The ClpXP and ClpAP proteases degrade proteins with carboxy-terminal peptide tails added by the SsrA-tagging system

High-throughput transposon mutagenesis in the family Enterobacteriaceae reveals core essential genes and rapid turnover of essentiality

The Enterobacteriaceae are a scientifically and medically important clade of bacteria, containing the model organism Escherichia coli, as well as major human pathogens including Salmonella enterica and Klebsiella pneumoniae. Essential gene sets have been determined for several members of the Enterobacteriaceae, with the Keio E. coli single-gene deletion library often regarded as a gold standard. However, it remains unclear how gene essentiality varies between related strains and species. To investigate this, we have assembled a collection of 13 sequenced high-density transposon mutant libraries from five genera within the Enterobacteriaceae. We first assess several gene essentiality prediction approaches, investigate the effects of transposon density on essentiality prediction, and identify biases in transposon insertion sequencing data. Based on these investigations, we develop a new classifier for gene essentiality. Using this new classifier, we define a core essential genome in the Enterobacteriaceae of 201 universally essential genes. Despite the presence of a large cohort of variably essential genes, we find an absence of evidence for genus-specific essential genes. A clear example of this sporadic essentiality is given by the set of genes regulating the σE extracytoplasmic stress response, which appears to have independently acquired essentiality multiple times in the Enterobacteriaceae. Finally, we compare our essential gene sets to the natural experiment of gene loss in obligate insect endosymbionts that have emerged from within the Enterobacteriaceae. This isolates a remarkably small set of genes absolutely required for survival and identifies several instances of essential stress responses masked by redundancy in free-living bacteria.IMPORTANCEThe essential genome, that is the set of genes absolutely required to sustain life, is a core concept in genetics. Essential genes in bacteria serve as drug targets, put constraints on the engineering of biological chassis for technological or industrial purposes, and are key to constructing synthetic life. Despite decades of study, relatively little is known about how gene essentiality varies across related bacteria. In this study, we have collected gene essentiality data for 13 bacteria related to the model organism Escherichia coli, including several human pathogens, and investigated the conservation of essentiality. We find that approximately a third of the genes essential in any particular strain are non-essential in another related strain. Surprisingly, we do not find evidence for essential genes unique to specific genera; rather it appears a substantial fraction of the essential genome rapidly gains or loses essentiality during evolution. This suggests that essentiality is not an immutable characteristic but depends crucially on the genomic context. We illustrate this through a comparison of our essential genes in free-living bacteria to genes conserved in 34 insect endosymbionts with naturally reduced genomes, finding several cases where genes generally regarded as being important for specific stress responses appear to have become essential in endosymbionts due to a loss of functional redundancy in the genome.

ProEnd: a comprehensive database for identifying HbYX motif-containing proteins across the tree of life

The proteasome plays a crucial role in cellular homeostasis by degrading misfolded, damaged, or unnecessary proteins. Understanding the regulatory mechanisms of proteasome activity is vital, particularly the interaction with activators containing the hydrophobic-tyrosine-any amino acid (HbYX) motif. Here, we present ProEnd, a comprehensive database designed to identify and catalog HbYX motif-containing proteins across the tree of life. Using a simple bioinformatics pipeline, we analyzed approximately 73 million proteins from 22,000 reference proteomes in the UniProt/SwissProt database. Our findings reveal the widespread presence of HbYX motifs in diverse organisms, highlighting their evolutionary conservation and functional significance. Notably, we observed an interesting prevalence of these motifs in viral proteomes, suggesting strategic interactions with the host proteasome. As validation two novel HbYX proteins found in this database were experimentally tested by pulldowns, confirming that they directly interact with the proteasome, with one of them directly activating it. ProEnd's extensive dataset and user-friendly interface enable researchers to explore the potential proteasomal regulator landscape, generating new hypotheses to advance proteasome biology. This resource is set to facilitate the discovery of novel therapeutic targets, enhancing our approach to treating diseases such as neurodegenerative disorders and cancer.

How the double-ring ClpAP protease motor grips the substrate to unfold and degrade stable proteins

Loops in the axial channels of ClpAP and other AAA+ proteases bind a short peptide degron connected by a linker to the N- or C-terminal residue of a native protein to initiate degradation. ATP hydrolysis then powers pore-loop movements that translocate these segments through the channel until a native domain is pulled against the narrow channel entrance, creating an unfolding force. Substrate unfolding is thought to depend on strong contacts between pore loops and a subset of amino acids in the unstructured sequence directly preceding the folded domain. Here, we identify such contact sequences that promote grip for ClpAP and use ClpA structures to place these sequences within ClpA's two AAA+ rings. The positions and chemical nature of certain residues within an unstructured segment that are positioned to interact with the D2 ring have major positive effects on substrate unfolding, whereas segments located within the D1 ring have little consequence. Within the D2-bound segment, two short elements are critical for accelerating degradation; one is at the 'top' of D2 and consists of at least two properly positioned non-slippery residues. In contrast, the second D2 element, which can be as short as one residue, is positioned to contact pore loops near the 'bottom' of this ring. Comparison with similar studies for ClpXP reveals that positioning a well-gripped substrate sequence within the major unfoldase motor is more important than its proximity to the folded domain and that charged, polar, and hydrophobic residues all contribute favorable contacts to substrate grip.

Functional investigation of the two ClpPs and three ClpXs in Myxococcus xanthus DK1622

ClpXP is a protease complex that plays important roles in protein quality control and cell cycle regulation, but the functions of multiple ClpXs and multiple ClpPs in M. xanthus remain unknown. The genome of Myxococcus xanthus DK1622 contains two clpPs and three clpXs. The clpP1 and clpX1 genes are cotranscribed and are both essential, while the other copies are isolated in the genome and are deletable. The deletion of clpX2 caused the mutant to be deficient in fruiting body development, while the clpX3 gene is involved in resistance to thermal stress. Both ClpPs possess catalytic active sites, but only ClpP1 shows in vitro peptidase activity on the typical substrate Suc-LY-AMC. All of these clpP and clpX genes exhibit strong transcriptional upregulation in the stationary phase, and the transcription of the three clpX genes appears to be coordinated. Our results demonstrated that multiple ClpPs and multiple ClpXs are functionally divergent and may assist in the environmental adaptation and functional diversification of M. xanthus.IMPORTANCEClpXP is an important protease complex of bacteria and is involved in various physiological processes. Myxococcus xanthus DK1622 possesses two ClpPs and three ClpXs with unclear functions. We investigated the functions of these genes and demonstrated the essential roles of clpP1 and clpX1. Only ClpP1 has in vitro peptidase activity on Suc-LY-AMC, and the isolated clpX copies participate in distinct cellular processes. All of these genes exhibited significant transcriptional upregulation in the stationary phase. Divergent functions appear in multiple ClpPs and multiple ClpXs in M. xanthus DK1622.

Effects of growth feedback on adaptive gene circuits: A dynamical understanding

The successful integration of engineered gene circuits into host cells remains a significant challenge in synthetic biology due to circuit-host interactions, such as growth feedback, where the circuit influences cell growth and vice versa. Understanding the dynamics of circuit failures and identifying topologies resilient to growth feedback are crucial for both fundamental and applied research. Utilizing transcriptional regulation circuits with adaptation as a paradigm, we systematically study more than four hundred topological structures and uncover various categories of failures. Three dynamical mechanisms of circuit failures are identified: continuous deformation of the response curve, strengthened or induced oscillations, and sudden switching to coexisting attractors. Our extensive computations also uncover a scaling law between a circuit robustness measure and the strength of growth feedback. Despite the negative effects of growth feedback on the majority of circuit topologies, we identify several circuits that maintain optimal performance as designed, a feature important for applications.

