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Localization of the equine IgG-binding domain in the fibrinogen-binding protein (FgBP) of Streptococcus equi subsp. equi

Mary Meehan^1,

Melanie J. Lewis^2,

¹ Department of Microbiology, Moyne Institute of Preventative Medicine, Trinity College, Dublin 2, Ireland
² Division of Medical Sciences, University of Dundee Medical School, Ninewells Hospital, Dundee DD1 9SY, UK

Fibrinogen-binding protein (FgBP, also termed SeM) is a cell-wall-associated anti-phagocytic M-like protein of the equine pathogen Streptococcus equi subsp. equi, and binds fibrinogen (Fg) and IgG. FgBP binds Fg avidly through residues located at the extreme N terminus of the molecule, whereas the IgG-binding site is more centrally located between the A and B repeats. FgBP binds equine IgG4 and IgG7 subclasses through interaction with the CH2–CH3 interdomain region of IgG-Fc, and possesses overlapping Fc-binding sites with protein A and protein G. In this study, FgBP truncates containing defined internal deletions were used to identify a stretch of 14 aa (residues 335–348) critical for IgG binding. Protein chimeras consisting of the non-IgG-binding -helical coiled-coil M5 protein fused to FgBP sequences were used to identify a minimal equine IgG-binding domain consisting of residues 329–360. Competition ELISA tests suggested that IgG does not compromise Fg binding and vice versa.

Correspondence
Mary Meehan
meehanmary{at}yahoo.co.uk

These authors contributed equally to this work.