Assembly mechanism of FCT region type 1 pili in serotype M6 Streptococcus pyogenes

Area of Science:

• Microbiology
• Molecular Biology
• Structural Biology

Background:

• * Streptococcus pyogenes is a human pathogen that produces various pili based on serotype.
• * Pili are crucial for bacterial adhesion and pathogenesis.
• * FCT type 1 pili are a significant virulence factor in certain serotypes.

Purpose of the Study:

• * To elucidate the assembly mechanism of FCT type 1 pili in Streptococcus pyogenes serotype M6.
• * To identify the roles of sortase enzymes (SrtA and SrtB) in pilus biogenesis.
• * To characterize the functional residues and motifs involved in T6 backbone protein polymerization.

Main Methods:

• * Investigation of FCT type 1 pili assembly in a serotype M6 strain.
• * Analysis of precursor proteins T6 (backbone) and FctX (ancillary).
• * Characterization of sortase enzyme functions (SrtA and SrtB) in pilus formation and anchoring.
• * Identification of novel pilin motifs and functional residues using genetic and biochemical approaches.
• * Immunogold electron microscopy to determine protein localization.

Main Results:

• * FCT type 1 pili assembly requires both T6 and FctX precursor proteins.
• * SrtB is essential for T6-FctX complex formation and T6 polymerization.
• * SrtA is primarily responsible for anchoring pili to the cell wall.
• * A novel VAKS pilin motif in T6, containing Lys-175, is critical for isopeptide linkage and polymerization.
• * FctX is likely located at the pilus tip, with its incorporation dependent on Lys-175 and cell wall sorting signals.

Conclusions:

• * FCT type 1 pili assembly is a complex, sortase-mediated process involving specific protein interactions and motifs.
• * The VAKS motif and Lys-175 play a pivotal role in T6 polymerization.
• * The findings provide insights into the structural organization and display of pili by Streptococcus pyogenes.