Extracellular tyrosyl-tRNA synthetase cleaved by plasma proteinases and stored in platelet α-granules: Potential role in monocyte activation

Background: Tyrosyl-tRNA synthetase (YRS) belongs to the family of enzymes that catalyzes the tRNA aminoacylation reaction for protein synthesis, and it has been recently shown to exert noncanonical functions. Although database results indicate extremely low levels of YRS mRNA in platelets, YRS protein is abundantly present. The source of YRS in platelets, as well as the physiological role of platelet-stored YRS, remains largely unknown. Objectives To clarify how YRS accumulates in platelets and determine the potential role of platelet-stored YRS. Methods: Recombinant YRS proteins with epitope tags were prepared and tested in vitro for proteolytic cleavage in human plasma. Fluorescent-labeled YRS was examined for uptake by platelets, as demonstrated by western blotting and confocal microscopy analysis. Using RAW-Dual reporter cells, Toll-like receptor and type I interferon activation pathways were analyzed after treatment with YRS. Results: Full-length YRS was cleaved by both elastase and matrix metalloproteinases in the plasma. The cleaved, N-terminal YRS fragment corresponds to the endogenous YRS detected in platelet lysate by western blotting. Both full-length and cleaved forms of YRS were taken up by platelets in vitro and stored in the alpha-granules. The N-terminal YRS fragment generated by proteolytic cleavage had monocyte activation comparable to that of the constitutive-active mutant YRS (YRSY341A) previously reported. Conclusion: Platelets take up both full-length YRS and the active form of cleaved YRS fragment from the plasma. The cleaved, N-terminal YRS fragment stored in alpha-granules may have potential to activate monocytes.