Inner membrane YfgM–PpiD heterodimer acts as a functional unit that associates with the SecY/E/G translocon and promotes protein translocation

Cryo-EM structure of the bacterial intramembrane metalloprotease RseP in the substrate-bound state

Site-2 proteases (S2Ps), conserved intramembrane metalloproteases that maintain cellular homeostasis, are associated with chronic infection and persistence leading to multidrug resistance in bacterial pathogens. A structural model of how S2Ps discriminate and accommodate substrates could help us develop selective antimicrobial agents. We previously proposed that the Escherichia coli S2P RseP unwinds helical substrate segments before cleavage, but the mechanism for accommodating a full-length membrane-spanning substrate remained unclear. Our present cryo-EM analysis of Aquifex aeolicus RseP (AaRseP) revealed that a substrate-like membrane protein fragment from the expression host occupied the active site while spanning a transmembrane cavity that is inaccessible via lateral diffusion. Furthermore, in vivo photocrosslinking supported that this substrate accommodation mode is recapitulated on the cell membrane. Our results suggest that the substrate accommodation by threading through a conserved membrane-associated region stabilizes the substrate-complex and contributes to substrate discrimination on the membrane.

A human gut bacterium antagonizes neighboring bacteria by altering their protein-folding ability

Antagonistic interactions play a key role in determining microbial community dynamics. Here, we report that one of the most widespread contact-dependent effectors in human gut microbiomes, Bte1, directly targets the PpiD-YfgM periplasmic chaperone complex in related microbes. Structural, biochemical, and genetic characterization of this interaction reveals that Bte1 reverses the activity of the chaperone complex, promoting substrate aggregation and toxicity. Using Bacteroides, we show that Bte1 is active in the mammalian gut, conferring a fitness advantage to expressing strains. Recipient cells targeted by Bte1 exhibit sensitivity to membrane-compromising conditions, and human gut microbes can use this effector to exploit pathogen-induced inflammation in the gut. Further, Bte1 allelic variation in gut metagenomes provides evidence for an arms race between Bte1-encoding and immunity-encoding strains in humans. Together, these studies demonstrate that human gut microbes alter the protein-folding capacity of neighboring cells and suggest strategies for manipulating community dynamics.

AFM observation of protein translocation mediated by one unit of SecYEG-SecA complex

Protein translocation across cellular membranes is an essential and nano-scale dynamic process. In the bacterial cytoplasmic membrane, the core proteins in this process are a membrane protein complex, SecYEG, corresponding to the eukaryotic Sec61 complex, and a cytoplasmic protein, SecA ATPase. Despite more than three decades of extensive research on Sec proteins, from genetic experiments to cutting-edge single-molecule analyses, no study has visually demonstrated protein translocation. Here, we visualize the translocation, via one unit of a SecYEG-SecA-embedded nanodisc, of an unfolded substrate protein by high-speed atomic force microscopy (HS-AFM). Additionally, the uniform unidirectional distribution of nanodiscs on a mica substrate enables the HS-AFM image data analysis, revealing dynamic structural changes in the polypeptide-crosslinking domain of SecA between wide-open and closed states depending on nucleotides. The nanodisc-AFM approach will allow us to execute detailed analyses of Sec proteins as well as visualize nano-scale events of other membrane proteins.

Translation arrest cancellation of VemP, a secretion monitor in Vibrio, is regulated by multiple cis and trans factors, including SecY

VemP is a secretory protein in the Vibrio species that monitors cellular protein-transport activity through its translation arrest, allowing expression of the downstream secD2-secF2 genes in the same operon, which encode components of the protein translocation machinery. When cellular protein-transport function is fully active, secD2/F2 expression remains repressed as VemP translation arrest is canceled immediately. The VemP arrest cancellation occurs on the SecY/E/G translocon in a late stage in the translocation process and requires both trans factors, SecD/F and PpiD/YfgM, and a cis element, Arg-85 in VemP; however, the detailed molecular mechanism remains elusive. This study aimed to elucidate how VemP passing through SecY specifically monitors SecD/F function. Genetic and biochemical studies showed that SecY is involved in the VemP arrest cancellation and that the arrested VemP is stably associated with a specific site in the protein-conducting pore of SecY. VemP-Bla reporter analyses revealed that a short hydrophobic segment adjacent to Arg-85 plays a critical role in the regulated arrest cancellation with its hydrophobicity correlating with the stability of the VemP arrest. We identified Gln-65 and Pro-67 in VemP as novel elements important for the regulation. We propose a model for the regulation of the VemP arrest cancellation by multiple cis elements and trans factors with different roles.

