Extreme sequence divergence but conserved ligand-binding specificity in Streptococcus pyogenes M protein

Area of Science:

• Microbiology and Immunology
• Structural Biology
• Protein Engineering

Background:

• Pathogenic bacteria frequently evade host immunity by altering surface protein sequences targeted by antibodies.
• Despite sequence variability, these protein regions often retain critical ligand-binding functions, crucial for bacterial survival.

Purpose of the Study:

• To investigate the sequence divergence limits within the ligand-binding hypervariable region (HVR) of Streptococcus pyogenes M protein.
• To characterize the HVRs responsible for binding human complement regulator C4b-binding protein (C4BP), a key factor in immune evasion.

Main Methods:

• Comparative analysis of C4BP-binding HVR sequences from Streptococcus pyogenes.
• Site-directed mutagenesis of specific residues within the M22 protein's HVR.
• Assessment of C4BP binding affinity and antigenic properties of mutated HVRs.

Main Results:

• No conserved residue identities were found across all C4BP-binding HVRs, indicating extreme sequence variability.
• Conserved Leu residues in M22 HVR are likely structural and not directly involved in C4BP binding.
• Mutations in conserved, solvent-exposed Glu residues significantly altered antigenic properties but not C4BP binding.

Conclusions:

• Streptococcus pyogenes M protein HVRs exhibit remarkable sequence divergence and antigenic variability.
• These HVRs can maintain specific C4BP binding function despite extensive sequence changes.
• Findings highlight a mechanism for immune evasion through adaptable protein structures in pathogenic bacteria.