Molecular characterization of the interaction between human IgG and the M-related proteins from Streptococcus...

Area of Science:

• Microbiology
• Immunology
• Structural Biology

Background:

• Group A Streptococcal M-related proteins (Mrps) are surface proteins crucial for bacterial virulence.
• Mrps bind human immunoglobulin G (IgG) via their A-repeat regions, aiding bacterial survival.
• Sequence diversity among Mrps raises questions about their conserved IgG-binding function.

Purpose of the Study:

• To investigate if sequence diversity in Mrps affects their interaction with human IgG subclasses.
• To characterize the binding affinity and stoichiometry of Mrp-IgG interactions.

Main Methods:

• Surface plasmon resonance (SPR) was used to quantify Mrp-IgG subclass binding affinities.
• Plasma pulldown assays followed by Western blotting assessed IgG binding in complex serum.
• Stoichiometry of the Mrp-IgG interaction was determined.

Main Results:

• Diverse Mrps exhibit nanomolar affinity for human IgG subclasses (3.7–11.1 nM for mixed IgG).
• All Mrps demonstrated significantly weaker binding to IgG3 compared to other IgG subclasses.
• Mrps bind IgG with a 1:1 stoichiometry and maintain binding in the presence of other serum proteins.

Conclusions:

• Despite sequence variation, Mrps maintain conserved, high-affinity binding to human IgG subclasses.
• Preferential binding and weaker affinity for IgG3 suggest specific roles in host immune evasion.
• The 1:1 stoichiometry provides a clearer understanding of this critical host-pathogen interaction.