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A highly variable region in members of the streptococcal M protein family binds the human complement regulator C4BP
E Johnsson1, A Thern, B Dahlbäck
1Department of Medical Microbiology, Lund University, Sweden.
Abstract:
Strains of Streptococcus pyogenes express one or more molecules that are members of the M protein family, a group of surface proteins implicated in virulence. A characteristic property of the molecules in this family is the presence of a highly variable N-terminal region, whose function is unknown. Here we show that human C4b-binding protein (C4BP), a regulatory component of the complement system, binds to the highly variable region of many members of the M protein family. Chimeric molecules, in which the N-terminal regions of four different C4BP-binding proteins were combined with the C-terminal part of the non-binding M5 protein, had intact C4BP-binding ability, as judged by binding assays and Scatchard analysis with highly purified molecules. Moreover, work with the C4BP-binding Arp4 protein showed that an N-terminal 52-residue fragment retained binding ability, and that a 21-residue synthetic peptide derived from the variable region completely inhibited the binding of C4BP. Computer-assisted analysis of the four C4BP-binding regions studied here (45-66 amino acid residues) indicated that they lack residue identities that could explain their ability to bind the same ligand, but differ from the nonbinding M5 protein in their lower propensity to form a coiled-coil. Thus, the variable C4BP-binding regions have an extraordinary capacity for sequence variation, while retaining the ability to bind C4BP. These data indicate that an important function of the variable region in members of the M protein family is to bind a host protein that down-regulates the complement system.
Insights
Streptococcus pyogenes M protein
Area of Science:
- Microbiology
- Immunology
- Structural Biology
Background:
- Streptococcus pyogenes produces M proteins, crucial virulence factors with a variable N-terminal region of unknown function.
- The complement system, a key immune defense, is regulated by C4b-binding protein (C4BP).
Purpose of the Study:
- To investigate the function of the variable N-terminal region of Streptococcus pyogenes M proteins.
- To determine if C4BP interacts with the variable region of M proteins and its implications for virulence.
Main Methods:
- Construction and testing of chimeric M proteins with variable N-terminal regions.
- Binding assays and Scatchard analysis to quantify C4BP-M protein interactions.
- Analysis of M protein fragments and synthetic peptides to map C4BP binding sites.
- Computer-assisted analysis of amino acid sequences and coiled-coil propensity.
Main Results:
- Human C4BP directly binds to the variable N-terminal region of multiple M protein family members.
- Chimeric proteins and N-terminal fragments retained C4BP-binding ability, confirming the region's role.
- A synthetic peptide from the variable region completely blocked C4BP binding.
- Variable regions exhibit sequence diversity but share a reduced propensity for coiled-coil formation compared to non-binding M proteins.
Conclusions:
- The variable N-terminal region of Streptococcus pyogenes M proteins functions to bind human C4BP.
- This interaction allows M proteins to recruit a host complement regulator, down-regulating the immune response.
- The M protein variable region demonstrates significant sequence plasticity while maintaining C4BP binding capability.