Protein H--a novel IgG binding bacterial protein.

Area of Science:

• Microbiology
• Immunology
• Protein Chemistry

Background:

• Group A Streptococcus (GAS) possesses surface proteins that can interact with host immune components.
• Understanding these interactions is crucial for elucidating bacterial pathogenesis and immune evasion strategies.

Purpose of the Study:

• To identify and characterize novel immunoglobulin-binding proteins from group A Streptococcus.
• To investigate the binding specificity and affinity of the identified protein for human immunoglobulin G (IgG).

Main Methods:

• Screening of forty-eight group A streptococcal strains for human IgG binding.
• Solubilization and purification of the IgG-binding molecule using mutanolysin and affinity chromatography.
• Cloning and expression of the encoding gene in E. coli.
• Characterization of binding specificity using various immunoglobulin isotypes and species, and determination of affinity constant.

Main Results:

• One M protein type 1 strain (AP1) showed significant human IgG binding.
• A 40 kDa protein, designated protein H, was purified and shown to bind human IgG almost exclusively.
• Protein H demonstrated high affinity (1.6 x 10^9) for human IgG heavy chains and Fc fragments, with no binding to other immunoglobulin isotypes (IgM, IgA, IgD, IgE).
• N-terminal sequencing revealed no homology to known Ig-binding proteins.

Conclusions:

• A novel human IgG-binding protein, protein H, has been identified in group A Streptococcus.
• Protein H exhibits specific and high-affinity binding to human IgG, suggesting a potential role in bacterial immune evasion.
• The unique characteristics of protein H warrant further investigation into its structure-function relationship and biological significance.