A role for the S4-domain containing protein YlmH in ribosome-associated quality control in Bacillus subtilis

Ribosomes trapped on mRNAs during protein synthesis need to be rescued for the cell to survive. The most ubiquitous bacterial ribosome rescue pathway is trans-translation mediated by tmRNA and SmpB. Genetic inactivation of trans-translation can be lethal, unless ribosomes are rescued by ArfA or ArfB alternative rescue factors or the ribosome-associated quality control (RQC) system, which in Bacillus subtilis involves MutS2, RqcH, RqcP and Pth. Using transposon sequencing in a trans-translation-incompetent B. subtilis strain we identify a poorly characterized S4-domain-containing protein YlmH as a novel potential RQC factor. Cryo-EM structures reveal that YlmH binds peptidyl-tRNA-50S complexes in a position analogous to that of S4-domain-containing protein RqcP, and that, similarly to RqcP, YlmH can co-habit with RqcH. Consistently, we show that YlmH can assume the role of RqcP in RQC by facilitating the addition of poly-alanine tails to truncated nascent polypeptides. While in B. subtilis the function of YlmH is redundant with RqcP, our taxonomic analysis reveals that in multiple bacterial phyla RqcP is absent, while YlmH and RqcH are present, suggesting that in these species YlmH plays a central role in the RQC.

ClpL Chaperone as a Possible Component of the Disaggregase Activity of Limosilactobacillus fermentum U-21

The L. fermentum U-21 strain, known for secreting chaperones into the extracellular milieu, emerges as a promising candidate for the development of novel therapeutics termed disaggregases for Parkinson's disease. Our study focuses on characterizing the secreted protein encoded by the C0965_000195 locus in the genome of this strain. Through sequence analysis and structural predictions, the protein encoded by C0965_000195 is identified as ClpL, homologs of which are known for their chaperone functions. The chaperone activity of ClpL from L. fermentum U-21 is investigated in vivo by assessing the refolding of luciferases with varying thermostabilities from Aliivibrio fischeri and Photorhabdus luminescens within Escherichia coli cells. The results indicate that the clpL gene from L. fermentum U-21 can compensate for the absence of the clpB gene, enhancing the refolding capacity of thermodenatured proteins in clpB-deficient cells. In vitro experiments demonstrate that both spent culture medium containing proteins secreted by L. fermentum U-21 cells, including ClpL, and purified heterologically expressed ClpL partially prevent the thermodenaturation of luciferases. The findings suggest that the ClpL protein from L. fermentum U-21, exhibiting disaggregase properties against aggregating proteins, may represent a key component contributing to the pharmabiotic attributes of this strain.

RqcH supports survival in the absence of non-stop ribosome rescue factors

Ribosomes frequently translate truncated or damaged mRNAs due to the extremely short half-life of mRNAs in bacteria. When ribosomes translate mRNA that lacks a stop codon (non-stop mRNA), specialized pathways are required to rescue the ribosome from the 3' end of the mRNA. The most highly conserved non-stop rescue pathway is trans-translation, which is found in greater than 95% of bacterial genomes. In all Proteobacteria that have been studied, the alternative non-stop ribosome rescue factors, ArfA and ArfB, are essential in the absence of trans-translation. Here, we investigate the interaction between non-stop rescue pathways and RqcH, a ribosome quality control factor that is broadly conserved outside of Proteobacteria. RqcH does not act directly on non-stop ribosomes but adds a degron tag to stalled peptides that obstruct the large ribosomal subunit, which allows the stalled peptide to be cleared from the ribosome by peptidyl-tRNA hydrolase (PTH). We show that Bacillus subtilis can survive without trans-translation and BrfA (Bacillus ArfA homolog), due to the presence of RqcH. We also show that expression of RqcH and its helper protein RqcP rescues the synthetic lethality of ΔssrAΔarfA in Escherichia coli. These results suggest that non-stop ribosome complexes can be disassembled and then cleared because of the tagging activity of RqcH, and that this process is essential in the absence of non-stop ribosome rescue pathways. Moreover, we surveyed the conservation of ribosome rescue pathways in >14,000 bacterial genomes. Our analysis reveals a broad distribution of non-stop rescue pathways, especially trans-translation and RqcH, and a strong co-occurrence between the ribosome splitting factor MutS2 and RqcH. Altogether, our results support a role for RqcH in non-stop ribosome rescue and provide a broad survey of ribosome rescue pathways in diverse bacterial species.

ProEnd: A Comprehensive Database for Identifying HbYX Motif-Containing Proteins Across the Tree of Life

The proteasome plays a crucial role in cellular homeostasis by degrading misfolded, damaged, or unnecessary proteins. Understanding the regulatory mechanisms of proteasome activity is vital, particularly the interaction with activators containing the hydrophobic-tyrosine-any amino acid (HbYX) motif. Here, we present ProEnd, a comprehensive database designed to identify and catalog HbYX motif-containing proteins across the tree of life. Using a simple bioinformatics pipeline, we analyzed approximately 73 million proteins from 22,000 reference proteomes in the UniProt/SwissProt database. Our findings reveal the widespread presence of HbYX motifs in diverse organisms, highlighting their evolutionary conservation and functional significance. Notably, we observed an interesting prevalence of these motifs in viral proteomes, suggesting strategic interactions with the host proteasome. As validation two novel HbYX proteins found in this database were tested and found to directly interact with the proteasome. ProEnd's extensive dataset and user-friendly interface enable researchers to explore the potential proteasomal regulator landscape, generating new hypotheses to advance proteasome biology. This resource is set to facilitate the discovery of novel therapeutic targets, enhancing our approach to treating diseases such as neurodegenerative disorders and cancer. Link: http://proend.org/.

Knockout Mouse Studies Show That Mitochondrial CLPP Peptidase and CLPX Unfoldase Act in Matrix Condensates near IMM, as Fast Stress Response in Protein Assemblies for Transcript Processing, Translation, and Heme Production

LONP1 is the principal AAA+ unfoldase and bulk protease in the mitochondrial matrix, so its deletion causes embryonic lethality. The AAA+ unfoldase CLPX and the peptidase CLPP also act in the matrix, especially during stress periods, but their substrates are poorly defined. Mammalian CLPP deletion triggers infertility, deafness, growth retardation, and cGAS-STING-activated cytosolic innate immunity. CLPX mutations impair heme biosynthesis and heavy metal homeostasis. CLPP and CLPX are conserved from bacteria to humans, despite their secondary role in proteolysis. Based on recent proteomic-metabolomic evidence from knockout mice and patient cells, we propose that CLPP acts on phase-separated ribonucleoprotein granules and CLPX on multi-enzyme condensates as first-aid systems near the inner mitochondrial membrane. Trimming within assemblies, CLPP rescues stalled processes in mitoribosomes, mitochondrial RNA granules and nucleoids, and the D-foci-mediated degradation of toxic double-stranded mtRNA/mtDNA. Unfolding multi-enzyme condensates, CLPX maximizes PLP-dependent delta-transamination and rescues malformed nascent peptides. Overall, their actions occur in granules with multivalent or hydrophobic interactions, separated from the aqueous phase. Thus, the role of CLPXP in the matrix is compartment-selective, as other mitochondrial peptidases: MPPs at precursor import pores, m-AAA and i-AAA at either IMM face, PARL within the IMM, and OMA1/HTRA2 in the intermembrane space.

