SipA is required for pilus formation in Streptococcus pyogenes serotype M3

Pili are a major surface feature of the human pathogen Streptococcus pyogenes (group A streptococcus [GAS]). The T3 pilus is composed of a covalently linked polymer of protein T3 (formerly Orf100 or Fct3) with an ancillary protein, Cpa, attached. A putative signal peptidase, SipA (also called LepA), has been identified in several pilus gene clusters of GAS. We demonstrate that the SipA2 allele of a GAS serotype M3 strain is required for synthesis of T3 pili. Heterologous expression in Escherichia coli showed that SipA2, along with the pilus backbone protein T3 and the sortase SrtC2, is required for polymerization of the T3 protein. In addition, we found that SipA2 is also required for linkage of the ancillary pilin protein Cpa to polymerized T3. Despite partial conservation of motifs of the type I signal peptidase family proteins, SipA lacks the highly conserved and catalytically important serine and lysine residues of these enzymes. Substitution of alanine for either of the two serine residues closest to the expected location of an active site serine demonstrated that these serine residues are both dispensable for T3 polymerization. Therefore, it seems unlikely that SipA functions as a signal peptidase. However, a T3 protein mutated at the P-1 position of the signal peptide cleavage site (alanine to arginine) was unstable in the presence of SipA2, suggesting that there is an interaction between SipA and T3. A possible chaperone-like function of SipA2 in T3 pilus formation is discussed.