F-actin coats purified platelet α-granules. (A) Platelets were subjected to nitrogen cavitation and subcellular fractionation as described in “Materials and methods.” Plasma membranes and OCS from biotin-labeled platelets were identified in fraction 1 by staining with HRP-avidin. Lysosomes and dense granules were identified primarily in fractions 2 and 4 by using anti-LAMP-1 antibody. α-Granules were identified in fraction 3 by using antibodies directed at the P-selectin. Dense granules were identified in fraction 4 following subcellular fractionation of ¹⁴C-serotonin-labeled platelets. (B) Fractions 1 (OCS and plasma membrane-enriched), 2 (lysosome-enriched), 3 (α-granule-enriched), 4 (dense granule-enriched), phosphatidylcholine (PC) micelles or phosphatidylserine (PS) micelles were incubated in the presence (□) or absence (▪) of 32 μM latrunculin A for 20 minutes. Samples were then incubated with 10 μM FITC-phalloidin for 20 minutes and analyzed by flow cytometry.