Structural and functional diversity of type IV secretion systems.

Considerable progress has been made in recent years in the structural and molecular biology of type IV secretion systems in Gram-negative bacteria. The latest advances have substantially improved our understanding of the mechanisms underlying the recruitment and delivery of DNA and protein substrates to the extracellular environment or target cells. In this Review, we aim to summarize these exciting structural and molecular biology findings and to discuss their functional implications for substrate recognition, recruitment and translocation, as well as the biogenesis of extracellular pili. We also describe adaptations necessary for deploying a breadth of processes, such as bacterial survival, host-pathogen interactions and biotic and abiotic adhesion. We highlight the functional and structural diversity that allows this extremely versatile secretion superfamily to function under different environmental conditions and in different bacterial species. Additionally, we emphasize the importance of further understanding the mechanism of type IV secretion, which will support us in combating antimicrobial resistance and treating type IV secretion system-related infections.

Substrate recruitment mechanism by gram-negative type III, IV, and VI bacterial injectisomes.

Bacteria use a wide arsenal of macromolecular substrates (DNA and proteins) to interact with or infect prokaryotic and eukaryotic cells. To do so, they utilize substrate-injecting secretion systems or injectisomes. However, prior to secretion, substrates must be recruited to specialized recruitment platforms and then handed over to the secretion apparatus for secretion. In this review, we provide an update on recent advances in substrate recruitment and delivery by gram-negative bacterial recruitment platforms associated with Type III, IV, and VI secretion systems.

Cryo-EM structure of a type IV secretion system.

Bacterial conjugation is the fundamental process of unidirectional transfer of DNAs, often plasmid DNAs, from a donor cell to a recipient cell1. It is the primary means by which antibiotic resistance genes spread among bacterial populations2,3. In Gram-negative bacteria, conjugation is mediated by a large transport apparatus-the conjugative type IV secretion system (T4SS)-produced by the donor cell and embedded in both its outer and inner membranes. The T4SS also elaborates a long extracellular filament-the conjugative pilus-that is essential for DNA transfer4,5. Here we present a high-resolution cryo-electron microscopy (cryo-EM) structure of a 2.8 megadalton T4SS complex composed of 92 polypeptides representing 8 of the 10 essential T4SS components involved in pilus biogenesis. We added the two remaining components to the structural model using co-evolution analysis of protein interfaces, to enable the reconstitution of the entire system including the pilus. This structure describes the exceptionally large protein-protein interaction network required to assemble the many components that constitute a T4SS and provides insights on the unique mechanism by which they elaborate pili.

Proteins DotY and DotZ modulate the dynamics and localization of the type IVB coupling complex of Legionella pneumophila.

Legionella pneumophila is an opportunistic pathogen infecting alveolar macrophages and protozoa species. Legionella utilizes a Type IV Secretion System (T4SS) to translocate over 300 effector proteins into its host cell. In a recent study, we have isolated and solved the cryo-EM structure of the Type IV Coupling Complex (T4CC), a large cytoplasmic determinant associated with the inner membrane that recruits effector proteins for delivery to the T4SS for translocation. The T4CC is composed of a DotLMNYZ hetero-pentameric core from which the flexible IcmSW module flexibly protrudes. The DotY and DotZ proteins were newly reported members of this complex and their role remained elusive. In this study, we observed the effect of deleting DotY and DotZ on T4CC stability and localization. Furthermore, we found these two proteins are co-dependent, whereby the deletion of DotY resulted in DotZ absence from the coupling complex, and vice versa. Additional cryo-EM data analysis revealed the dynamic movement of the IcmSW module is modified by the DotY/Z proteins. We therefore determined the likely function of DotY and DotZ and revealed their importance on T4CC function.

Mechanism of effector capture and delivery by the type IV secretion system from Legionella pneumophila.

Legionella pneumophila is a bacterial pathogen that utilises a Type IV secretion (T4S) system to inject effector proteins into human macrophages. Essential to the recruitment and delivery of effectors to the T4S machinery is the membrane-embedded T4 coupling complex (T4CC). Here, we purify an intact T4CC from the Legionella membrane. It contains the DotL ATPase, the DotM and DotN proteins, the chaperone module IcmSW, and two previously uncharacterised proteins, DotY and DotZ. The atomic resolution structure reveals a DotLMNYZ hetero-pentameric core from which the flexible IcmSW module protrudes. Six of these hetero-pentameric complexes may assemble into a 1.6-MDa hexameric nanomachine, forming an inner membrane channel for effectors to pass through. Analysis of multiple cryo EM maps, further modelling and mutagenesis provide working models for the mechanism for binding and delivery of two essential classes of Legionella effectors, depending on IcmSW or DotM, respectively.

Structural insights and activating mutations in diverse pathologies define mechanisms of deregulation for phospholipase C gamma enzymes.

