Location of endogenous Arp2/3 in spread platelets.

(A) Three different spread platelets fixed in 4% paraformaldehyde containing 0.25% Triton and stained with αArp2. Arp2/3 displays a speckled cytoplasmic staining and a row of immunofluorescent dots at the periphery. (B) Double label of a single platelet fixed as in panel A and stained for Arp2/3 with αp34 (Cy3, red) and for actin filaments (FITC–phalloidin, green). Superimposition of the double label shows Arp2/3 at the periphery. Bar = 2 μm. (C) Triple image of a single platelet fixed without detergent to permit imaging by phase microscopy and then stained with αArp2 (red) and phalloidin (green). Note that Arp2/3 is found in a bright arc internal to the actin frill. Arrows indicate weak staining at the outermost edge of the lamellipodia. Also note that the platelet contours imaged by phalloidin correspond to those imaged by phase microscopy. Bar = 2 μm.