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Streptococcal protein G. Gene structure and protein binding properties.
U Sjöbring1, L Björck, W Kastern
1Department of Medical Microbiology, University of Lund, Sweden.
The Journal of Biological Chemistry
|January 5, 1991
Summary
This study reveals variations in Protein G from Streptococcus strains, impacting its binding affinity for immunoglobulin G (IgG) and human serum albumin (HSA). Genetic analysis identified specific gene deletions correlating with altered binding properties, crucial for understanding streptococcal virulence factors.
Area of Science:
- Microbiology
- Molecular Biology
- Immunology
Background:
- Protein G from Streptococcus strains is a surface protein with binding capabilities for immunoglobulin G (IgG) and human serum albumin (HSA).
- Variations in Protein G structure and function among different streptococcal strains are not fully understood.
- Understanding these variations is important for characterizing streptococcal pathogenesis and developing diagnostic or therapeutic strategies.
Purpose of the Study:
- To investigate the structural diversity of Protein G among human group C and G streptococcal strains.
- To correlate observed structural differences with variations in binding affinities for IgG and HSA.
- To identify the specific genetic regions responsible for these altered binding properties.
Main Methods:
- Solubilization of Protein G using mutanolysin from 31 streptococcal strains.
- Analysis of protein size and binding patterns using SDS-PAGE and Western blot.
- Cloning and sequencing of Protein G genes from representative strains (C40, G43, G148).
- Genetic mapping and characterization of IgG- and HSA-binding domains using PCR and synthetic peptides.
Main Results:
- Three distinct Protein G variants (65-kDa, 58-kDa, 40-kDa) were identified based on molecular weight and binding profiles.
- The 65-kDa and 58-kDa variants bound both IgG and HSA, while the 40-kDa variant bound only IgG.
- Genetic analysis revealed deletions in the IgG-binding region of the G43 and C40 genes, explaining reduced IgG affinity.
- A specific fragment deletion in the G43 gene accounted for its lack of HSA binding.
- A 11.5-kDa peptide corresponding to a single IgG-binding unit demonstrated high affinity for human IgG (8.0 x 10(7) M-1).
Conclusions:
- Structural variations in Streptococcus Protein G directly influence its binding specificity and affinity for IgG and HSA.
- Specific deletions within the Protein G gene, particularly in the IgG-binding region, are responsible for altered functional properties.
- These findings provide a molecular basis for understanding the heterogeneity of Protein G and its role in streptococcal interactions.