Structure and activity of Streptococcus pyogenes SipA: a signal peptidase-like protein essential for pilus polymerisation

PLoS One. 2014 Jun 9;9(6):e99135. doi: 10.1371/journal.pone.0099135. eCollection 2014.

Abstract

The pili expressed on the surface of the human pathogen Streptococcus pyogenes play an important role in host cell attachment, colonisation and pathogenesis. These pili are built from two or three components, an adhesin subunit at the tip, a major pilin that forms a polymeric shaft, and a basal pilin that is attached to the cell wall. Assembly is carried out by specific sortase (cysteine transpeptidase) enzyme. These components are encoded in a small gene cluster within the S. pyogenes genome, often together with another protein, SipA, whose function is unknown. We show through functional assays, carried out by expressing the S. pyogenes pilus components in Lactococcus lactis, SipA from the clinically important M1T1 strain is essential for pilus assembly, and that SipA function is likely to be conserved in all S. pyogenes. From the crystal structure of SipA we confirm that SipA belongs to the family of bacterial signal peptidases (SPases), which process the signal-peptides of secreted proteins. In contrast to a previous arm-swapped SipA dimer, this present structure shows that its principal domain closely resembles the catalytic domain of SPases and has a very similar peptide-binding cleft, but it lacks the catalytic Ser and Lys residues characteristic of SPases. In SipA these are replaced by Asp and Gly residues, which play no part in activity. We propose that SipA functions by binding a key component at the bacterial cell surface, in a conformation that facilitates pilus assembly.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Crystallography, X-Ray
  • Fimbriae, Bacterial*
  • Humans
  • Membrane Proteins / chemistry*
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Protein Structure, Tertiary
  • Serine Endopeptidases / chemistry*
  • Serine Endopeptidases / genetics
  • Serine Endopeptidases / metabolism
  • Streptococcus pyogenes / chemistry
  • Streptococcus pyogenes / enzymology*
  • Streptococcus pyogenes / genetics

Substances

  • Bacterial Proteins
  • Membrane Proteins
  • Serine Endopeptidases
  • type I signal peptidase

Grant support

This work was supported by the Health Research Council of New Zealand. Access to the Australian Synchrotron was funded by the New Zealand Synchrotron Group Ltd. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.