Tunable force transduction through the Escherichia coli cell envelope

Structural architecture of TolQ-TolR inner membrane protein complex from opportunistic pathogen Acinetobacter baumannii

Gram-negative bacteria harness the proton motive force (PMF) within their inner membrane (IM) to uphold the integrity of their cell envelope, an indispensable aspect for both division and survival. The IM TolQ-TolR complex is the essential part of the Tol-Pal system, serving as a conduit for PMF energy transfer to the outer membrane. Here we present cryo-EM reconstructions of Acinetobacter baumannii TolQ in apo and TolR- bound forms at atomic resolution. The apo TolQ configuration manifests as a symmetric pentameric pore, featuring a trans-membrane funnel leading towards a cytoplasmic chamber. In contrast, the TolQ-TolR complex assumes a proton non-permeable stance, characterized by the TolQ pentamer's flexure to accommodate the TolR dimer, where two protomers undergo a translation-based relationship. Our structure-guided analysis and simulations support the rotor-stator mechanism of action, wherein the rotation of the TolQ pentamer harmonizes with the TolR protomers' interplay. These findings broaden our mechanistic comprehension of molecular stator units empowering critical functions within the Gram-negative bacterial cell envelope.

Teaser

Apo TolQ and TolQ-TolR structures depict structural rearrangements required for cell envelope organization in bacterial cell division.

Discovery and structural characterization of the D-box, a conserved TonB motif that couples an inner-membrane motor to outer-membrane transport

Gram-negative bacteria use TonB-dependent transport to take up nutrients from the external environment, employing the Ton complex to import a variety of nutrients that are either scarce or too large to cross the outer membrane unaided. The Ton complex contains an inner-membrane motor (ExbBD) that generates force, as well as nutrient-specific transport proteins on the outer membrane. These two components are coupled by TonB, which transmits the force from the inner to the outer membrane. TonB contains an N-terminus anchored in the inner membrane, a C-terminal domain that binds the outer-membrane transporter, and a proline-rich linker connecting the two. While much is known about the interaction between TonB and outer-membrane transporters, the critical interface between TonB and ExbBD is less well understood. Here, we identify a conserved motif within TonB that we term the D-box, which serves as an attachment point for ExbD. We characterize the interaction between ExbD and the D-box both functionally and structurally, showing that a homodimer of ExbD captures one copy of the D-box peptide via beta-strand recruitment. We additionally show that both the D-box motif and ExbD are conserved in a range of Gram-negative bacteria, including members of the ESKAPE group of pathogens. The ExbD:D-box interaction is likely to represent an important aspect of force transduction between the inner and outer membranes. Given that TonB-dependent transport is an important contributor to virulence, this interaction is an intriguing potential target for novel antibacterial therapies.