Presence of myosin IIA at the surface of platelet organelles. (A) Confocal observation of isolated human platelet organelles immunolabeled with anti-VWF antibody and anti-myosin IIA antibody showing the presence of myosin at the surface of VWF-positive α granules. (B) Immunogold labeling (anti-myosin IIA, 10 nm) of WT mouse platelets, showing the presence of myosin IIA at the cytoplasmic face of organelles. Inset, α granule labeled with 2 gold particles. (C) Quantification of the numbers of gold particles according to their localization in WT mouse platelets, either cytoplasmic or at the cytoplasmic face of granules or mitochondria. Results are mean ± SEM; n = 20-22 platelets. (D) Quantification of gold particles in human platelets treated or not with latrunculin A (100 µM). Data are mean ± SEM; n = 20. (E) Western blots performed on granules isolated from human platelets pretreated or not with latrunculin A (100 µM). While latrunculin A treatment removed F-actin from the surface of organelles, myosin IIA remained associated with organelles. Prohibitin, a mitochondrial protein, was used as a loading control and blots are representative of 3 separate experiments. cyto, cytoplasmic; G/M, granules or mitochondria; IgG, immunoglobulin G (isotype-matched control antibody); Lat A, latrunculin A; Myo, anti-myosin IIA antibody.