Evolution of nacre: biochemistry and proteomics of the shell organic matrix of the cephalopod Nautilus macromphalus.

In mollusks, one of the most widely studied shell textures is nacre, the lustrous aragonitic layer that constitutes the internal components of the shells of several bivalves, a few gastropods,and one cephalopod: the nautilus. Nacre contains a minor organic fraction, which displays a wide range of functions in relation to the biomineralization process. Here, we have biochemically characterized the nacre matrix of the cephalopod Nautilus macromphalus. The acid-soluble matrix contains a mixture of polydisperse and discrete proteins and glycoproteins, which interact with the formation of calcite crystals. In addition, a few bind calcium ions. Furthermore, we have used a proteomic approach,which was applied to the acetic acid-soluble and -insoluble shell matrices, as well as to spots obtained after 2D gel electrophoresis. Our data demonstrate that the insoluble and soluble matrices, although different in their bulk monosaccharide and amino acid compositions, contain numerous shared peptides. Strikingly, most of the obtained partial sequences are entirely new. A few only partly match with bivalvian nacre proteins.Our findings have implications for knowledge of the long-term evolution of molluskan nacre matrices.