Nautilin-63, a novel acidic glycoprotein from the shell nacre of Nautilusmacromphalus

In molluscs, and more generally in metazoan organisms, the production of a calcified skeleton is a complex molecular process that is regulated by the secretion of an extracellular organic matrix. This matrix constitutes a cohesive and functional macromolecular assemblage, containing mainly proteins, glycoproteins and polysaccharides that, together, control the biomineral formation. These macromolecules interact with the extruded precursor mineral ions, mainly calcium and bicarbonate, to form complex organo-mineral composites of well-defined microstructures. For several reasons related to its remarkable mechanical properties and to its high value in jewelry, nacre is by far the most studied molluscan shell microstructure and constitutes a key model in biomineralization research. To understand the molecular mechanism that controls the formation of the shell nacreous layer, we have investigated the biochemistry of Nautilin-63, one of the main nacre matrix proteins of the cephalopod Nautilusmacromphalus. After purification of Nautilin-63 by preparative electrophoresis, we demonstrate that this soluble protein is glycine-aspartate-rich, that it is highly glycosylated, that its sugar moieties are acidic, and that it is able to bind chitin invitro. Interestingly, Nautilin-63 strongly interacts with the morphology of CaCO3 crystals precipitated invitro but, unexpectedly, it exhibits an extremely weak ability to inhibit invitro the precipitation of CaCO3. The partial resolution of its amino acid sequence by denovo sequencing of its tryptic peptides indicates that Nautilin-63 exhibits short collagenous-like domains. Owing to specific polyclonal antibodies raised against the purified protein, Nautilin-63 was immunolocalized mainly in the intertabular nacre matrix. In conclusion, Nautilin-63 exhibits 'hybrid' biochemical properties that are found both in the soluble and insoluble proteins, rendering it difficult to classify according to the standard view on nacre proteins. Database The protein sequences of N63appear on the UniProt Knowledgebase under accession number . To understand the molecular mechanism that controls the formation of the shell nacreous layer, we have investigated here the biochemistry of Nautilin-63, one of the main nacre matrix proteins of the cephalopod Nautilusmacromphalus. In conclusion, Nautilin-63 exhibits 'hybrid' biochemical properties, found both in soluble and insoluble proteins, a fact that renders it difficult to classify according to t