· H. Remaut, R.J. Rose, T.J. Hannan, S.J.
Hultgren, S.E. Radford, A.E. Ashcroft, and G. Waksman
Donor-strand exchange in chaperone-assisted pilus assembly proceeds through a
concerted beta-strand displacement mechanism
Molecular Cell. 22:
831-842. (2006)
Gram-negative pathogens commonly use the chaperone-usher pathway
to assemble adhesive multi-subunit fibres on their surface. In the periplasm,
subunits are stabilized by a chaperone that donates a beta-strand to complement
the subunits’ truncated immunoglobulin-like fold. Pilus assembly proceeds
through a “donor-strand exchange”
(DSE) mechanism whereby this complementary beta-strand is replaced by
the N-terminal extension (Nte) of an incoming pilus subunit. Using X-ray
crystallography and real-time electrospray ionisation mass spectrometry, we
demonstrate that DSE requires the formation of a transient ternary complex
between the chaperone-subunit complex and the Nte of the next subunit to be
assembled. The process is crucially dependent on an initiation site (the P5
pocket) needed to recruit the incoming Nte. The data also suggest a capping
reaction displacing DSE towards product formation. These results support a
zip-in-zip-out mechanism for donor-strand exchange and a catalytic role for the
usher, the molecular platform at which pili are assembled.