title: The molecular switch that activates the cell wall anchoring step of pilus assembly in gram-positive bacteria.
abstract: Cell surface pili in gram-positive bacteria orchestrate the colonization of host tissues, evasion of immunity, and the development of biofilms. Recent work revealed that pilus assembly is a biphasic process wherein pilus polymerization is catalyzed by a pilus-specific sortase followed by cell wall anchoring of the pilus that is promoted by the housekeeping sortase. Here, we present molecular genetic and biochemical studies of a heterotrimeric pilus in Corynebacterium diphtheriae, uncovering the molecular switch that terminates pilus polymerization in favor of cell wall anchoring. The prototype pilus contains a major pilin (SpaA) forming the shaft, a tip pilin (SpaC), and another minor pilin (SpaB). Cells lacking SpaB form pilus fibers, but they are largely secreted in the medium, a phenotype also observed when cells lack the housekeeping sortase. Furthermore, the average pilus length is greatly increased in the absence of SpaB. Remarkably, a SpaB mutant that lacks the cell wall sorting signal but contains a critical lysine residue is incorporated in the pilus. However, the resulting pili fail to anchor to the cell wall. We propose that a specific minor pilin acts as the terminal subunit in pilus assembly. Cell wall anchoring ensues when the pilus polymer assembled on the pilus-specific sortase is transferred to the minor pilin presented by the housekeeping sortase via lysine-mediated transpeptidation.
authors.affiliation: Department of Molecular, Microbial, and Structural Biology, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030, USA.
authors.forename: Anjali
authors.initials: A
authors.lastname: Mandlik
authors.forename: Asis
authors.initials: A
authors.lastname: Das
authors.forename: Hung
authors.initials: H
authors.lastname: Ton-That
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001704:Biopolymers / Q000378:metabolism; D002473:Cell Wall / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D003353:Corynebacterium diphtheriae / Q000166:cytology* / Q000235:genetics / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000235:genetics / Q000378:metabolism; D010861:Fimbriae, Bacterial / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D046529:Microscopy, Electron, Transmission; D016253:Microscopy, Immunoelectron
chemical_list: D001426:Bacterial Proteins; D001704:Biopolymers; D008565:Membrane Proteins; D003546:Cysteine Endopeptidases
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