Proteomics of Sternaspis chinensis to understand phosphate biomineralization in annelids

Organisms with phosphate hard tissues contribute to the evolutionary history of life and global biogeochemical cycles. The annelid Sternaspidae is an important group of phosphate-mineralizing organisms, known for its unique amorphous phosphate ventro-caudal shield. However, the molecular mechanisms underlying the formation of their phosphate shields remain unclear. In this study, we identified shield matrix proteins (ShiMPs) from Sternaspis chinensis (SC group with hard shield) and Sternaspis liui syn (SL group with soft shield) using liquid chromatography-tandem mass spectrometry (LC-MS/MS). A total of 1360 ShiMPs were identified, with 237 differentially expressed proteins between the two morphologies. The differential expression of collagen, extracellular matrix proteins with new EGF-VWA domains, and calcium-binding proteins in different morphological shields suggested that they may be associated with the variation in shield phenotypes. Furthermore, the phenoloxidase innate immune cascade pathway was found to be upregulated in the SL group. Compared with other biomineral proteins, sternaspid shields lacked typical chitin-related proteins but were rich in collagen proteins containing EGF domains, suggesting that collagen may serve as a scaffold for amorphous phosphate deposition. This study provided a molecular dataset for the formation of sternaspid shields and revealed the molecular regulatory features of different shield morphologies. SIGNIFICANCE: Phosphate is one of the important types of biominerals in invertebrates. However, the molecular mechanisms underlying its formation in these organisms remain poorly understood, and no studies have yet explored phosphate biomineralization in annelids. In this study, we performed proteomic analysis of matrix proteins in the phosphate shields of the annelid Sternaspidae to provide new insights into phosphate biomineralization in this group. Comparative analysis of the matrix proteins in different morphological shields from Sternaspis chinensis revealed the regulation of collagen, extracellular matrix proteins, calcium-binding proteins, and immune-related proteins. By comparing with other biominerals, we proposed that collagen may serve as a scaffold for amorphous phosphate deposition in sternaspid shields. This study advances our understanding of phosphate biomineralization in annelids and provides a foundation for future research on the formation mechanisms of phosphate biominerals in invertebrates.