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M1 protein and protein H: IgGFc- and albumin-binding streptococcal surface proteins encoded by adjacent genes
P Akesson1, K H Schmidt, J Cooney
1Department of Medical and Physiological Chemistry, Lund University, Sweden.
Abstract:
M1 protein and Protein H are surface proteins simultaneously present at the surface of certain strains of Streptococcus pyogenes, important pathogenic bacteria in humans. The present study concerns the structure, protein-binding properties and relationship between these two molecules. The gene encoding M1 protein (emm1) was found immediately upstream of the Protein H gene (sph). Both genes were preceded by a promoter region. Comparison of the sequences revealed a high degree of similarity in the signal peptides, the C repeats located in the central parts of the molecules and in the C-terminal cell-wall-attached regions, whereas the N-terminal sequences showed no significant similarity. Protein H has affinity for the Fc region of IgG antibodies. Also M1 protein, isolated from streptococcal culture supernatants or from Escherichia coli expressing emm1, was found to bind human IgGFc. When tested against polyclonal IgG from eight other mammalian species, M1 protein and Protein H both showed affinity for baboon, rabbit and pig IgG. M1 protein also reacted with guinea-pig IgG, whereas both streptococcal proteins were negative in binding experiments with rat, mouse, bovine and horse IgG. The two proteins were also tested against other members of the immunoglobulin super family: human IgM, IgA, IgD, IgE, beta 2-microglobulin, and major histocompatibility complex (MHC) class-I and class-II antigens. M1 protein showed no affinity for any of these molecules whereas Protein H reacted with MHC class-II antigens. M1 protein is known to bind albumin and fibrinogen also. The binding sites for these two plasma proteins and for IgGFc were mapped to different sites on M1 protein. Thus albumin bound to the C repeats and IgGFc to a region (S) immediately N-terminal of the C repeats. Finally, fibrinogen bound further towards the N-terminus but close to the IgGFc-binding site. On the fibrinogen molecule, fragment D was found to mediate binding to M1 protein. The IgGFc-binding region of M1 protein showed no similarity to that of Protein H. Still, competitive binding experiments demonstrated that the two streptococcal proteins bound to overlapping sites on IgGFc.
Insights
Streptococcus pyogenes M1 protein and Protein H bind human IgG Fc, with distinct but overlapping binding sites. M1 protein also binds albumin and fibrinogen, while Protein H interacts with MHC class-II antigens.
Area of Science:
- Microbiology and Immunology
- Molecular Biology
- Structural Biology
Background:
- M1 protein and Protein H are surface proteins of Streptococcus pyogenes, a significant human pathogen.
- These proteins play roles in bacterial virulence and host immune evasion.
- Understanding their structure and binding properties is crucial for developing therapeutic strategies.
Purpose of the Study:
- To investigate the structure, protein-binding properties, and relationship between M1 protein and Protein H.
- To identify the specific binding sites and affinities of these proteins for host molecules, particularly immunoglobulins.
- To compare the molecular characteristics and binding specificities of M1 protein and Protein H.
Main Methods:
- Sequence comparison of emm1 and sph genes and their encoded proteins.
- Protein purification from bacterial sources and recombinant expression systems (E. coli).
- Binding assays using various human and mammalian immunoglobulins (IgG, IgM, IgA, etc.), albumin, fibrinogen, and MHC antigens.
- Competitive binding experiments to determine overlapping binding sites.
- Mapping of binding sites on M1 protein using different protein fragments and ligands.
Main Results:
- M1 protein and Protein H share sequence similarities in signal peptides, C repeats, and cell-wall-attached regions, but not in N-terminal sequences.
- Both M1 protein and Protein H bind to the Fc region of human IgG, with affinity for IgG from baboon, rabbit, and pig.
- M1 protein binds albumin and fibrinogen at distinct sites, and its IgG Fc binding site is N-terminal to the C repeats.
- Protein H binds to MHC class-II antigens, unlike M1 protein.
- Competitive binding assays reveal overlapping binding sites for M1 protein and Protein H on human IgG Fc.
Conclusions:
- M1 protein and Protein H exhibit distinct yet partially overlapping molecular interactions with host proteins, contributing to Streptococcus pyogenes pathogenesis.
- The differential binding specificities suggest varied roles in immune evasion and host colonization.
- Further structural and functional studies are warranted to fully elucidate the mechanisms of interaction and their implications.