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Two major classes in the M protein family in group A streptococci
P W O'Toole1, P O'Toole, L Stenberg
1Department of Medical Microbiology, University of Lund, Sweden.
Abstract:
The M protein family of molecules in the group A streptococcus comprises a number of cell surface proteins that interact with the immune system of the host. One of the proteins in this family is the IgA receptor Arp4, which has C repeats similar to those that characterize the known M proteins. The streptococcal strain expressing Arp4 also expresses a second immunoglobulin-binding protein, Mrp4, which is shown here to be encoded by a gene located immediately upstream of the gene for Arp4. In addition to binding IgG, Mrp4 also binds fibrinogen, a property ascribed to M proteins. DNA sequence analysis demonstrated that the Mrp4 protein indeed is a member of the M protein family, but it was unexpectedly found to have a type of repeat that is identical to the A repeat described for FcRA76, a partially sequenced streptococcal Fc receptor. Purified FcRA76 was shown to bind fibrinogen and IgG, like Mrp4. These data show that the known molecules in the M protein family can be divided into two classes, A and C, according to the type of repeat region found. Hybridization studies with a panel of clinical isolates indicate that many streptococcal strains express class A and class C proteins, whereas some strains express only class C proteins. Class A molecules show amino-terminal sequence variation, like class C molecules, which suggests that proteins of both classes are targets for the immune response.
Insights
Group A Streptococcus M protein family includes IgA receptor Arp4 and immunoglobulin-binding protein Mrp4. These proteins can be classified into two distinct groups, A and C, based on their repeat regions, influencing host immune responses.
Area of Science:
- Microbiology
- Immunology
- Molecular Biology
Background:
- Group A Streptococcus utilizes M protein family molecules on its cell surface to interact with the host immune system.
- The IgA receptor Arp4, a member of this family, possesses C repeats characteristic of known M proteins.
- A second immunoglobulin-binding protein, Mrp4, is co-expressed and shares upstream gene location with Arp4.
Purpose of the Study:
- To investigate the structural and functional characteristics of the immunoglobulin-binding protein Mrp4.
- To determine the relationship of Mrp4 to the M protein family and its repeat structures.
- To classify M protein family members based on repeat region types and assess their prevalence in clinical isolates.
Main Methods:
- DNA sequence analysis of the Mrp4 gene.
- Biochemical characterization of purified FcRA76 protein.
- Hybridization studies using a panel of clinical streptococcal isolates.
Main Results:
- Mrp4 binds both IgG and fibrinogen, a known M protein property.
- DNA sequence analysis revealed Mrp4 contains A-type repeats, distinct from Arp4's C repeats, and similar to FcRA76.
- FcRA76 also binds fibrinogen and IgG.
- M protein family members are classifiable into A and C types based on repeat regions.
- Many clinical isolates express both class A and C proteins, while some express only class C.
- Both classes exhibit amino-terminal sequence variation, suggesting immune system targeting.
Conclusions:
- The M protein family of Group A Streptococcus can be divided into class A and class C based on repeat region types.
- Both classes of M proteins exhibit sequence variation, indicating they are targets for the host immune response.
- Understanding these protein classes is crucial for developing effective immune-based strategies against Streptococcus infections.