Toxin-based screening of C-terminal tags in Escherichia coli reveals the exceptional potency of ssrA-like degrons

All bacteria possess ATP-dependent proteases that destroy cytosolic proteins. These enzymes help cells mitigate proteotoxic stress, adapt to changing nutrient availability, regulate virulence phenotypes, and transition to pathogenic lifestyles. Moreover, ATP-dependent proteases have emerged as promising antibacterial and antivirulence targets in a variety of pathogens. The physiological roles of these proteases are largely defined by the complement of proteins that they degrade. Substrates are typically recognized in a highly selective manner, often via short unstructured sequences termed degrons. While a few degrons have been identified and rigorously characterized, we lack a systematic understanding of how proteases select valid degrons from the vast complexity of protein sequence space. Here, we describe a novel high-throughput screening approach in Escherichia coli that couples proteolysis of a protein toxin to cell survival. We used this method to screen a combinatorial library of C-terminal pentapeptide sequences for functionality as proteolytic degrons in wild type E. coli, and in strains lacking components of the ClpXP and ClpAP proteases. By examining the competitive enrichment of sequences over time, we found that about one percent of pentapeptide tags lead to toxin proteolysis. Interestingly, the most enriched degrons were ClpXP-dependent and highly similar to the ssrA tag, one of the most extensively characterized degrons in bacteria. Among ssrA-like sequences, we observed that specific upstream residues correlate with successful recognition. The lack of diversity among strongly enriched sequences suggests that ssrA-like tags comprise a uniquely potent class of short C-terminal degron in E. coli. Efficient proteolysis of substrates lacking such degrons likely requires adaptors or multivalent interactions. These findings broaden our understanding of the constraints that shape the bacterial proteolytic landscape. Our screening approach may be broadly applicable to probing aspects of proteolytic substrate selection in other bacterial systems.

Fluorescent tools for the standardized work in Gram-negative bacteria

Standardized and thoroughly characterized genetic tools are a prerequisite for studying cellular processes to ensure the reusability and consistency of experimental results. The discovery of fluorescent proteins (FPs) represents a milestone in the development of genetic reporters for monitoring transcription or protein localization in vivo. FPs have revolutionized our understanding of cellular dynamics by enabling the real-time visualization and tracking of biological processes. Despite these advancements, challenges remain in the appropriate use of FPs, specifically regarding their proper application, protein turnover dynamics, and the undesired disruption of cellular functions. Here, we systematically compared a comprehensive set of 15 FPs and assessed their performance in vivo by focusing on key parameters, such as signal over background ratios and protein stability rates, using the Gram-negative model organism Salmonella enterica as a representative host. We evaluated four protein degradation tags in both plasmid- and genome-based systems and our findings highlight the necessity of introducing degradation tags to analyze time-sensitive cellular processes. We demonstrate that the gain of dynamics mediated by the addition of degradation tags impacts the cell-to-cell heterogeneity of plasmid-based but not genome-based reporters. Finally, we probe the applicability of FPs for protein localization studies in living cells using standard and super-resolution fluorescence microscopy. In summary, our study underscores the importance of careful FP selection and paves the way for the development of improved genetic reporters to enhance the reproducibility and reliability of fluorescence-based research in Gram-negative bacteria and beyond.

State-of-the-art in engineering small molecule biosensors and their applications in metabolic engineering

Genetically encoded biosensors are crucial for enhancing our understanding of how molecules regulate biological systems. Small molecule biosensors, in particular, help us understand the interaction between chemicals and biological processes. They also accelerate metabolic engineering by increasing screening throughput and eliminating the need for sample preparation through traditional chemical analysis. Additionally, they offer significantly higher spatial and temporal resolution in cellular analyte measurements. In this review, we discuss recent progress in in vivo biosensors and control systems-biosensor-based controllers-for metabolic engineering. We also specifically explore protein-based biosensors that utilize less commonly exploited signaling mechanisms, such as protein stability and induced degradation, compared to more prevalent transcription factor and allosteric regulation mechanism. We propose that these lesser-used mechanisms will be significant for engineering eukaryotic systems and slower-growing prokaryotic systems where protein turnover may facilitate more rapid and reliable measurement and regulation of the current cellular state. Lastly, we emphasize the utilization of cutting-edge and state-of-the-art techniques in the development of protein-based biosensors, achieved through rational design, directed evolution, and collaborative approaches.

Degron-Controlled Protein Degradation in Escherichia coli: New Approaches and Parameters

Protein degron tags have proven to be uniquely useful for the characterization of gene function. Degrons can mediate quick depletion, usually within minutes, of a protein of interest, allowing researchers to characterize cellular responses to the loss of function. To develop a general-purpose degron tool in Escherichia coli, we sought to build upon a previously characterized system of SspB-dependent inducible protein degradation. For this, we created a family of expression vectors containing a destabilized allele of SspB, capable of a rapid and nearly perfect "off-to-on" induction response. Using this system, we demonstrated excellent control over several DNA metabolism enzymes. However, other substrates did not respond to degron tagging in such an ideal manner, indicating the apparent limitations of SspB-dependent systems. Several degron-tagged proteins were degraded too slowly to be completely depleted during active growth, whereas others appeared to be completely refractory to degron-promoted degradation. Thus, only a minority of our, admittedly biased, selection of degron substrates proved to be amenable to efficient SspB-catalyzed degradation. We also uncovered an apparent stalling and/or disengagement of ClpXP from a degron-tagged allele of beta-galactosidase (beta-gal). While a degron-containing fusion peptide attached to the carboxy-terminus of beta-gal was degraded quantitatively, no reductions in beta-gal activity or concentration were detected, demonstrating an apparently novel mechanism of protease resistance. We conclude that substrate-dependent effects of the SspB system present a continued challenge to the widespread adoption of this degron system. For substrates that prove to be degradable, we provide a series of titratable SspB-expression vehicles.

The membrane-cytoplasmic linker defines activity of FtsH proteases in Pseudomonas aeruginosa clone C

Pandemic Pseudomonas aeruginosa clone C strains encode two inner-membrane associated ATP-dependent FtsH proteases. PaftsH1 is located on the core genome and supports cell growth and intrinsic antibiotic resistance, whereas PaftsH2, a xenolog acquired through horizontal gene transfer from a distantly related species, is unable to functionally replace PaftsH1. We show that purified PaFtsH2 degrades fewer substrates than PaFtsH1. Replacing the 31-amino acid-extended linker region of PaFtsH2 spanning from the C-terminal end of the transmembrane helix-2 to the first seven highly divergent residues of the cytosolic AAA+ ATPase module with the corresponding region of PaFtsH1 improves hybrid-enzyme substrate processing in vitro and enables PaFtsH2 to substitute for PaFtsH1 in vivo. Electron microscopy indicates that the identity of this linker sequence influences FtsH flexibility. We find membrane-cytoplasmic (MC) linker regions of PaFtsH1 characteristically glycine-rich compared to those from FtsH2. Consequently, introducing three glycines into the membrane-proximal end of PaFtsH2's MC linker is sufficient to elevate its activity in vitro and in vivo. Our findings establish that the efficiency of substrate processing by the two PaFtsH isoforms depends on MC linker identity and suggest that greater linker flexibility and/or length allows FtsH to degrade a wider spectrum of substrates. As PaFtsH2 homologs occur across bacterial phyla, we hypothesize that FtsH2 is a latent enzyme but may recognize specific substrates or is activated in specific contexts or biological niches. The identity of such linkers might thus play a more determinative role in the functionality of and physiological impact by FtsH proteases than previously thought.

