AFFINITY PURIFICATION AND CHARACTERIZATION OF A FIBRINOGEN-BINDING PROTEIN COMPLEX WHICH PROTECTS MICE AGAINST LETHAL CHALLENGE WITH STREPTOCOCCUS-EQUI SUBSP EQUI

Cell-wall-associated proteins from Streptococcus egui subsp. equi, the causative agent of strangles, were analysed with a view to identifyin g a potential protective antigen. Preparations of these proteins, isol ated from mutanolysin extracts of cell walls, were shown to contain on e major high-M-r protein species (apparent M-r 220000 and 550000 when analysed by SDS-PAGE and gel-filtration chromatography, respectively). The high-M-r protein bound horse fibrinogen and was purified under no n-denaturing conditions using fibrinogen affinity chromatography. The fibrinogen-binding protein (FgBP) reacted with serum taken from horses recovering from strangles and protected mice against lethal challenge from S. equi subsp. equi. The sequence of the corresponding gene (fbp ) was determined and shown to encode a mature protein (M-r 54597) with predicted coiled-coil structure. An FgBP truncate, lacking the C-term inal cell wall/membrane anchor domain, was overexpressed in and purifi ed from Escherichia coli and was shown to behave in an analogous fashi on to the wild-type product in terms of M-r estimation, fibrinogen bin ding and seroreactivity.