Ska), extracellular protein factor ( epf), or protein F2 ( prtF2) (B) were tested for surface PK/FXIIa activity after incubation with normal human plasma. (C) The PK/FXIIa activity at the surfaces of M49 wild-type and ska mutant strains was examined after exposure to normal or plasminogen-deficient plasma. Kaolin served as a positive control for plasmin-independent contact system activation. (D) P