Light-inducible protein degradation in E. coli with the LOVdeg tag

Molecular tools for optogenetic control allow for spatial and temporal regulation of cell behavior. In particular, light-controlled protein degradation is a valuable mechanism of regulation because it can be highly modular, used in tandem with other control mechanisms, and maintain functionality throughout growth phases. Here, we engineered LOVdeg, a tag that can be appended to a protein of interest for inducible degradation in Escherichia coli using blue light. We demonstrate the modularity of LOVdeg by using it to tag a range of proteins, including the LacI repressor, CRISPRa activator, and the AcrB efflux pump. Additionally, we demonstrate the utility of pairing the LOVdeg tag with existing optogenetic tools to enhance performance by developing a combined EL222 and LOVdeg system. Finally, we use the LOVdeg tag in a metabolic engineering application to demonstrate post-translational control of metabolism. Together, our results highlight the modularity and functionality of the LOVdeg tag system and introduce a powerful new tool for bacterial optogenetics.

Tunable Dynamics in a Multistrain Transcriptional Pulse Generator

One challenge in synthetic biology is the tuning of regulatory components within gene circuits to elicit a specific behavior. This challenge becomes more difficult in synthetic microbial consortia since each strain's circuit must function at the intracellular level and their combination must operate at the population level. Here we demonstrate that circuit dynamics can be tuned in synthetic consortia through the manipulation of strain fractions within the community. To do this, we construct a microbial consortium comprised of three strains of engineered Escherichia coli that, when cocultured, use homoserine lactone-mediated intercellular signaling to create a multistrain incoherent type-1 feedforward loop (I1-FFL). Like naturally occurring I1-FFL motifs in gene networks, this engineered microbial consortium acts as a pulse generator of gene expression. We demonstrate that the amplitude of the pulse can be easily tuned by adjusting the relative population fractions of the strains. We also develop a mathematical model for the temporal dynamics of the microbial consortium. This model allows us to identify population fractions that produced desired pulse characteristics, predictions that were confirmed for all but extreme fractions. Our work demonstrates that intercellular gene circuits can be effectively tuned simply by adjusting the starting fractions of each strain in the consortium.

Degron-controlled protein degradation in Escherichia coli: New Approaches and Parameters

Protein degron tags have proven uniquely useful for characterization of gene function. Degrons mediate quick depletion, usually within minutes, of a protein of interest - allowing researchers to characterize cellular responses to the loss of function. To develop a general purpose degron tool in E. coli, we sought to build upon a previously characterized system of SspB-dependent inducible protein degradation. For this, we created a family of expression vectors containing a destabilized allele of SspB, capable of a rapid and nearly perfect "off-to-on" induction response. Using this system, we demonstrated control over several enzymes of DNA metabolism, but also found with other substates apparent limitations of a SspB-dependent system. Several degron target proteins were degraded too slowly to affect their complete depletion during active growth, whereas others appeared completely refractory to degron-promoted degradation. We demonstrated that a model substrate, beta-galactosidase, was positively recognized as a degron substrate, but failed to be degraded by the ClpXP protease - demonstrating an apparently unknown mechanism of protease resistance. Thus, only a minority of our, admittedly biased, selection of degron substates proved amenable to rapid SspB-catalyzed degradation. We conclude that substrate-dependence of the SspB system remains a critical factor for the success of this degron system. For substrates that prove degradable, we provide a series of titratable SspB-expression vehicles.

Antibiotic that inhibits trans-translation blocks binding of EF-Tu to tmRNA but not to tRNA

IMPORTANCE

Elongation factor thermo-unstable (EF-Tu) is a universally conserved translation factor that mediates productive interactions between tRNAs and the ribosome. In bacteria, EF-Tu also delivers transfer-messenger RNA (tmRNA)-SmpB to the ribosome during trans-translation. We report the first small molecule, KKL-55, that specifically inhibits EF-Tu activity in trans-translation without affecting its activity in normal translation. KKL-55 has broad-spectrum antibiotic activity, suggesting that compounds targeted to the tmRNA-binding interface of EF-Tu could be developed into new antibiotics to treat drug-resistant infections.

Light inducible protein degradation in E. coli with the LOVdeg tag

Molecular tools for optogenetic control allow for spatial and temporal regulation of cell behavior. In particular, light controlled protein degradation is a valuable mechanism of regulation because it can be highly modular, used in tandem with other control mechanisms, and maintain functionality throughout growth phases. Here, we engineered LOVdeg, a tag that can be appended to a protein of interest for inducible degradation in Escherichia coli using blue light. We demonstrate the modularity of LOVdeg by using it to tag a range of proteins, including the LacI repressor, CRISPRa activator, and the AcrB efflux pump. Additionally, we demonstrate the utility of pairing the LOVdeg tag with existing optogenetic tools to enhance performance by developing a combined EL222 and LOVdeg system. Finally, we use the LOVdeg tag in a metabolic engineering application to demonstrate post-translational control of metabolism. Together, our results highlight the modularity and functionality of the LOVdeg tag system, and introduce a powerful new tool for bacterial optogenetics.

Molecular Highway Patrol for Ribosome Collisions

During translation, messenger RNAs (mRNAs) are decoded by ribosomes which can stall for various reasons. These include chemical damage, codon composition, starvation, or translation inhibition. Trailing ribosomes can collide with stalled ribosomes, potentially leading to dysfunctional or toxic proteins. Such aberrant proteins can form aggregates and favor diseases, especially neurodegeneration. To prevent this, both eukaryotes and bacteria have evolved different pathways to remove faulty nascent peptides, mRNAs and defective ribosomes from the collided complex. In eukaryotes, ubiquitin ligases play central roles in triggering downstream responses and several complexes have been characterized that split affected ribosomes and facilitate degradation of the various components. As collided ribosomes signal translation stress to affected cells, in eukaryotes additional stress response pathways are triggered when collisions are sensed. These pathways inhibit translation and modulate cell survival and immune responses. Here, we summarize the current state of knowledge about rescue and stress response pathways triggered by ribosome collisions.

Ubiquitin-independent proteasomal degradation driven by C-degron pathways

Although most eukaryotic proteins are targeted for proteasomal degradation by ubiquitination, a subset have been demonstrated to undergo ubiquitin-independent proteasomal degradation (UbInPD). However, little is known about the molecular mechanisms driving UbInPD and the degrons involved. Utilizing the GPS-peptidome approach, a systematic method for degron discovery, we found thousands of sequences that promote UbInPD; thus, UbInPD is more prevalent than currently appreciated. Furthermore, mutagenesis experiments revealed specific C-terminal degrons required for UbInPD. Stability profiling of a genome-wide collection of human open reading frames identified 69 full-length proteins subject to UbInPD. These included REC8 and CDCA4, proteins which control proliferation and survival, as well as mislocalized secretory proteins, suggesting that UbInPD performs both regulatory and protein quality control functions. In the context of full-length proteins, C termini also play a role in promoting UbInPD. Finally, we found that Ubiquilin family proteins mediate the proteasomal targeting of a subset of UbInPD substrates.

