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Immunochemical analysis of intact M protein secreted from cell wall-less streptococci
Abstract:
M protein is a major virulence factor of group A streptococci, which provides these organisms with protection against phagocytosis in the absence of specific antibody. To gain insight into the nature of the native M-protein molecule, type 12 M protein was isolated and purified from the extracellular supernatants of a group A streptococcal L form and stabilized protoplasts. The intact purified M protein from both sources had a molecular weight of 58,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This is in contrast to the 32,0000-dalton molecule isolated from the parent type 12 organism by using a nonionic detergent. The purified secretory M protein removed opsonic antibodies from type 12 rabbit immune serum, as demonstrated by a bactericidal assay. Therefore, it appears that either previous nondestructive methods of M-protein isolation have not removed intact M protein from cell walls or part of the molecule is fragmented during its association with cell walls.
Insights
Group A streptococci M protein is crucial for virulence. Researchers isolated a larger, intact M protein (58,000 Da) from L forms, differing from smaller cell-associated forms, suggesting fragmentation during isolation.
Area of Science:
- Microbiology
- Immunology
- Molecular Biology
Background:
- Group A streptococci (GAS) possess M protein, a major virulence factor.
- M protein confers resistance to phagocytosis in the absence of specific antibodies.
- Understanding the native M protein structure is key to understanding GAS pathogenesis.
Purpose of the Study:
- To characterize the native M protein molecule of type 12 group A streptococci.
- To compare the molecular weight of M protein isolated from different sources.
Main Methods:
- Isolation and purification of type 12 M protein from GAS L forms and protoplasts.
- Molecular weight determination using SDS-PAGE.
- Bactericidal assays to assess the function of purified M protein.
Main Results:
- Intact M protein isolated from supernatants had a molecular weight of 58,000 Da.
- This differs from the 32,000 Da molecule obtained from parent organisms using nonionic detergents.
- Purified secretory M protein removed opsonic antibodies from immune serum.
Conclusions:
- Previous M protein isolation methods may not have extracted intact molecules.
- M protein may fragment during its association with cell walls.
- The larger M protein form appears to be the biologically relevant molecule.