’ and emphasizes the fact that it is built by the co-option of ancient and recent functions. In a counterintuitive manner, proteomic data suggest that the skeletal matrix is less evolutionary constrained than suspected, exhibits a true ‘evolvability’, while maintaining stable and perennial the associated microstructures. |