Molecular docking and molecular simulation studies for N-degron selectivity of chloroplastic ClpS from Chlamydomonas reinhardtii

Regarding the importance of N-degron pathway in protein degradation network, the adaptor protein ClpS recognizes the substrates bearing classical N-degrons, and delivers them to caseinolytic protease complex ClpAP for degradation. Interestingly, the majority of N-degrons located near the N-terminus of protein substrate are belonged to the hydrophobic type amino acids. Chloroplast, an important organelle for plant photosynthesis, contain a diversified Clp degradation system. Despite several studies have confirmed that chloroplastic ClpS is able to interact with classical N-degrons derived from prokaryotes, whereas, the molecular mechanism underlying how the chloroplastic ClpS protein could recognize the substrate tagged by N-degrons is still unclear until now. Chlamydomonas reinhardtii is a kind of unicellular model organism for photosynthesis researches, which possesses a large cup-shaped chloroplast, and the corresponding genome data indicates that it owns bacterial homologous adaptor protein, named CrClpS1. However, the relevant biochemical knowledges, and protein structure researches for CrClpS1 adaptor aren't reported up to date. The molecular interactions between CrClpS1 and possible N-degrons are undefined as well. Here, we build a reliable homology model of CrClpS1 and find a hydrophobic pocket for N-degron binding. We combine molecular docking, molecular dynamic simulations, and MM/PBSA, MM/GBSA binding free energy estimations to elucidate the molecular properties of CrClpS1-N-degron interactions. Besides, we investigate the conformational changes for CrClpS1-apo in water-solvent environment and analyze its possible biological significances through a long time molecular dynamic simulation. Specifically, the adaptor CrClpS1 displays the stronger interactions with Phe, Trp, Tyr, His and Ile with respect to other amino acids. Using the residue decomposition analysis, the interactions between CrClpS1 and N-degrons are heavily depended on several conservative residues, which are located around the hydrophobic pocket, implying that chloroplast isolated from Chlamydomonas reinhadtii adopts a relatively conservative N-degron recognition mode. Besides, the opening-closure of hydrophobic pocket of CrClpS1 might be beneficial for the N-degron selectivity.

Folding of heterologous proteins in bacterial cell factories: Cellular mechanisms and engineering strategies

The expression of correctly folded and functional heterologous proteins is important in many biotechnological production processes, whether it is enzymes, biopharmaceuticals or biosynthetic pathways for production of sustainable chemicals. For industrial applications, bacterial platform organisms, such as E. coli, are still broadly used due to the availability of tools and proven suitability at industrial scale. However, expression of heterologous proteins in these organisms can result in protein aggregation and low amounts of functional protein. This review provides an overview of the cellular mechanisms that can influence protein folding and expression, such as co-translational folding and assembly, chaperone binding, as well as protein quality control, across different model organisms. The knowledge of these mechanisms is then linked to different experimental methods that have been applied in order to improve functional heterologous protein folding, such as codon optimization, fusion tagging, chaperone co-production, as well as strain and protein engineering strategies.

Targeting caseinolytic protease P and its AAA1 chaperone for tuberculosis treatment

Caseinolytic protease P with its AAA1 chaperone, known as Mycobacterium tuberculosis (Mtb)ClpP1P2 proteolytic machinery, maintains protein homeostasis in Mtb cells and is essential for bacterial survival. It is regarded as an important biological target with the potential to address the increasingly serious issue of multidrug-resistant (MDR) TB. Over the past 10 years, many MtbClpP1P2-targeted modulators have been identified and characterized, some of which have shown potent anti-TB activity. In this review, we describe current understanding of the substrates, structure and function of MtbClpP1P2, classify the modulators of this important protein machine into several categories based on their binding subunits or pockets, and discuss their binding details; Such information provides insights for use in candidate drug research and development of TB treatments by targeting MtbClpP1P2 proteolytic machinery.

The SspB adaptor drives structural changes in the AAA+ ClpXP protease during ssrA-tagged substrate delivery

Energy-dependent protein degradation by the AAA+ ClpXP protease helps maintain protein homeostasis in bacteria and eukaryotic organelles of bacterial origin. In Escherichia coli and many other proteobacteria, the SspB adaptor assists ClpXP in degrading ssrA-tagged polypeptides produced as a consequence of tmRNA-mediated ribosome rescue. By tethering these incomplete ssrA-tagged proteins to ClpXP, SspB facilitates their efficient degradation at low substrate concentrations. How this process occurs structurally is unknown. Here, we present a cryo-EM structure of the SspB adaptor bound to a GFP-ssrA substrate and to ClpXP. This structure provides evidence for simultaneous contacts of SspB and ClpX with the ssrA tag within the tethering complex, allowing direct substrate handoff concomitant with the initiation of substrate translocation. Furthermore, our structure reveals that binding of the substrate·adaptor complex induces unexpected conformational changes within the spiral structure of the AAA+ ClpX hexamer and its interaction with the ClpP tetradecamer.

FtsH degrades kinetically stable dimers of cyclopropane fatty acid synthase via an internal degron

Targeted protein degradation plays important roles in stress responses in all cells. In E. coli, the membrane-bound AAA+ FtsH protease degrades cytoplasmic and membrane proteins. Here, we demonstrate that FtsH degrades cyclopropane fatty acid (CFA) synthase, whose synthesis is induced upon nutrient deprivation and entry into stationary phase. We find that neither the disordered N-terminal residues nor the structured C-terminal residues of the kinetically stable CFA-synthase dimer are required for FtsH recognition and degradation. Experiments with fusion proteins support a model in which an internal degron mediates FtsH recognition as a prelude to unfolding and proteolysis. These findings elucidate the terminal step in the life cycle of CFA synthase and provide new insight into FtsH function.

Prophage excision switches the primary ribosome rescue pathway and rescue-associated gene regulations in Escherichia coli

Escherichia coli has multiple pathways to release nonproductive ribosome complexes stalled at the 3' end of nonstop mRNA: tmRNA (SsrA RNA)-mediated trans-translation and stop codon-independent termination by ArfA/RF2 or ArfB (YaeJ). The arfA mRNA lacks a stop codon and its expression is repressed by trans-translation. Therefore, ArfA is considered to complement the ribosome rescue activity of trans-translation, but the physiological situations in which ArfA is expressed have not been elucidated. Here, we found that the excision of CP4-57 prophage adjacent to E. coli ssrA leads to the inactivation of tmRNA and switches the primary rescue pathway from trans-translation to ArfA/RF2. This "rescue-switching" rearranges not only the proteome landscape in E. coli but also the phenotype such as motility. Furthermore, among the proteins with significantly increased abundance in the ArfA⁺ cells, we found ZntR, whose mRNA is transcribed together as the upstream part of nonstop arfA mRNA. Repression of ZntR and reconstituted model genes depends on the translation of the downstream nonstop ORFs that trigger the trans-translation-coupled exonucleolytic degradation by polynucleotide phosphorylase (PNPase). Namely, our studies provide a novel example of trans-translation-dependent regulation and re-define the physiological roles of prophage excision.