Rcs signal transduction system in Escherichia coli: Composition, related functions, regulatory mechanism, and applications

The regulator of capsule synthesis (Rcs) system, an atypical two-component system prevalent in numerous gram-negative bacteria, serves as a sophisticated regulatory phosphorylation cascade mechanism. It plays a pivotal role in perceiving environmental stress and regulating the expression of downstream genes to ensure host survival. During the signaling transduction process, various proteins participate in phosphorylation to further modulate signal inputs and outputs. Although the structure of core proteins related to the Rcs system has been partially well-defined, and two models have been proposed to elucidate the intricate molecular mechanisms underlying signal sensing, a systematic characterization of the signal transduction process of the Rcs system remains challenging. Furthermore, exploring its corresponding regulator outputs is also unremitting. This review aimed to shed light on the regulation of bacterial virulence by the Rcs system. Moreover, with the assistance of the Rcs system, biosynthesis technology has developed high-value target production. Additionally, via this review, we propose designing chimeric Rcs biosensor systems to expand their application as synthesis tools. Finally, unsolved challenges are highlighted to provide the basic direction for future development of the Rcs system.

Structural analysis of the dynamic ribosome-translocon complex

The protein translocon at the endoplasmic reticulum comprises the Sec61 translocation channel and numerous accessory factors that collectively facilitate the biogenesis of secretory and membrane proteins. Here, we leveraged recent advances in cryo-electron microscopy (cryo-EM) and structure prediction to derive insights into several novel configurations of the ribosome-translocon complex. We show how a transmembrane domain (TMD) in a looped configuration passes through the Sec61 lateral gate during membrane insertion; how a nascent chain can bind and constrain the conformation of ribosomal protein uL22; and how the translocon-associated protein (TRAP) complex can adjust its position during different stages of protein biogenesis. Most unexpectedly, we find that a large proportion of translocon complexes contains RAMP4 intercalated into Sec61's lateral gate, widening Sec61's central pore and contributing to its hydrophilic interior. These structures lead to mechanistic hypotheses for translocon function and highlight a remarkably plastic machinery whose conformations and composition adjust dynamically to its diverse range of substrates.

Involvement of the Cell Division Protein DamX in the Infection Process of Bacteriophage T4

The molecular mechanism of how the infecting DNA of bacteriophage T4 passes from the capsid through the bacterial cell wall and enters the cytoplasm is essentially unknown. After adsorption, the short tail fibers of the infecting phage extend from the baseplate and trigger the contraction of the tail sheath, leading to a puncturing of the outer membrane by the tail tip needle composed of the proteins gp5.4, gp5 and gp27. To explore the events that occur in the periplasm and at the inner membrane, we constructed T4 phages that have a modified gp27 in their tail tip with a His-tag. Shortly after infection with these phages, cells were chemically cross-linked and solubilized. The cross-linked products were affinity-purified on a nickel column and the co-purified proteins were identified by mass spectrometry, and we found that predominantly the inner membrane proteins DamX, SdhA and PpiD were cross-linked. The same partner proteins were identified when purified gp27 was added to Escherichia coli spheroplasts, suggesting a direct protein-protein interaction.

Dynamic coupling of fast channel gating with slow ATP-turnover underpins protein transport through the Sec translocon

The Sec translocon is a highly conserved membrane assembly for polypeptide transport across, or into, lipid bilayers. In bacteria, secretion through the core channel complex-SecYEG in the inner membrane-is powered by the cytosolic ATPase SecA. Here, we use single-molecule fluorescence to interrogate the conformational state of SecYEG throughout the ATP hydrolysis cycle of SecA. We show that the SecYEG channel fluctuations between open and closed states are much faster (~20-fold during translocation) than ATP turnover, and that the nucleotide status of SecA modulates the rates of opening and closure. The SecY variant PrlA4, which exhibits faster transport but unaffected ATPase rates, increases the dwell time in the open state, facilitating pre-protein diffusion through the pore and thereby enhancing translocation efficiency. Thus, rapid SecYEG channel dynamics are allosterically coupled to SecA via modulation of the energy landscape, and play an integral part in protein transport. Loose coupling of ATP-turnover by SecA to the dynamic properties of SecYEG is compatible with a Brownian-rachet mechanism of translocation, rather than strict nucleotide-dependent interconversion between different static states of a power stroke.

A unifying mechanism for protein transport through the core bacterial Sec machinery

Encapsulation and compartmentalization are fundamental to the evolution of cellular life, but they also pose a challenge: how to partition the molecules that perform biological functions-the proteins-across impermeable barriers into sub-cellular organelles, and to the outside. The solution lies in the evolution of specialized machines, translocons, found in every biological membrane, which act both as gate and gatekeeper across and into membrane bilayers. Understanding how these translocons operate at the molecular level has been a long-standing ambition of cell biology, and one that is approaching its denouement; particularly in the case of the ubiquitous Sec system. In this review, we highlight the fruits of recent game-changing technical innovations in structural biology, biophysics and biochemistry to present a largely complete mechanism for the bacterial version of the core Sec machinery. We discuss the merits of our model over alternative proposals and identify the remaining open questions. The template laid out by the study of the Sec system will be of immense value for probing the many other translocons found in diverse biological membranes, towards the ultimate goal of altering or impeding their functions for pharmaceutical or biotechnological purposes.