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conformational rotation of plug and NTD must be powered from the energy of
binding the chaperone-adhesin complex.
While the periplasmic domains NTD and CTD are freely mobile in the apo
state ( Remaut et al., 2008 ), once plug has swung into the periplasm and formed
a complex with NTD ( Phan et al., 2011 ; Volkan et al . , 2012 ), the periplasmic
domains must make deliberate and concerted movements for the efficient
catalysis of pilus growth. Unfortunately, nothing is known about the dynam-
ics involved in subsequent steps in pilus assembly. In our modeling efforts, a
stationary in silico model of FimC-FimG docked at the NTD of the FimD-
FimC-FimH complex shows steric clashes between the two chaperone-subunit
complexes. The Nte of FimF, the incoming subunit, is almost but not quite
properly oriented above the P5 pocket of FimH for DSE, suggesting an undis-
covered usher conformation that partakes in the DSE reaction. Presumably, the
chaperone-subunit complexes shuffle across these periplasmic domains so that
the fiber can ascend through the channel and so that periplasmic domains are
freed up for incoming chaperone-subunit complexes. Whether this shuffling
occurs by rigid body movements of the NTD, CTDs, or both toward each other
is unknown. Further work is needed to assess the dynamics and conformational
changes within usher domains, which will greatly benefit our understanding of
DSE and catalysis of pilus assembly.
Working model
The results and associated interpretations presented above suggest the following
working model of CU pilus biogenesis ( Figure 12.5 ):
1. The usher adopts a gated conformation in its inactive state, in which its plug
domain lies in the center of the kidney-shaped β-barrel. NTD and CTD lie
disordered in the periplasm, moving rapidly and randomly.
2. Once NTD binds the initiator of pilus assembly, the chaperone-adhesin
complex, the binding energy induces movement in the NTD linker that
thereby transduces conformational changes in the β-barrel domain, relax-
ing its shape from ovular to circular and preventing plug from re-entering
the channel lumen once it has already entered the periplasm. Concurrently,
NTD may swing to transfer its bound chaperone-adhesin complex to CTDs.
Once NTD has unloaded its cargo, it binds the now periplasmic plug domain
to form the NTD-plug complex, which can then bind and dock other incom-
ing chaperone-subunit complexes.
3. Alternatively or concomitantly, the CTDs may swing over together to the
NTD-plug complex to catalyze the dissociation of the chaperone-adhesin
complex. A minor conformational species in the FimD-FimC-FimH com-
plex as measured by electron paramagnetic resonance (EPR) ( Phan et al.,
2011 ) suggests the occurrence of an uncharacterized transient binding
interaction between CTD2 and the chaperone-adhesin complex that may
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