Antibiotic Acyldepsipeptides Stimulate the Streptomyces Clp-ATPase/ClpP Complex for Accelerated Proteolysis

Clp proteases consist of a proteolytic, tetradecameric ClpP core and AAA+ Clp-ATPases. Streptomycetes, producers of a plethora of secondary metabolites, encode up to five different ClpP homologs, and the composition of their unusually complex Clp protease machinery has remained unsolved. Here, we report on the composition of the housekeeping Clp protease in Streptomyces, consisting of a heterotetradecameric core built of ClpP1, ClpP2, and the cognate Clp-ATPases ClpX, ClpC1, or ClpC2, all interacting with ClpP2 only. Antibiotic acyldepsipeptides (ADEP) dysregulate the Clp protease for unregulated proteolysis. We observed that ADEP binds Streptomyces ClpP1, but not ClpP2, thereby not only triggering the degradation of nonnative protein substrates but also accelerating Clp-ATPase-dependent proteolysis. The explanation is the concomitant binding of ADEP and Clp-ATPases to opposite sides of the ClpP1P2 barrel, hence revealing a third, so far unknown mechanism of ADEP action, i.e., the accelerated proteolysis of native protein substrates by the Clp protease. IMPORTANCE Clp proteases are antibiotic and anticancer drug targets. Composed of the proteolytic core ClpP and a regulatory Clp-ATPase, the protease machinery is important for protein homeostasis and regulatory proteolysis. The acyldepsipeptide antibiotic ADEP targets ClpP and has shown promise for treating multiresistant and persistent bacterial infections. The molecular mechanism of ADEP is multilayered. Here, we present a new way how ADEP can deregulate the Clp protease system. Clp-ATPases and ADEP bind to opposite sides of Streptomyces ClpP, accelerating the degradation of natural Clp protease substrates. We also demonstrate the composition of the major Streptomyces Clp protease complex, a heteromeric ClpP1P2 core with the Clp-ATPases ClpX, ClpC1, or ClpC2 exclusively bound to ClpP2, and the killing mechanism of ADEP in Streptomyces.

Degradation Mechanism of AAA+ Proteases and Regulation of Streptomyces Metabolism

Hundreds of proteins work together in microorganisms to coordinate and control normal activity in cells. Their degradation is not only the last step in the cell's lifespan but also the starting point for its recycling. In recent years, protein degradation has been extensively studied in both eukaryotic and prokaryotic organisms. Understanding the degradation process is essential for revealing the complex regulatory network in microorganisms, as well as further artificial reconstructions and applications. This review will discuss several studies on protein quality-control family members Lon, FtsH, ClpP, the proteasome in Streptomyces, and a few classical model organisms, mainly focusing on their structure, recognition mechanisms, and metabolic influences.

Cellular functions of the ClpP protease impacting bacterial virulence

Proteostasis mechanisms significantly contribute to the sculpting of the proteomes of all living organisms. ClpXP is a central AAA+ chaperone-protease complex present in both prokaryotes and eukaryotes that facilitates the unfolding and subsequent degradation of target substrates. ClpX is a hexameric unfoldase ATPase, while ClpP is a tetradecameric serine protease. Substrates of ClpXP belong to many cellular pathways such as DNA damage response, metabolism, and transcriptional regulation. Crucially, disruption of this proteolytic complex in microbes has been shown to impact the virulence and infectivity of various human pathogenic bacteria. Loss of ClpXP impacts stress responses, biofilm formation, and virulence effector protein production, leading to decreased pathogenicity in cell and animal infection models. Here, we provide an overview of the multiple critical functions of ClpXP and its substrates that modulate bacterial virulence with examples from several important human pathogens.

Observing protein degradation in solution by the PAN-20S proteasome complex: Astate-of-the-art example of bio-macromolecular TR-SANS

In the present book chapter we illustrate the state-of-the-art of time-resolved small-angle neutron scattering (TR-SANS) by a concrete example of a dynamic bio-macromolecular system, i.e., regulated protein degradation by the archaeal PAN-20S proteasome complex. We present the specific and unique structural information that can be obtained by this approach, in combination with bio-macromolecular deuteration and online spectrophotometric measurements of a fluorescent substrate (GFP). The complementarity with atomic-resolution structural biology techniques (SAXS, NMR, crystallography and cryo-EM) and with the advent of atomic structure prediction are discussed, as well as the respective limitations and future perspectives.

Mitochondrial matrix proteases: quality control and beyond

To ensure correct function, mitochondria have developed several mechanisms of protein quality control (QC). Protein homeostasis highly relies on chaperones and proteases to maintain proper folding and remove damaged proteins that might otherwise form cell-toxic aggregates. Besides quality control, mitochondrial proteases modulate and regulate many essential functions, such as trafficking, processing and activation of mitochondrial proteins, mitochondrial dynamics, mitophagy and apoptosis. Therefore, the impaired function of mitochondrial proteases is associated with various pathological conditions, including cancer, metabolic syndromes and neurodegenerative disorders. This review recapitulates and discusses the emerging roles of two major proteases of the mitochondrial matrix, LON and ClpXP. Although commonly acknowledge for their protein quality control role, recent advances have uncovered several highly regulated processes controlled by the LON and ClpXP connected to mitochondrial gene expression and respiratory chain function maintenance. Furthermore, both proteases have been lately recognized as potent targets for anticancer therapies, and we summarize those findings.

Tag-Dependent Substrate Selection of ClpX Underlies Secondary Differentiation of Chlamydia trachomatis

Despite having a highly reduced genome, Chlamydia trachomatis undergoes a complex developmental cycle in which the bacteria differentiate between the following two functionally and morphologically distinct forms: the infectious, nonreplicative elementary body (EB) and the noninfectious, replicative reticulate body (RB). The transitions between EBs and RBs are not mediated by division events that redistribute intracellular proteins. Rather, both primary (EB to RB) and secondary (RB to EB) differentiation likely require bulk protein turnover. One system for targeted protein degradation is the trans-translation system for ribosomal rescue, where polypeptides stalled during translation are marked with an SsrA tag encoded by a hybrid tRNA-mRNA, tmRNA. ClpX recognizes the SsrA tag, leading to ClpXP-mediated degradation. We hypothesize that ClpX functions in chlamydial differentiation through targeted protein degradation. We found that mutation of a key residue (R230A) within the specific motif in ClpX associated with the recognition of SsrA-tagged substrates resulted in abrogated secondary differentiation while not reducing chlamydial replication or developmental cycle progression as measured by transcripts. Furthermore, inhibition of trans-translation through chemical and targeted genetic approaches also impeded chlamydial development. Knockdown of tmRNA and subsequent complementation with an allele mutated in the SsrA tag closely phenocopied the overexpression of ClpX_R230A, thus suggesting that ClpX recognition of SsrA-tagged substrates plays a critical function in secondary differentiation. Taken together, these data provide mechanistic insight into the requirements for transitions between chlamydial developmental forms. IMPORTANCE Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infections and preventable infectious blindness. This unique organism undergoes developmental transitions between infectious, nondividing forms and noninfectious, dividing forms. Therefore, the chlamydial developmental cycle is an attractive target for Chlamydia-specific antibiotics, which would minimize effects of broad-spectrum antibiotics on the spread of antibiotic resistance in other organisms. However, the lack of knowledge about chlamydial development on a molecular level impedes the identification of specific, druggable targets. This work describes a mechanism through which both the fundamental processes of trans-translation and proteomic turnover by ClpXP contribute to chlamydial differentiation, a critical facet of chlamydial growth and survival. Given the almost universal presence of trans-translation and ClpX in eubacteria, this mechanism may be conserved in developmental cycles of other bacterial species. Additionally, this study expands the fields of trans-translation and Clp proteases by emphasizing the functional diversity of these systems throughout bacterial evolution.

