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Size variation of the M protein in group A streptococci
Abstract:
In addition to the type-specific antigenic variation that is a well-known characteristic for the group A streptococcal M protein, we have now found that the M molecules vary with respect to their molecular size, both between M types and within an M type. By the use of an M6 monoclonal antibody, which crossreacts with 20 different M protein types, and antibodies to the N-acetyl glucosamine determinant of the cell wall, we have been able to identify the M protein molecules released from the streptococcal cell wall with muralytic enzymes, particularly group C phage-associated lysin. Immunoblot analysis of the cell extract identified M protein molecules bound to various cell wall fragments, suggesting a peptidoglycan linkage for the M molecule. M protein extracted from 20 different streptococcal serotypes revealed size variations from 41,000 to 80,000 in molecular weight. This extreme variation is unusual for related proteins. Similar size variations in the M molecule were also found in random clinical isolates of type 6 streptococci. No size change was seen in M6 protein isolated from: (a) strains within a limited epidemic, (b) a strain passaged in mice 192 times, and (c) a strain passaged in the laboratory for 156 generations, suggesting that the observed variation is not a rapid process. The results indicate that, within the broad limits observed in this study, the size of the M protein may not be critical to the antiphagocytic activity of the molecule.
Insights
Group A Streptococcus M proteins exhibit significant molecular size variation between and within types. This size variability, observed across various serotypes, may not impact the M protein's antiphagocytic function.
Area of Science:
- Microbiology
- Immunology
- Molecular Biology
Background:
- Group A Streptococcus (GAS) M protein is known for type-specific antigenic variation.
- The M protein is crucial for GAS virulence, mediating antiphagocytic activity.
Purpose of the Study:
- To investigate molecular size variations in GAS M protein.
- To determine if M protein size variation occurs between and within M types.
- To explore the potential linkage of M protein to the streptococcal cell wall.
Main Methods:
- Utilized an M6 monoclonal antibody cross-reactive with 20 M types and cell wall antibodies.
- Employed muralytic enzymes, specifically group C phage-associated lysin, to release M protein.
- Conducted immunoblot analysis to identify M protein and assess its molecular weight and cell wall association.
Main Results:
- M protein molecules were identified bound to cell wall fragments, suggesting a peptidoglycan linkage.
- Extracted M protein from 20 serotypes showed molecular weight variations from 41,000 to 80,000 Da.
- Similar size variations were observed in clinical isolates of type 6 Streptococcus, but not in laboratory-passaged or epidemic strains, indicating variation is not rapid.
Conclusions:
- Group A Streptococcus M protein exhibits significant molecular size heterogeneity, both between and within M types.
- The observed size variation in M protein is not a rapid evolutionary process.
- M protein size may not be critical for its antiphagocytic activity within the studied range.