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EMBO reports 7, 7, 734–738 (2006)
doi:10.1038/sj.embor.7400722
AOP Published online: 9 June 2006
Mechanism of fibre assembly through the chaperone–usher pathway
Michael Vetsch†, Denis Erilov, Noël Molière, Mireille Nishiyama, Oleksandr Ignatov & Rudi Glockshuber
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Institute for Molecular Biology and Biophysics, ETH Zurich, Schafmattstrasse 20, 8093 Zurich, Switzerland
To whom correspondence should be addressed
Rudi Glockshuber Tel: +41 1 633 6819; Fax: +41 1 633 1036; E-mail: rudi@mol.biol.ethz.ch
† Present address: Novartis Pharma AG, 4002 Basel, Switzerland
Received 21 February 2006; Accepted 27 April 2006; Published online 9 June 2006.
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Abstract
The chaperone–usher pathway directs the formation of adhesive surface fibres in numerous pathogenic Gram-negative bacteria. The fibres or pili consist exclusively of protein subunits that, before assembly, form transient complexes with a chaperone in the periplasm. In these chaperone:subunit complexes, the chaperone donates one  -strand to complete the imperfect immunoglobulin-like fold of the subunit. During pilus assembly, the chaperone is replaced by a polypeptide extension of another subunit in a process termed 'donor strand exchange' (DSE). Here we show that DSE occurs in a concerted reaction in which a chaperone-bound acceptor subunit is attacked by another chaperone-bound donor subunit. We provide evidence that efficient DSE requires interactions between the reacting subunits in addition to those involving the attacking donor strand. Our results indicate that the pilus assembly platforms in the outer membrane, referred to as ushers, catalyse fibre formation by increasing the effective concentrations of donor and acceptor subunits.
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