Single molecule microscopy reveals diverse actions of substrate sequences that impair ClpX AAA+ ATPase function

AAA+ (ATPases Associated with diverse cellular Activities) proteases unfold substrate proteins by pulling the substrate polypeptide through a narrow pore. To overcome the barrier to unfolding, substrates may require extended association with the ATPase. Failed unfolding attempts can lead to a slip of grip, which may result in substrate dissociation, but how substrate sequence affects slippage is unresolved. Here, we measured single molecule dwell time using total internal reflection fluorescence microscopy, scoring time-dependent dissociation of engaged substrates from bacterial AAA+ ATPase unfoldase/translocase ClpX. Substrates comprising a stable domain resistant to unfolding and a C-terminal unstructured tail, tagged with a degron for initiating translocase insertion, were used to determine dwell time in relation to tail length and composition. We found greater tail length promoted substrate retention during futile unfolding. Additionally, we tested two tail compositions known to frustrate unfolding. A poly-glycine tract (polyG) promoted release, but only when adjacent to the folded domain, whereas glycine-alanine repeats (GAr) did not promote release. A high complexity motif containing polar and charged residues also promoted release. We further investigated the impact of these and related motifs on substrate degradation rates and ATP consumption, using the unfoldase-protease complex ClpXP. Here, substrate domain stability modulates the effects of substrate tail sequences. polyG and GAr are both inhibitory for unfolding, but act in different ways. GAr motifs only negatively affected degradation of highly stable substrates, which is accompanied by reduced ClpXP ATPase activity. Together, our results specify substrate characteristics that affect unfolding and degradation by ClpXP.

Proteomic analysis of the regulatory networks of ClpX in a model cyanobacterium Synechocystis sp. PCC 6803

Protein homeostasis is tightly regulated by protein quality control systems such as chaperones and proteases. In cyanobacteria, the ClpXP proteolytic complex is regarded as a representative proteolytic system and consists of a hexameric ATPase ClpX and a tetradecameric peptidase ClpP. However, the functions and molecular mechanisms of ClpX in cyanobacteria remain unclear. This study aimed to decipher the unique contributions and regulatory networks of ClpX in the model cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis). We showed that the interruption of clpX led to slower growth, decreased high light tolerance, and impaired photosynthetic cyclic electron transfer. A quantitative proteomic strategy was employed to globally identify ClpX-regulated proteins in Synechocystis cells. In total, we identified 172 differentially expressed proteins (DEPs) upon the interruption of clpX. Functional analysis revealed that these DEPs are involved in diverse biological processes, including glycolysis, nitrogen assimilation, photosynthetic electron transport, ATP-binding cassette (ABC) transporters, and two-component signal transduction. The expression of 24 DEPs was confirmed by parallel reaction monitoring (PRM) analysis. In particular, many hypothetical or unknown proteins were found to be regulated by ClpX, providing new candidates for future functional studies on ClpX. Together, our study provides a comprehensive ClpX-regulated protein network, and the results serve as an important resource for understanding protein quality control systems in cyanobacteria.

ATP hydrolysis tunes specificity of a AAA+ protease

In bacteria, AAA+ proteases such as Lon and ClpXP degrade substrates with exquisite specificity. These machines capture the energy of ATP hydrolysis to power unfolding and degradation of target substrates. Here, we show that a mutation in the ATP binding site of ClpX shifts protease specificity to promote degradation of normally Lon-restricted substrates. However, this ClpX mutant is worse at degrading ClpXP targets, suggesting an optimal balance in substrate preference for a given protease that is easy to alter. In vitro, wild-type ClpXP also degrades Lon-restricted substrates more readily when ATP levels are reduced, similar to the shifted specificity of mutant ClpXP, which has altered ATP hydrolysis kinetics. Based on these results, we suggest that the rates of ATP hydrolysis not only power substrate unfolding and degradation, but also tune protease specificity. We consider various models for this effect based on emerging structures of AAA+ machines showing conformationally distinct states.

Frequency dependent growth of bacteria in living materials

The fusion of living bacteria and man-made materials represents a new frontier in medical and biosynthetic technology. However, the principles of bacterial signal processing inside synthetic materials with three-dimensional and fluctuating environments remain elusive. Here, we study bacterial growth in a three-dimensional hydrogel. We find that bacteria expressing an antibiotic resistance module can take advantage of ambient kinetic disturbances to improve growth while encapsulated. We show that these changes in bacterial growth are specific to disturbance frequency and hydrogel density. This remarkable specificity demonstrates that periodic disturbance frequency is a new input that engineers may leverage to control bacterial growth in synthetic materials. This research provides a systematic framework for understanding and controlling bacterial information processing in three-dimensional living materials.

FtsH degrades dihydrofolate reductase by recognizing a partially folded species

AAA+ proteolytic machines play essential roles in maintaining and rebalancing the cellular proteome in response to stress, developmental cues, and environmental changes. Of the five AAA+ proteases in Escherichia coli, FtsH is unique in its attachment to the inner membrane and its function in degrading both membrane and cytosolic proteins. E. coli dihydrofolate reductase (DHFR) is a stable and biophysically well-characterized protein, which a previous study found resisted FtsH degradation despite the presence of an ssrA degron. By contrast, we find that FtsH degrades DHFR fused to a long peptide linker and ssrA tag. Surprisingly, we also find that FtsH degrades DHFR with shorter linkers and ssrA tag, and without any linker or tag. Thus, FtsH must be able to recognize a sequence element or elements within DHFR. We find that FtsH degradation of DHFR is noncanonical in the sense that it does not rely upon recognition of an unstructured polypeptide at or near the N-terminus or C-terminus of the substrate. Results using peptide-array experiments, mutant DHFR proteins, and fusion proteins suggest that FtsH recognizes an internal sequence in a species of DHFR that is partially unfolded. Overall, our findings provide insight into substrate recognition by FtsH and indicate that its degradation capacity is broader than previously reported.

Regulation of flagellar motility and biosynthesis in enterohemorrhagic Escherichia coli O157:H7

ABSTARCTEnterohemorrhagic Escherichia coli (EHEC) O157:H7 is a human pathogen that causes a variety of diseases, such as hemorrhagic colitis and lethal hemolytic uremic syndrome. Flagellum-dependent motility plays diverse roles in the pathogenesis of EHEC O157:H7, including its migration to an optimal host site, adherence and colonization, survival at the infection site, and post-infection dispersal. However, it is very expensive for cellular economy in terms of the number of genes and the energy required for flagellar biosynthesis and functioning. Furthermore, the flagellar filament bears strong antigenic properties that induce a strong host immune response. Consequently, the flagellar gene expression and biosynthesis are highly regulated to occur at the appropriate time and place by different regulatory influences. The present review focuses on the regulatory mechanisms of EHEC O157:H7 motility and flagellar biosynthesis, especially in terms of flagellar gene regulation by environmental factors, regulatory proteins, and small regulatory RNAs.

