Enteric bacteria such as Escherichia coli must tolerate high levels of bile salts, powerful detergents that disrupt biological membranes. The outer membrane barrier of gram-negative bacteria plays an important role in this resistance, but ultimately it can only retard the influx of bile salts. We therefore examined whether E. coli possessed an energy-dependent efflux mechanism for these compounds. Intact cells of E. coli K-12 appeared to pump out chenodeoxycholate, since its intracellular accumulation increased more than twofold upon deenergization of the cytoplasmic membrane by a proton conductor. Growth inhibition by bile salts and accumulation levels of chenodeoxycholate increased when mutations inactivating the acrAB and emrAB gene clusters were introduced. The AcrAB system especially appeared to play a significant role in bile acid efflux. However, another efflux system(s) also plays an important role, since the accumulation level of chenodeoxycholate increased strongly upon deenergization of acrA emrB double mutant cells. Everted membrane vesicles accumulated taurocholate in an energy-dependent manner, apparently consuming ⌬pH without affecting ⌬⌿.The efflux thus appears to be catalyzed by a proton antiporter. Accumulation by the everted membrane vesicles was not decreased by mutations in acr and emrB genes and presumably reflects activity of the unknown system seen in intact cells. It followed saturation kinetics with V max and K m values in the neighborhood of 0.3 nmol min ؊1 mg of protein ؊1 and 50 M, respectively.

Type 1 pili are the archetypal representative of a widespread class of adhesive multisubunit fibres in Gram-negative bacteria. During pilus assembly, subunits dock as chaperone-bound complexes to an usher, which catalyzes their polymerization and mediates pilus translocation across the outer membrane. We report the crystal structure of the full-length FimD usher bound to the FimC:FimH chaperone:adhesin complex and that of the unbound form of the FimD translocation domain. The FimD:FimC:FimH structure shows FimH inserted inside the FimD 24-stranded β-barrel translocation channel. FimC:FimH is held in place through interactions with the two C-terminal periplasmic domains of FimD, a binding mode confirmed in solution by electron paramagnetic resonance spectroscopy. To accommodate FimH, the usher plug domain is displaced from the barrel lumen to the periplasm, concomitant with a dramatic conformational change in the β-barrel. The N-terminal domain of FimD is observed in an ideal position to catalyse incorporation of a newly recruited chaperone:subunit complex. The FimD:FimC:FimH structure provides unique insights into the pilus subunit incorporation cycle, and captures the first view of a protein transporter in the act of secreting its cognate substrate.

Gram-negative pathogens commonly exhibit adhesive pili on their surfaces that mediate specific attachment to the host. A major class of pili is assembled via the chaperone/usher pathway. Here, the structural basis for pilus fiber assembly and secretion performed by the outer membrane assembly platform--the usher--is revealed by the crystal structure of the translocation domain of the P pilus usher PapC and single particle cryo-electron microscopy imaging of the FimD usher bound to a translocating type 1 pilus assembly intermediate. These structures provide molecular snapshots of a twinned-pore translocation machinery in action. Unexpectedly, only one pore is used for secretion, while both usher protomers are used for chaperone-subunit complex recruitment. The translocating pore itself comprises 24 beta strands and is occluded by a folded plug domain, likely gated by a conformationally constrained beta-hairpin. These structures capture the secretion of a virulence factor across the outer membrane of gram-negative bacteria.

The biogenesis of diverse adhesive structures in a variety of Gram-negative bacterial species is dependent on the chaperone/usher pathway. Very little is known about how the usher protein translocates protein subunits across the outer membrane or how assembly of these adhesive structures occurs. We have discovered several mechanisms by which the usher protein acts to regulate the ordered assembly of type 1 pili, specifically through critical interactions of the chaperone-adhesin complex with the usher. A study of association and dissociation events of chaperone-subunit complexes with the usher in real time using surface plasmon resonance revealed that the chaperone-adhesin complex has the tightest and fastest association with the usher. This suggests that kinetic partitioning of chaperone-adhesin complexes to the usher is a defining factor in tip localization of the adhesin in the pilus. Furthermore, we identified and purified a chaperoneadhesin-usher assembly intermediate that was formed in vivo. Trypsin digestion assays showed that the usher in this complex was in an altered conformation, which was maintained during pilus assembly. The data support a model in which binding of the chaperoneadhesin complex to the usher stabilizes the usher in an assembly-competent conformation and allows initiation of pilus assembly.

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