The formation of the molluscan shell nacre 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, glycoproteins and polysaccharides that is assembled and occluded within the mineral phase during the calcification process. Better molecular-level characterization of the substances that regulate nacre formation is still required. Notable advances in expressed tag sequencing of freshwater mussels, such as Elliptio complanata and Villosa lienosa, provide a pre-requisite to further characterize bivalve nacre proteins by a proteomic approach. In this study, we have identified a total of 48 different proteins from the insoluble matrices of the nacre, 31 of which are common to both E. complanata and V. lienosa. A few of these proteins, such as PIF, MSI60, CA, shematrin-like, Kunitz-like, LamG, chitin-binding-containing proteins, together with A-, D-, G-, M- and Q-rich proteins, appear to be analogues, if not true homologues, of proteins previously described from the pearl oyster or the edible mussel nacre matrices, thus forming a remarkable list of deeply conserved nacre proteins. This work constitutes a comprehensive nacre proteomic study of non-pteriomorphid bivalves that has enabled us to describe the molecular basis of a deeply conserved biomineralization toolkit among nacreous shell-bearing bivalves, with regard to proteins associated with other shell microstructures, with those of other mollusc classes (gastropods, cephalopods) and, finally, with other lophotrochozoans (brachiopods).
Electronic supplementary material is available online at https://dx.doi.org/10.6084/m9.figshare.c.3655688.
- Received October 21, 2016.
- Accepted December 20, 2016.
- © 2017 The Author(s)
Published by the Royal Society. All rights reserved.