TAB1 and TAB2 are required for activated PAR1-induced p38 activation. (A) Tab1 WT (+/+) and null (−/−) MEFs were stimulated with 10 nM α-Th, and phosphorylation of p38 and ERK1/2 was determined. The data (mean ± SD [error bars], n = 3) were analyzed using a Student’s t test (**, P < 0.01). (B) PAR1 HeLa cells were transiently transfected with ns, TAB1, TAB2, MKK3, or MKK6 siRNAs, and stimulated with 10 nM α-Th. p38 phosphorylation was then determined. The data (mean ± SD [error bars], n = 3) were analyzed using a Student’s t test (*, P < 0.05). See Fig. S4 E. (C) Endothelial cells transfected with ns, TAB1, and TAB2 siRNA were stimulated with 10 nM α-Th, and p38 phosphorylation was examined. The data (mean ± SD [error bars], n = 3) were analyzed using a Student’s t test (*, P < 0.05; **, P < 0.01). (D) PAR1 HeLa cells were cotransfected with ns or TAB1 and TAB2 siRNA together with pcDNA vector (lanes 1–4); siRNA-resistant TAB1 WT, TAB2 WT, or CA mutant (lanes 5–6); or siRNA-resistant TAB1 P412A mutant, TAB2 WT, or CA mutant (lanes 7–8); and stimulated with 10 nM α-Th. The data (mean ± SD [error bars], n = 3) were analyzed using a Student’s t test (*, P < 0.05; **, P < 0.01; ***, P < 0.001).