Outside-in αIIbβ3 signals require C-terminal β3 motifs. (A) Platelet spreading on immobilized fibrinogen mediated by wild-type and mutant β3 integrins. Outside-in signaling was measured using platelet spreading after 45 minutes of incubation on a fibrinogen-coated surface. Adherent platelets were visualized using actin staining with Alexa-Fluor 594 phalloidin (top) and green fluorescence to detect β3-expressing platelets (middle). The overlay of actin and GFP staining is shown on the bottom row. (B) Relative outside-in signaling by platelets expressing wild-type and mutant β3 integrins. The area occupied by adherent platelets was measured using the Image J program. P values are shown for comparison of values obtained in platelets expressing wild-type and each mutant β3 integrin type. Asterisk indicates P < .01; n = 100 to 1000 individual platelets from 7 animals analyzed for each type of platelet; error bars indicate mean ± standard deviation.