BACKGROUND: PLCγ enzymes are key nodes in cellular signal transduction and their mutated and rare variants have been recently implicated in development of a range of diseases with unmet need including cancer, complex immune disorders, inflammation and neurodegenerative diseases. However, molecular nature of activation and the impact and dysregulation mechanisms by mutations, remain unclear; both are critically dependent on comprehensive characterization of the intact PLCγ enzymes. METHODS: For structural studies we applied cryo-EM, cross-linking mass spectrometry and hydrogen-deuterium exchange mass spectrometry. In parallel, we compiled mutations linked to main pathologies, established their distribution and assessed their impact in cells and in vitro. FINDINGS: We define structure of a complex containing an intact, autoinhibited PLCγ1 and the intracellular part of FGFR1 and show that the interaction is centred on the nSH2 domain of PLCγ1. We define the architecture of PLCγ1 where an autoinhibitory interface involves the cSH2, spPH, TIM-barrel and C2 domains; this relative orientation occludes PLCγ1 access to its substrate. Based on this framework and functional characterization, the mechanism leading to an increase in PLCγ1 activity for the largest group of mutations is consistent with the major, direct impact on the autoinhibitory interface. INTERPRETATION: We reveal features of PLCγ enzymes that are important for determining their activation status. Targeting such features, as an alternative to targeting the PLC active site that has so far not been achieved for any PLC, could provide new routes for clinical interventions related to various pathologies driven by PLCγ deregulation. FUND: CR UK, MRC and AstaZeneca.

The structure of an elongation factor G-ribosome complex captured in the absence of inhibitors.

During translation's elongation cycle, elongation factor G (EF-G) promotes messenger and transfer RNA translocation through the ribosome. Until now, the structures reported for EF-G-ribosome complexes have been obtained by trapping EF-G in the ribosome. These results were based on use of non-hydrolyzable guanosine 5'-triphosphate (GTP) analogs, specific inhibitors or a mutated EF-G form. Here, we present the first cryo-electron microscopy structure of EF-G bound to ribosome in the absence of an inhibitor. The structure reveals a natural conformation of EF-G·GDP in the ribosome, with a previously unseen conformation of its third domain. These data show how EF-G must affect translocation, and suggest the molecular mechanism by which fusidic acid antibiotic prevents the release of EF-G after GTP hydrolysis.

A Genetic Tool to Quantify trans-Translation Activity in Vivo.

In bacteria, trans-translation is the main quality control mechanism for rescuing ribosomes arrested during translation. This key process is universally conserved and plays a critical role in the viability and virulence of many pathogens. We developed a reliable in vivo double-fluorescence reporter system for the simultaneous quantification of both trans-translation and the associated proteolysis activities in bacteria. The assay was validated using mutant bacteria lacking tmRNA, SmpB, and the ClpP protease. Both antisense tmRNA-binding RNA and a peptide mimicking the SmpB C-terminal tail proved to be potent inhibitors of trans-translation in vivo. The double-fluorescent reporter was also tested with KKL-35, an oxadiazole derivative that is supposed to be a promising trans-translation inhibitor, and it surprisingly turns out that trans-translation is not the only target of KKL-35 in vivo.

Origins of tmRNA: the missing link in the birth of protein synthesis?

The RNA world hypothesis refers to the early period on earth in which RNA was central in assuring both genetic continuity and catalysis. The end of this era coincided with the development of the genetic code and protein synthesis, symbolized by the apparition of the first non-random messenger RNA (mRNA). Modern transfer-messenger RNA (tmRNA) is a unique hybrid molecule which has the properties of both mRNA and transfer RNA (tRNA). It acts as a key molecule during trans-translation, a major quality control pathway of modern bacterial protein synthesis. tmRNA shares many common characteristics with ancestral RNA. Here, we present a model in which proto-tmRNAs were the first molecules on earth to support non-random protein synthesis, explaining the emergence of early genetic code. In this way, proto-tmRNA could be the missing link between the first mRNA and tRNA molecules and modern ribosome-mediated protein synthesis.

[Protein synthesis by the ribosome: a pathway full of pitfalls]. / La synthèse des protéines par le ribosome - Un chemin semé d'embûches.

Protein synthesis is accomplished through a process known as translation and is carried out by the ribosome, a large macromolecular complex found in every living organism. Given the huge amount of biological data that must be deciphered, it is not uncommon for ribosomes to regularly stall during the process of translation. Any disruption of this finely tuned process will jeopardize the viability of the cell. In bacteria, the main quality-control mechanism for rescuing ribosomes that undergo arrest during translation is trans-translation, which is performed by transfer-messenger RNA (tmRNA) in association with small protein B (SmPB). However, other rescue systems have been discovered recently, revealing a far more complicated network of factors dedicated to ribosome rescue. These discoveries make it possible to consider inhibition of these pathways as a very promising target for the discovery of new antibiotics.

Trans-translation exposed: understanding the structures and functions of tmRNA-SmpB.

Ribosome stalling is a serious issue for cell survival. In bacteria, the primary rescue system is trans-translation, performed by tmRNA and its protein partner small protein B (SmpB). Since its discovery almost 20 years ago, biochemical, genetic, and structural studies have paved the way to a better understanding of how this sophisticated process takes place at the cellular and molecular levels. Here we describe the molecular details of trans-translation, with special mention of recent cryo-electron microscopy and crystal structures that have helped explain how the huge tmRNA-SmpB complex targets and delivers stalled ribosomes without interfering with canonical translation.