Bacterial degrons in synthetic circuits

Bacterial proteases are a promising post-translational regulation strategy in synthetic circuits because they recognize specific amino acid degradation tags (degrons) that can be fine-tuned to modulate the degradation levels of tagged proteins. For this reason, recent efforts have been made in the search for new degrons. Here we review the up-to-date applications of degradation tags for circuit engineering in bacteria. In particular, we pay special attention to the effects of degradation bottlenecks in synthetic oscillators and introduce mathematical approaches to study queueing that enable the quantitative modelling of proteolytic queues.

The mitochondrial unfolded protein response (UPRmt): shielding against toxicity to mitochondria in cancer

Mitochondria are essential for tumor growth and progression. However, the heavy demand for mitochondrial activity in cancer leads to increased production of mitochondrial reactive oxygen species (mtROS), accumulation of mutations in mitochondrial DNA, and development of mitochondrial dysfunction. If left unchecked, excessive mtROS can damage and unfold proteins in the mitochondria to an extent that becomes lethal to the tumor. Cellular systems have evolved to combat mtROS and alleviate mitochondrial stress through a quality control mechanism called the mitochondrial unfolded protein response (UPR^mt). The UPR^mt system is composed of chaperones and proteases, which promote protein folding or eliminate mitochondrial proteins damaged by mtROS, respectively. UPR^mt is conserved and activated in cancer in response to mitochondrial stress to maintain mitochondrial integrity and support tumor growth. In this review, we discuss how mitochondria become dysfunctional in cancer and highlight the tumor-promoting functions of key components of the UPR^mt.

Tailored phenyl esters inhibit ClpXP and attenuate Staphylococcus aureus α-hemolysin secretion

Novel strategies against multidrug‐resistant bacteria are urgently needed in order to overcome the current silent pandemic. Manipulation of toxin production in pathogenic species serves as a promising approach to attenuate virulence and prevent infections. In many bacteria such as Staphylococcus aureus or Listeria monocyotgenes, serine protease ClpXP is a key contributor to virulence and thus represents a prime target for antimicrobial drug discovery. The limited stability of previous electrophilic warheads has prevented a sustained effect of virulence attenuation in bacterial culture. Here, we systematically tailor the stability and inhibitory potency of phenyl ester ClpXP inhibitors by steric shielding of the ester bond and fine‐tuning the phenol leaving group. Out of 17 derivatives, two (MAS‐19 and MAS‐30) inhibited S. aureus ClpP peptidase and ClpXP protease activities by >60 % at 1 μM. Furthermore, the novel inhibitors did not exhibit pronounced cytotoxicity against human and bacterial cells. Unlike the first generation phenylester AV170, these molecules attenuated S. aureus virulence markedly and displayed increased stability in aqueous buffer compared to the previous benchmark AV170.

Structure and function of ClpXP, a AAA+ proteolytic machine powered by probabilistic ATP hydrolysis

ClpXP is an archetypical AAA+ protease, consisting of ClpX and ClpP. ClpX is an ATP-dependent protein unfoldase and polypeptide translocase, whereas ClpP is a self-compartmentalized peptidase. ClpXP is currently the only AAA+ protease for which high-resolution structures exist, the molecular basis of recognition for a protein substrate is understood, extensive biochemical and genetic analysis have been performed, and single-molecule optical trapping has allowed direct visualization of the kinetics of substrate unfolding and translocation. In this review, we discuss our current understanding of ClpXP structure and function, evaluate competing sequential and probabilistic mechanisms of ATP hydrolysis, and highlight open questions for future exploration.

Update on the Protein Homeostasis Network in Bacillus subtilis

Protein homeostasis is fundamental to cell function and survival. It relies on an interconnected network of processes involving protein synthesis, folding, post-translational modification and degradation as well as regulators of these processes. Here we provide an update on the roles, regulation and subcellular localization of the protein homeostasis machinery in the Gram-positive model organism Bacillus subtilis. We discuss emerging ideas and current research gaps in the field that, if tackled, increase our understanding of how Gram-positive bacteria, including several human pathogens, maintain protein homeostasis and cope with stressful conditions that challenge their survival.

ClpX/P-Dependent Degradation of Novel Substrates in Streptococcus mutans

Regulated proteolysis is where AAA+ ATPases (ClpX, ClpC, and ClpE) are coupled to a protease subunit (ClpP) to facilitate degradation of misfolded and native regulatory proteins in the cell. The process is intricately linked to protein quality control and homeostasis and modulates several biological processes. In streptococci, regulated proteolysis is vital to various functions, including virulence expression, competence development, bacteriocin production, biofilm formation, and stress responses. Among the various Clp ATPases, ClpX is the major one that recognizes specific amino acid residues in its substrates and delivers them to the ClpP proteolytic chamber for degradation. While multiple ClpX substrates have been identified in Escherichia coli and other bacteria, little is known about the identity of these substrates in streptococci. Here, we used a preliminary proteomic analysis to identify putative ClpX substrates using Streptococcus mutans as a model organism. SMU.961 is one such putative substrate where we identified the Glu-Lue-Gln (ELQ) motif at the C terminus that is recognized by ClpX/P. We identified several other proteins, including MecA, which also harbor ELQ and are degraded by ClpX/P. This is surprising since MecA is known to be degraded by ClpC/P in Bacillus subtilis; however, ClpX/P-mediated MecA degradation is unknown. We also identified Glu and Gln as the crucial residues for ClpX recognition. Our data indicate a species and perhaps strain-specific recognition of ELQ by streptococcal ClpX/P. At present, we do not know whether this species-dependent degradation by ClpX/P is unique to S. mutans, and we are currently examining the phenomenon in other pathogenic streptococci. IMPORTANCE ClpX/P is a major intracellular proteolytic complex that is responsible for protein quality control in the cell. ClpX, an AAA+ ATPase, distinguishes the potential substrates by recognizing short motifs at the C-terminal end of proteins and delivers the substrates for degradation by ClpP protease. The identity of these ClpX substrates, which varies greatly among bacteria, is known only for a few well-studied species. Here, we used Streptococcus mutans as a model organism to identify ClpX substrates. We found that a short motif of three residues is successfully recognized by ClpX/P. Interestingly, the motif is not present at the ultimate C-terminal end; rather it is present close to the end. This result suggests that streptococcal ClpX ATPase can recognize internal motifs.

Improving the Robustness of Engineered Bacteria to Nutrient Stress Using Programmed Proteolysis

The use of short peptide tags in synthetic genetic circuits allows for the tuning of gene expression dynamics and release of amino acid resources through targeted protein degradation. Here, we use elements of the Escherichia coli and Mesoplasma florum transfer-mRNA (tmRNA) ribosome rescue systems to compare endogenous and foreign proteolysis systems in E. coli. We characterize the performance and burden of each and show that, while both greatly shorten the half-life of a tagged protein, the endogenous system is approximately 10 times more efficient. On the basis of these results we then demonstrate using mathematical modeling and experiments how proteolysis can improve cellular robustness through targeted degradation of a reporter protein in auxotrophic strains, providing a limited secondary source of essential amino acids that help partially restore growth when nutrients become scarce. These findings provide avenues for controlling the functional lifetime of engineered cells once deployed and increasing their tolerance to fluctuations in nutrient availability.