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Differential recognition of surface proteins in Streptococcus pyogenes by two sortase gene homologs
Timothy C Barnett1, June R Scott
1Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Abstract:
The interaction of Streptococcus pyogenes (group A streptococcus [GAS]) with its human host requires several surface proteins. In this study, we isolated mutations in a gene required for the surface localization of protein F by transposon mutagenesis of the M6 strain JRS4. This gene (srtA) encodes a protein homologous to Staphylococcus aureus sortase, which covalently links proteins containing an LPXTG motif to the cell wall. The GAS srtA mutant was defective in anchoring the LPXTG-containing proteins M6, protein F, ScpA, and GRAB to the cell surface. This phenotype was complemented when a wild-type srtA gene was provided in trans. The surface localization of T6, however, was unaffected by the srtA mutation. The M1 genome sequence contains a second open reading frame with a motif characteristic of sortase proteins. Inactivation of this gene (designated srtB) in strain JRS4 affected the surface localization of T6 but not M6, protein F, ScpA, or GRAB. This phenotype was complemented by srtB in trans. An srtA probe hybridized with DNA from all GAS strains tested (M types 1, 3, 4, 5, 6, 18, 22, and 50 and nontypeable strain 64/14) and from streptococcal groups C and G, while srtB hybridized with DNA from only a few GAS strains. We conclude that srtA and srtB encode sortase enzymes required for anchoring different subsets of proteins to the cell wall. It seems likely that the multiple sortase homologs in the genomes of other gram-positive bacteria have a similar substrate-specific role.
Insights
Streptococcus pyogenes uses two sortase enzymes, SrtA and SrtB, to anchor different surface proteins essential for host interaction. SrtA anchors M6, protein F, ScpA, and GRAB, while SrtB anchors T6.
Area of Science:
- Microbiology
- Molecular Biology
- Bacterial Pathogenesis
Background:
- Streptococcus pyogenes (group A Streptococcus [GAS]) utilizes surface proteins for host interaction.
- Understanding the mechanisms of GAS surface protein anchoring is crucial for deciphering its pathogenesis.
Purpose of the Study:
- To identify and characterize genes responsible for anchoring specific surface proteins in GAS.
- To investigate the roles of sortase enzymes in the cell wall anchoring of GAS proteins.
Main Methods:
- Transposon mutagenesis was employed to isolate mutants in GAS strain JRS4.
- Gene sequencing and complementation assays were used to identify and confirm gene functions.
- Southern hybridization was performed to assess gene distribution across different GAS strains.
Main Results:
- A mutation in srtA disrupted the surface localization of M6, protein F, ScpA, and GRAB.
- A mutation in srtB affected the surface localization of T6.
- SrtA was detected in all tested GAS strains and other streptococcal groups, whereas SrtB was found in fewer GAS strains.
Conclusions:
- GAS possesses at least two sortase enzymes, SrtA and SrtB, with distinct substrate specificities.
- SrtA and SrtB are essential for anchoring different subsets of surface proteins to the GAS cell wall.
- This substrate specificity likely extends to sortase homologs in other Gram-positive bacteria.
