Chaperone-usher pathways: diversity and pilus assembly mechanism

Abstract

Up to eight different types of secretion systems, and several more subtypes, have been described in Gram-negative bacteria. Here, we focus on the diversity and assembly mechanism of one of the best-studied secretion systems, the widespread chaperone-usher pathway known to assemble and secrete adhesive surface structures, called pili or fimbriae, which play essential roles in targeting bacterial pathogens to the host.

Figure 1.

A schematic of (a) P and (b) type 1 pili assembled by the Pap and Fim systems, respectively. The chaperones attached to the last subunit to be incorporated into each pilus are shown in yellow. Periplasmic chaperones assist in folding pilus subunits and targeting them to the OM usher. P pili are terminated at the OM by the termination subunit, PapH. No such subunit is known in the Fim system. The periplasmic NTD and CTDs, respectively, of the usher are indicated with the letters N and C.

Figure 2.

Donor-strand complementation (DSC) and exchange. (a) A topology diagram of the FimH pilin domain (FimHp; orange) complemented via DSC with the G1 strand of the chaperone FimC (yellow). Arrows and cylinders represent β-strands and α-helices, respectively. The C- and the N-terminus are indicated. (b) A topology diagram of donor-strand exchange (DSE) between FimHp (orange) and FimG (red). The red arrow in the upper diagram represents the N-terminal extension (Nte) of the incoming subunit FimG, complementing in trans the hydrophobic groove of FimHp. The blue arrow in the lower panel represents the Nte of the incoming FimF subunit. (c) A surface and stick representation of the FimH pilin domain (orange) in complex with chaperone FimC (yellow) during DSC. The empty P5 pocket where the incoming subunit starts the zip-in, zip-out mechanism is shown. For clarity, only the G1 strand of FimC is shown.

Figure 3.

Structures of the apo and activated (FimC–FimH-engaged) FimD usher pore. (a) Top and (b) side view ribbon representations of the superimposed apo-FimD (slate) and activated FimD (green) β-barrel. The plug domain in the channel lumen in apo FimD (magenta) rotates into the periplasm following FimD activation (pink). (c) Top view surface representation of the apo-FimD. (d) Activated FimD with FimH lectin domain (FimH_L, orange) in the translocation pore. The plug and FimH_L are coloured magenta and orange, respectively.

Figure 4.

Usher-mediated pilus biogenesis. (a) Schematic of the domains of the chaperone FimC, the usher FimD and the adhesin FimH. Numbers indicate residues where the respective domains start and end. (b) FimD–FimC–FimH complex. Side view ribbon representation of FimD (green), FimC (yellow) and surface representation of FimH (orange). The NTD, CTD1 and CTD2 are coloured light blue, grey and purple, respectively; the plug is coloured magenta. The FimC G1 strand complementing the groove (P1–P4 pockets) of FimHp is coloured cyan. (c) Superposition of FimD–FimC–FimH complex with the FimD_N–FimC–FimF structure (PDB 3BWU) leading to the proposed chaperone-subunit incorporation cycle at the FimD usher (see text and figure 5). The Nte of the incoming modelled subunit FimG occupying the P5 pocket is coloured purple.

Figure 5.

Schematic of the proposed chaperone–subunit incorporation cycle. Initial targeting of the periplasmic chaperone–subunit complex (FimC′–FimG) to the NTD of the outer membrane usher (FimD), followed by DSE between the previously assembled subunit (FimH) and the incoming subunit FimG via a ‘zip-in–zip-out’ mechanism, which releases the chaperone FimC from FimH. Subsequently, the incoming FimG–FimC′ complex is handed over from the NTD to the CTDs, and the nascent pilus is secreted.