(A) Ribbon representation of the PapC_130–640 translocation channel viewed from the extracellular side. The β-barrel, plug domain, β5-6 hairpin, and helix α1 are colored blue, magenta, orange, and yellow, respectively. β strands are labeled β1 through β24, the labels N and C indicate the N and C termini of the translocation channel.
(B) Stereo representation of PapC_130–640 viewed from the side. Structural elements are colored as in (A). The N and C termini, helix α1, strand β4, and the β5-6 hairpin are labeled.
(C) Topology diagram of PapC_130–640. Residues in β strand, loop, or α-helical topology are indicated by squares, circles, or rectangles, respectively; they are colored blue, orange, and yellow for the β-barrel, β5-6 hairpin, and plug domain, respectively. The extracellular space, the expected OM spanning area, and periplasmic space are indicated as E, OM, and P, respectively. Strands are labeled 1–24 and βA–βF in the β-barrel and plug domain, respectively.
(D) Top (left) and side (right) views of the PapC_130–640 dimer. The β-barrel in the right protomer is shown in surface representation colored by electrostatic potential, from−3 kT/e⁻ (blue) to 8 kT/e⁻ (red). The N and C termini are labeled. Two C8E4 molecules in the dimer interface are shown in stick representation (C and O atoms are colored cyan and red, respectively). Figures 2 and 3 were generated using PyMol and APBS (http://www.pymol.org/ and http://apbs.sourceforge.net/).

(A) Gel filtration profile of FimD₂:C_His:F:G:H complexes.
(B) Coomassie blue-stained gel of the isolated tip complex. Lanes 1 and 2 represent the peak fractions from the gel filtration column as indicated in (A). The identity and molecular weight of each Fim subunit is indicated on the right, and the molar ratio of each subunit relative to FimH—estimated by densitometry of lane 1—is indicated on the left.
(C) The purified FimD:tip complexes were incubated in SDS sample buffer at 25° or 95°C, separated by gel electrophoresis, and immunoblotted with anti-FimC:F, anti-FimC:G, or anti-FimC:H polyclonal antibodies, as indicated. The FimF, G, and H subunits are engaged in donor-strand exchange, as revealed by the disappearance of bands corresponding to the pilin monomers and the appearance of a band reacting with all three antibodies in samples incubated at 25°C.
(D) Negatively stained EM image of the purified tip complexes from the fraction shown in (B), lane 1, and a single class average (inset). Individual particles are indicated with white circles.

(A) Integrated, hypothetical model of pilus subunit incorporation at the OM usher. The model shows the FimD₂:C:F:G:H complex as derived by cryo-EM (Figure 5C and main text), colored blue, dark blue, yellow, red, orange, and green for FimD usher pore 1, 2, the FimC chaperone, FimF, FimG, and the FimH adhesin, respectively. In addition, the model shows the tentative position (shaded in light gray) of an incoming FimC:A complex bound to usher 2’s N-terminal domain (FimD_N2:C:A, colored dark blue:yellow:cyan and labeled N2, FimC, and FimA, respectively). N1 indicates the N-terminal domain of usher 1.
(B) Schematic diagram of pilus assembly integrating available biological and biochemical data with the new PapC_130–640 structure and FimD₂:C:F:G:H model. The diagram shows the PapC twinned pores (in blue and dark blue for usher 1 and 2, respectively), the ushers’ N-terminal domains (N1 and N2 for usher 1 and 2 in blue and dark blue, respectively), the plug domain (P in usher 2 and P′ and P″ in usher 1 for the two alternative models of gating [see text]; in magenta), the FimH adhesin (H; in green), FimG (G; in orange), FimC:F (C1:F; in yellow:red), and an incoming FimC:A complex (C2:A; in yellow:cyan). For clarity, the C-terminal domains of ushers 1 and 2 are not shown. At left, the FimD₂:C:F:G:H complex recruits an incoming FimC:A complex through binding to the N-terminal domain of usher 2 (N2; step 1). The complex is brought within donor-strand exchange of FimF, resulting in the release of the FimF-bound chaperone (C1) and the dissociation of the N-terminal domain of usher 1 (N1) (middle panel; steps 2 and 3). N1 is now free to recruit another FimC:A complex (labeled C:A′; right panel, step 4), and bring the complex within proximity of the N2-bound FimC:A complex (step 5). Donor-strand exchange then releases N2 for recruitment of the next chaperone-subunit complex (step 6). Iteration of alternating binding to released usher N-terminal domains, followed by donor-strand exchange with the penultimate chaperone-subunit complex leads to stepwise growth of the pilus fiber (steps 1 through 6).

(A) Multisubunit structure of E. coli P pili (left) and type 1 pili (right). Subunits are represented as oval shapes colored by subunit type. The subunit’s N-terminal extensions are represented as short rectangular protrusions.
(B) Schematic diagram of the CU assembly pathway. The outer membrane, extracellular, and periplasmic spaces are labeled OM, E, and P, respectively. The periplasmic chaperone and the OM usher are colored yellow and blue, respectively; subunits are represented as in (A). Blowout topology diagrams of donor-strand complemented (lower right) and donor-strand exchanged (upper right) subunits are shown. The subunit’s N-terminal extension is labeled (Nte).
(C) Predicted domain organization in OM ushers. The OM translocation and middle domain crystallized in this study are represented in blue and magenta, respectively.

(A) Close-up of the site of inward orientation of the β5-6 hairpin. Residues in the β4 bulge and hairpin involved in the hairpin curvature are shown in stick representation. Color scheme is according to Figure 1.
(B) Close-up of the β5-6 hairpin interactions with the plug domain, helix α1, and the channel wall. Residues involved in the interaction are shown in stick representation.
(C) Charge interactions between the plug domain and the β-barrel, viewed from the periplasmic side of the membrane. Side chains of neighboring residues are labeled by residue number.

(A) Cryo-EM image of the purified tip complexes from the fraction shown in lane 1 in Figure 4B. Three individual tip complex particles are marked by a white circle.
(B) A comparison of selected 2D class averages in rows 1 and 3 with their corresponding reprojections from the 3D map in rows 2 and 4.
(C) Front (top left), side (top right), periplasmic (bottom left), and extracellular (bottom right) views of the 3D reconstruction of the FimD:tip complex rendered at 2σ (white surface), and additionally at 4.5 σ (gray), and 5σ (green) in the upper left view in order to show the cryo-EM density’s center of mass. The density regions corresponding to the outer membrane, extracellular space and periplasm (OM, E and P) are labeled as 1, 2 and 3, respectively. Available structures or closely related models (see Experimental Procedures for details) were docked into the cryo-EM density as follows: FimH adhesin (green), FimD translocation pore 1 and 2 (blue and dark blue), FimD_N:C:F (blue:yellow:red), and FimG (orange).