In-depth proteomic analyses of (greenlip abalone) nacre and prismatic organic shell matrix.

Background: The shells of various species have served as models of invertebrate biomineralization and physical shell properties for more than 20 years. A focus of this research has been the nacreous inner layer of the shell with its conspicuous arrangement of aragonite platelets, resembling in cross-section a brick-and-mortar wall. In comparison, the outer, less stable, calcitic prismatic layer has received much less attention. One of the first molluscan shell proteins to be characterized at the molecular level was Lustrin A, a component of the nacreous organic matrix of . This was soon followed by the C-type lectin perlucin and the growth factor-binding perlustrin, both isolated from nacre, and the crystal growth-modulating AP7 and AP24, isolated from nacre. Mass spectrometry-based proteomics was subsequently applied to to biomineralization research with the analysis of the shell matrix and yielded 14 different shell-associated proteins. That study was the most comprehensive for a species to date.

Methods: The shell proteomes of nacre and prismatic layer of the marine gastropod were analyzed combining mass spectrometry-based proteomics and next generation sequencing.

Results: We identified 297 proteins from the nacreous shell layer and 350 proteins from the prismatic shell layer from the green lip abalone . Considering the overlap between the two sets we identified a total of 448 proteins. Fifty-one nacre proteins and 43 prismatic layer proteins were defined as major proteins based on their abundance at more than 0.2% of the total. The remaining proteins occurred at low abundance and may not play any significant role in shell fabrication. The overlap of major proteins between the two shell layers was 17, amounting to a total of 77 major proteins.

Conclusions: The shell proteome shares moderate sequence similarity at the protein level with other gastropod, bivalve and more distantly related invertebrate biomineralising proteomes. Features conserved in and other molluscan shell proteomes include short repetitive sequences of low complexity predicted to lack intrinsic three-dimensional structure, and domains such as tyrosinase, chitin-binding, and carbonic anhydrase. This catalogue of shell proteins represents the most comprehensive for a haliotid and should support future efforts to elucidate the molecular mechanisms of shell assembly.