Article

Fiber Formation across the Bacterial Outer Membrane by the Chaperone/Usher Pathway

Institute of Structural Molecular Biology, University College London and Birkbeck College, Malet Street, London, WC1E 7HX, United Kingdom.
Cell (Impact Factor: 32.24). 06/2008; 133(4):640-52. DOI: 10.1016/j.cell.2008.03.033
Source: PubMed

ABSTRACT

Gram-negative pathogens commonly exhibit adhesive pili on their surfaces that mediate specific attachment to the host. A major class of pili is assembled via the chaperone/usher pathway. Here, the structural basis for pilus fiber assembly and secretion performed by the outer membrane assembly platform--the usher--is revealed by the crystal structure of the translocation domain of the P pilus usher PapC and single particle cryo-electron microscopy imaging of the FimD usher bound to a translocating type 1 pilus assembly intermediate. These structures provide molecular snapshots of a twinned-pore translocation machinery in action. Unexpectedly, only one pore is used for secretion, while both usher protomers are used for chaperone-subunit complex recruitment. The translocating pore itself comprises 24 beta strands and is occluded by a folded plug domain, likely gated by a conformationally constrained beta-hairpin. These structures capture the secretion of a virulence factor across the outer membrane of gram-negative bacteria.

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    • "The membrane protects the transmembrane portions of the protein from protease cleavage and more compact forms of the membrane protein can be produced. Such an approach was used to generate a stable 55 kDa construct of the P pilus PapC that was subsequently used for crystal structure determination (Remaut et al., 2008). "
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    • "To illustrate guidance provided by the physical model, consider the mechanisms by which the chaperone-subunit complex of a diverse group of Gram-negative bacteria [28] gains passage through the Papc usher channel at the outer membrane. Here, an approaching chaperone complex triggers a response by which outer membrane usher passage unblocks. "
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