Adhesive pili of the chaperone-usher family - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 359

USHERS
Domain function and selectivity
The usher of the CU pathway catalyzes the translocation and assembly of
the multisubunit pilus fiber across the outer membrane, while maintain-
ing membrane integrity. The usher is an outer membrane protein comprised
of five domains: a 24-stranded beta barrel channel, plug, N-terminal domain
(NTD), and C-terminal domains (CTD1, CTD2) ( Thanassi et al., 2002 ; Nishi-
yama et al., 2003 ; Capitani et al., 2006 ). In the apo state of the FimD usher,
the plug resides in the lumen of the transmembrane channel, preventing flow
of molecules across the outer membrane ( Remaut et al., 2008 ; Huang et al.,
2009 ); in the active form, the plug swings away from the channel, creating
an unobstructed opening through which pilins translocate ( Figure 12.4 ). This
FIGURE 12.4 Structures of the gated and open FimDusher. (A) In the gated apo state, the plug
(blue, surface representation) resides in the lumen of the usher pore (orange, ribbon diagram), pre-
venting flow of molecules across the OM and maintaining membrane integrity. The remaining usher
domains are not depicted since a FimD truncate was used to solve the apo structure [PDB code:
3OHN]. (B) In the FimD-FimC-FimH complex, the plug swings into the periplasm and binds NTD
(light blue), while FimC (green) docks at CTD1 (yellow) and CTD2 (purple) with its bound FimH
(magenta). The lectin domain of FimH now resides in the lumen of the usher. Note the unoccupied
P5 pocket (light pink) in the FimH pilin domain, which is targeted by the Nte of the incoming
subunit FimG for DSE [PDB code: 3RFZ]. Rotation of the usher by 90°, such that the usher is seen
from the top, shows a kidney-shaped pore in the plug-gated usher but a nearly circular pore in the
secreting usher. ECF, extracellular fluid; OM, outer membrane; PP, periplasm.