SipA is required for pilus formation in Streptococcus pyogenes serotype M3

Area of Science:

• Microbiology
• Molecular Biology
• Structural Biology

Background:

• Pili are critical surface structures of Streptococcus pyogenes (GAS), a human pathogen.
• The T3 pilus is a complex structure involving the backbone protein T3 and ancillary protein Cpa.
• SipA, a putative signal peptidase, has been identified in GAS pilus gene clusters.

Purpose of the Study:

• To investigate the role of the SipA2 allele in T3 pilus synthesis in GAS.
• To determine the function of SipA2 in the polymerization of T3 protein and the attachment of Cpa.
• To explore the potential enzymatic or chaperone activity of SipA2 in pilus biogenesis.

Main Methods:

• Heterologous expression of GAS pilus components in Escherichia coli.
• Analysis of T3 pilus formation and protein polymerization.
• Site-directed mutagenesis of SipA2 and T3 proteins to assess functional roles.

Main Results:

• SipA2 is indispensable for T3 pilus synthesis in GAS.
• SipA2, T3, and SrtC2 are required for T3 protein polymerization.
• SipA2 is necessary for the linkage of Cpa to polymerized T3.
• SipA2 does not appear to function as a signal peptidase, as key catalytic residues are absent and dispensable.
• Mutated T3 protein showed instability with SipA2, suggesting a direct interaction.

Conclusions:

• SipA2 plays a crucial role in multiple steps of T3 pilus assembly.
• The findings suggest SipA2 may function as a chaperone rather than a signal peptidase.
• Further investigation into the chaperone-like function of SipA2 in T3 pilus formation is warranted.