Microstructure and in-depth proteomic analysis of Perna viridis shell

For understanding the structural characteristics and the proteome of Perna shell, the microstructure, polymorph, and protein composition of the adult Perna viridis shell were investigated. The P. viridis shell have two distinct mineral layers, myostracum and nacre, with the same calcium carbonate polymorph of aragonite, determined by scanning electron microscope, Fourier transform infrared spectroscopy, and x-ray crystalline diffraction. Using Illumina sequencing, the mantle transcriptome of P. viridis was investigated and a total of 69,859 unigenes was generated. Using a combined proteomic/transcriptomic approach, a total of 378 shell proteins from P. viridis shell were identified, in which, 132 shell proteins identified with more than two matched unique peptides. Of the 132 shell proteins, 69 are exclusive to the nacre, 12 to the myostracum, and 51 are shared by both. The Myosin-tail domain containing proteins, Filament-like proteins, and Chitin-binding domain containing proteins represent the most abundant molecules. In addition, the shell matrix proteins (SMPs) containing biomineralization-related domains, such as Kunitz, A2M, WAP, EF-hand, PDZ, VWA, Collagen domain, and low complexity regions with abundant certain amino acids, were also identified from P. viridis shell. Collagenase and chitinase degradation can significantly change the morphology of the shell, indicating the important roles of collagen and chitin in the shell formation and the muscle-shell attachment. Our results present for the first time the proteome of P. viridis shell and increase the knowledge of SMPs in this genus.