Normal actin polymerization, Rac activation, and PAK phosphorylation in β1 hypomorphic platelets. (A) Platelets from wild-type (WT), β1-null (KO), and hypomorphic (Hpm) mice were stimulated with 5 µg/mL fibrillar collagen for 60 seconds in the presence of 0.5 mM RGDS peptide, and the relative F-actin content was measured by flow cytometry. Curves represent mean ± SEM increase in FITC phalloidin MFI (n = 3, significance level are indicated). (B) Same platelet populations were stimulated with 5 µg/mL fibrillar collagen for 30 and 60 seconds lysed and subjected to Rac-1 GTP pulldown experiments. Total Rac-1 loading is shown below. (C) Quantification of GTP-bound active Rac-1 pulldown experiments at 60 seconds (bars represent mean values ± SEM; significance level are indicated; *P < .05; **P < .01; wild-type (WT), n = 4; β1-null (KO), n = 3; hypomorphic (Hpm), n = 4). (D) Platelets from WT, KO, and Hpm mice were stimulated with 5 µg/mL fibrillar collagen for the indicated time points, lysed and subjected to immunoblotting for p-Thr432/402 PAK1/2 and p-Ser19 myosin light chain (pMLC). Total MLC is shown as loading control.