Structural differences between the Streptococcus agalactiae housekeeping and pilus-specific sortases: SrtA and SrtC1.

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Sortases are transpeptidase enzymes essential for pilus assembly on Gram-positive bacterial cell walls.
• Streptococcus agalactiae utilizes pilus-specific sortases (SrtC1, SrtC2) and a housekeeping sortase (SrtA) for pilus formation and cell wall anchoring.

Purpose of the Study:

• To elucidate the structural basis of sortase function in Streptococcus agalactiae.
• To understand how structural variations in sortases dictate their specific roles in pili biogenesis.

Main Methods:

• Determined crystal structures of Streptococcus agalactiae SrtC1 and SrtA.
• Analyzed the active site architecture and compared structural features with known sortases.
• Determined the crystal structure of a SrtC1 mutant with an altered sorting signal motif.

Main Results:

• Both SrtC1 and SrtA possess an eight-stranded beta-barrel core with distinct active site variations.
• SrtA displays a catalytic triad arrangement similar to S. pyogenes SrtA but differs from S. aureus SrtA.
• SrtC1 features an N-terminal helical domain and an active site 'lid', differing subtly from S. pneumoniae pilus-specific sortases.

Conclusions:

• Structural differences in the active site and 'lid' region of sortases are critical for their specific functions in pili biogenesis.
• The study provides insights into the molecular mechanisms underlying sortase-mediated pilus assembly and cell wall anchoring.