polyP enhances FXI activation by α-thrombin. In all panels, initial rates of FXI activation were quantified at 37°C in reactions containing 30nM FXI, 5nM α-thrombin, and polyP or dextran sulfate. Data are mean ± SE (n = 4). (A) Concentration dependence of polyP-mediated enhancement of FXI activation by α-thrombin, tested with 4 different polyP polymer lengths: 22mer (●), 65mer (▿), 167mer (■), and 350mer (◊). (B) PolyP polymer length dependence of the enhancement of FXI activation by α-thrombin, using size-fractionated polyP preparations at 4μM phosphate (0 indicates no polyP). (C) EcPPXc abrogates the ability of polyP, but not dextran sulfate, to enhance FXI activation by α-thrombin. Rates of FXI activation were quantified in the absence (open bars) or presence (solid bars) of EcPPXc. Reaction conditions included no polyanion (control), with 1 μg/mL dextran sulfate (DS) or with 4μM polyP (80mer or 255mer, as indicated).