Repeating covalent structure of streptococcal M protein

Area of Science:

• Microbiology
• Immunology
• Structural Biology

Background:

• Group A streptococci possess M proteins that induce type-specific protective immunity.
• The M protein's exact structure and its role in immune evasion are not fully understood.

Purpose of the Study:

• To elucidate the covalent structure of the M protein responsible for type-specific immunity.
• To identify the specific regions within the M protein that confer antiphagocytic properties.

Main Methods:

• Extraction and purification of M protein from type 24 streptococci.
• Cleavage of M protein using cyanogen bromide and purification of resulting peptides.
• Automated Edman degradation to determine the primary amino acid sequences of peptides.

Main Results:

• Seven M protein peptides were purified, accounting for the entire amino acid content.
• Each peptide contained the type-specific determinant inhibiting opsonic antibodies.
• Two peptides shared identical amino-terminal sequences with the uncleaved M protein, while the other five had distinct sequences.

Conclusions:

• The M protein's type-specific immunoreactivity and antiphagocytic function are likely due to repeating amino acid sequences.
• These findings provide insights into the structural basis of M protein-mediated immune evasion in group A streptococci.