Decoupling Filamentous Phage Uptake and Energy of the TolQRA Motor in Escherichia coli

Structure, Biology, and Applications of Filamentous Bacteriophages

The closely related Escherichia coli Ff filamentous phages (f1, fd, and M13) have taken a fantastic journey over the past 60 years, from the urban sewerage from which they were first isolated, to their use in high-end technologies in multiple fields. Their relatively small genome size, high titers, and the virions that tolerate fusion proteins make the Ffs an ideal system for phage display. Folding of the fusions in the oxidizing environment of the E. coli periplasm makes the Ff phages a platform that allows display of eukaryotic surface and secreted proteins, including antibodies. Resistance of the Ffs to a broad range of pH and detergents facilitates affinity screening in phage display, whereas the stability of the virions at ambient temperature makes them suitable for applications in material science and nanotechnology. Among filamentous phages, only the Ffs have been used in phage display technology, because of the most advanced state of knowledge about their biology and the various tools developed for E. coli as a cloning host for them. Filamentous phages have been thought to be a rather small group, infecting mostly Gram-negative bacteria. A recent discovery of more than 10 thousand diverse filamentous phages in bacteria and archaea, however, opens a fascinating prospect for novel applications. The main aim of this review is to give detailed biological and structural information to researchers embarking on phage display projects. The secondary aim is to discuss the yet-unresolved puzzles, as well as recent developments in filamentous phage biology, from a viewpoint of their impact on current and future applications.

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.

Direct interaction between fd phage pilot protein pIII and the TolQ-TolR proton-dependent motor provides new insights into the import of filamentous phages

Filamentous phages are one of the simplest examples of viruses with a protein capsid that protects a circular single-stranded DNA genome. The infection is very specific, nonlytic, and can strongly affect the physiology or provide new pathogenic factors to its bacterial host. The infection process is proposed to rely on a pore-forming mechanism similar to that of certain nonenveloped eukaryotic viruses. The Ff coliphages (including M13, fd, and f1) have been intensively studied and were used to establish the sequence of events taking place for efficient crossing of the host envelope structure. However, the mechanism involved in the penetration of the cell inner membrane is not well understood. Here, we identify new host players involved in the phage translocation mechanism. Interaction studies by a combination of in vivo biochemical methods demonstrate that the adhesion protein pIII located at the tip of the phage binds to TolQ and TolR, two proteins that form a conserved proton-dependent molecular motor in the inner membrane of the host cell. Moreover, in vivo cysteine cross-linking studies reveal that the interactions between the pIII and TolQ or TolR occur between their transmembrane helix domains and may be responding to the proton motive force status of the cell. These results allow us to propose a model for the late stage of filamentous phage translocation mediated by multiple interactions with each individual component of the host TolQRA complex.

Energization of Outer Membrane Transport by the ExbB ExbD Molecular Motor

The outer membranes (OM) of Gram-negative bacteria contain a class of proteins (TBDTs) that require energy for the import of nutrients and to serve as receptors for phages and protein toxins. Energy is derived from the proton motif force (pmf) of the cytoplasmic membrane (CM) through the action of three proteins, namely, TonB, ExbB, and ExbD, which are located in the CM and extend into the periplasm. The leaky phenotype of exbB exbD mutants is caused by partial complementation by homologous tolQ tolR. TonB, ExbB, and ExbD are genuine components of an energy transmission system from the CM into the OM. Mutant analyses, cross-linking experiments, and most recently X-ray and cryo-EM determinations were undertaken to arrive at a model that describes the energy transfer from the CM into the OM. These results are discussed in this paper. ExbB forms a pentamer with a pore inside, in which an ExbD dimer resides. This complex harvests the energy of the pmf and transmits it to TonB. TonB interacts with the TBDT at the TonB box, which triggers a conformational change in the TBDT that releases bound nutrients and opens the pore, through which nutrients pass into the periplasm. The structurally altered TBDT also changes the interactions of its periplasmic signaling domain with anti-sigma factors, with the consequence being that the sigma factors initiate transcription.

Cryo-electron microscopy of the f1 filamentous phage reveals insights into viral infection and assembly

Phages are viruses that infect bacteria and dominate every ecosystem on our planet. As well as impacting microbial ecology, physiology and evolution, phages are exploited as tools in molecular biology and biotechnology. This is particularly true for the Ff (f1, fd or M13) phages, which represent a widely distributed group of filamentous viruses. Over nearly five decades, Ffs have seen an extraordinary range of applications, yet the complete structure of the phage capsid and consequently the mechanisms of infection and assembly remain largely mysterious. In this work, we use cryo-electron microscopy and a highly efficient system for production of short Ff-derived nanorods to determine a structure of a filamentous virus including the tips. We show that structure combined with mutagenesis can identify phage domains that are important in bacterial attack and for release of new progeny, allowing new models to be proposed for the phage lifecycle.

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

First Report of Filamentous Phages Isolated from Tunisian Orchards to Control Erwinia amylovora

Newly discovered Erwinia amylovora phages PEar1, PEar2, PEar4 and PEar6 were isolated from three different orchards in North Tunisia to study their potential as biocontrol agents. Illumina sequencing revealed that the PEar viruses carry a single-strand DNA genome between 6608 and 6801 nucleotides and belong to the Inoviridae, making them the first described filamentous phages of E. amylovora. Interestingly, phage-infected cells show a decreased swimming and swarming motility and a cocktail of the four phages can significantly reduce infection of E. amylovora in a pear bioassay, potentially making them suitable candidates for phage biocontrol.