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Characterization of the Teeth Skeletal Matrix from Arbacia lixula

The teeth of sea urchins are highly complex composite structures, composed predominantly of high magnesium calcite, and of a minor heterogeneous assemblage of organic macromolecules that are occluded within the mineral. The organic matrix fulfils important functions in mineralization, in addition to giving the mineral phase peculiar mechanical properties, different from that of purely inorganic calcite. Nevertheless, the composition and function of individual components of the organic matrix still remains largely unknown. Up to now, the detailed protein repertoire of teeth from a single sea urchin species (Strongylocentrotus purpuratus, order Camarodonta) was investigated. In this study, we characterized for the first time the teeth skeletal matrix of another sea urchin, Arbacia lixula (order Arbacioida). The acetic acid soluble and acetic acid insoluble matrices, namely ASM and AIM respectively, were extracted and characterized with different biochemical methods including mono-dimensional SDS-PAGE, FT-IR spectroscopy, HPAE-PAD for monosaccharide analysis, and finally, proteomics. In spite of the paucity of peptide data, several of them displayed a high abundance of hydrophobic residues, i.e., alanine, glycine and valine, and of the apolar proline. We assert that the alanine- and proline-rich domains are important features of some of the matrix proteins associated to the teeth of sea urchins. None of the known skeletal matrix proteins from S. purpuratus teeth were identified in the organic matrix of A. lixula teeth. This might suggest major differences in teeth matrix protein repertoires of these two species belonging to orders that diverged in the Mesozoic times.