Bacterial pili are long, multi-subunit protein assemblies that extend from bacterial surfaces, mediating adhesion and colonisation. The recently characterised pili expressed by Gram-positive pathogens represent a novel variation; completely covalent polymers in which sortase-mediated isopeptide bonds link successive pilin subunits. Recent structural studies of the component pilins have revealed a common pattern of tandem immunoglobulin (Ig)-like domains, joined end-on-end. This long thin assembly is further stabilised by autocatalytically generated isopeptide bond crosslinks within the domains, joining Lys and Asn(or Asp) side chains. Specialised subunits at the tip and the base complete the assembly, with the tip pilins presenting novel adhesive structures.
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