Dimerization is not a determining factor for functional high affinity human plasminogen binding by the group A...

Area of Science:

• Microbiology
• Protein Biochemistry
• Molecular Biology

Background:

• Group A Streptococcus pyogenes (GAS) utilizes M-like proteins (PAM) to interact with human plasma proteins.
• GAS PAM binds human plasminogen (hPg) and plasmin (hPm) through specific protein domains.
• Previous studies indicated weaker binding of monomeric PAM fragments compared to full-length dimeric PAM.

Purpose of the Study:

• To investigate the role of PAM dimerization in its binding affinity to hPg.
• To identify the minimal PAM peptide sequence required for dimerization and high-affinity binding.
• To determine the specific residues within the PAM N-terminal a1a2 region essential for hPg interaction and GAS virulence.

Main Methods:

• Peptide synthesis and characterization to define minimal dimerization length.
• Surface plasmon resonance (SPR) to measure binding kinetics (KD) between PAM peptides and K2hPg.
• Site-directed mutagenesis of key residues in the PAM a1a2 region.
• In vivo GAS virulence assays comparing wild-type, mutated, and gene-knockout strains.

Main Results:

• VEK64, the smallest α-helical dimer peptide, showed high affinity binding to K2hPg (KD = 1-2 nm).
• VEK32, a monomeric peptide, also exhibited high-affinity binding (similar to dimeric PAM), indicating dimerization is not solely responsible.
• Five specific residues (Arg113, His114, Glu116, Arg126, His127) in the PAM a1a2 region were critical for high-affinity hPg binding.
• Mutation of these residues significantly reduced hPg binding affinity (∼1000-fold) and GAS virulence.

Conclusions:

• High-affinity binding of GAS PAM to hPg is mediated by five key residues in the a1a2 domain, not by PAM dimerization.
• These critical residues are essential for enhancing GAS virulence through interaction with hPg.
• Targeting these residues could represent a novel strategy to reduce GAS pathogenicity.