Transcriptome and proteome analysis revealed tyrosine oxidation involved in the periostracum formation of the green mussel Perna viridis (Linnaeus)

Mollusk secretes a periostracum layer prior to the underlying calcified shell. This organic membrane serves as the first line of protection and primary template for shell orchestration. However, the chemical composition and formation mechanism of the periostracum layer is largely unknown. In this study, we applied transcriptomic, proteomics, physical and chemical analysis to unravel the mysteries of the periostracum formation in the green mussel Perna viridis (Linnaeus). Scanning electron microscopy examination and FTIR analysis showed that the periostracum layer was a multilayered organic membrane composed of polysaccharides, lipids and proteins. Interestingly, proteomic study identified components enriched in tyrosine and some enzymes evolved in tyrosine oxidation, indicating that tyrosine oxidation might play an important role in the periostracum formation. Moreover, comparative transcriptomics suggested that tyrosine-rich proteins were intensively synthesize in the periostracum groove. After being secreted, the periostracum proteins were gradually tanned by oxidation in the sea water, and the level of crosslink increased significantly as revealed by the ATR-FTIR. Our present study sheds light on the chemical composition and putative tanning mechanism of the periostracum layer in bivalve mollusk. Mature and healthy mussel individuals were collected from the coast of Zhuhai city in China. Animals with shell length around 6-7 cm were selected for subsequent analysis. The shell valves were opened by cutting the adductor muscles with a scalpel. Tissues of the periostracum groove (PG) and submarginal zone (SZ) were cut and placed in 1.5 mL Eppendorf tube. Each sample contained tissues from six animals, and three biological replicates were set in the RNA- seq.