Characterization of the platelet granule proteome: evidence of the presence of MHC1 in alpha-granules.

In the present study, we performed an extensive qualitative characterization of the platelet granule proteome using subcellular fractionation followed by mass spectrometry analysis and functional annotation. Eight-hundred-and-twenty-seven proteins were identified, most of them being associated to granules and to the granule's secretory machinery. Functional pathway analysis revealed 30 pathways, including the major histocompatibility complex class 1 (MHC I) presenting antigen pathway. This pathway was of particular interest for its potential interrelation between platelets and the immune system. Key proteins belonging to this metabolic route such as β-2-microglobulin, 26S protease regulatory subunit 10B from the proteasome and proteins 1 and 2 of the transporter associated with antigen processing were shown to co-localize with von Willebrand factor in resting platelets and to be located on the plasma membrane when platelets were activated. Key proteins of the MHC1 antigen-presenting pathway are located in platelet alpha-granules. These results suggest a possible functional role of platelet granules in platelet-related immune modulation. BIOLOGICAL SIGNIFICANCE: In this study, we described the largest dataset related to platelet granule proteins. We performed a functional pathway analysis that evidenced several expected granule-related pathways. We also highlighted the "Antigen processing and presentation" pathway that has drawn our attention. Using immunofluorescence technique, we confirmed the presence of several key proteins for antigen presentation in platelet granules. This study suggests a putative functional role of MHC1 and platelet granules in the immune modulation.