Molecular characterisation of the interaction between human IgG and the M-related proteins from Streptococcus pyogenes

Group A Streptococcal M-related proteins (Mrp) are dimeric α-helical-coiled-coil cell membrane-bound surface proteins. During infection, Mrp recruit the fragment crystallisable region of human immunoglobulin G (IgG-Fc) via their A-repeat regions to the bacterial surface, conferring upon the bacteria enhanced phagocytosis resistance and augmented growth in human blood. However, Mrps show a high degree of sequence diversity, and it is currently not known whether this diversity affects the Mrp-IgG interaction. Herein, we report that diverse Mrps all bind human IgG subclasses with nanomolar affinity, with differences in affinity which ranged from 3.7-11.1 nM for mixed IgG. Using surface plasmon resonance, we confirmed Mrps display preferential IgG-subclass binding. All Mrps were found to have a significantly weaker affinity for IgG3 (p<0.05) compared to all other IgG-subclasses. Furthermore, plasma pulldown assays analysed via western blotting revealed that all Mrp were able to bind IgG in the presence of other serum proteins at both 25°C and 37°C. Finally, we report for the first time that dimeric Mrps bind to IgG with a 1:1 stoichiometry, enhancing our understanding of this important host-pathogen interaction.