Proteolytic adaptor for transfer-messenger RNA-tagged proteins from alpha-proteobacteria

Applications of Bacterial Degrons and Degraders - Toward Targeted Protein Degradation in Bacteria

A repertoire of proteolysis-targeting signals known as degrons is a necessary component of protein homeostasis in every living cell. In bacteria, degrons can be used in place of chemical genetics approaches to interrogate and control protein function. Here, we provide a comprehensive review of synthetic applications of degrons in targeted proteolysis in bacteria. We describe recent advances ranging from large screens employing tunable degradation systems and orthogonal degrons, to sophisticated tools and sensors for imaging. Based on the success of proteolysis-targeting chimeras as an emerging paradigm in cancer drug discovery, we discuss perspectives on using bacterial degraders for studying protein function and as novel antimicrobials.

Identification of New Degrons in Streptococcus mutans Reveals a Novel Strategy for Engineering Targeted, Controllable Proteolysis

Recently, controllable, targeted proteolysis has emerged as one of the most promising new strategies to study essential genes and otherwise toxic mutations. One of the principal limitations preventing the wider adoption of this approach is due to the lack of easily identifiable species-specific degrons that can be used to trigger the degradation of target proteins. Here, we report new advancements in the targeted proteolysis concept by creating the first prokaryotic N-terminal targeted proteolysis system. We demonstrate how proteins from the LexA-like protein superfamily can be exploited as species-specific reservoirs of N- and/or C-degrons, which are easily identifiable due to their proximity to strictly conserved residues found among LexA-like proteins. Using the LexA-like regulator HdiR of Streptococcus mutans, we identified two separate N-degrons derived from HdiR that confer highly efficient constitutive proteolysis upon target proteins when added as N-terminal peptide tags. Both degrons mediate degradation via AAA+ family housekeeping proteases with one degron primarily targeting FtsH and the other targeting the ClpP-dependent proteases. To modulate degron activity, our approach incorporates a hybrid N-terminal protein tag consisting of the ubiquitin-like protein NEDD8 fused to an HdiR degron. The NEDD8 fusion inhibits degron function until the NEDD8-specific endopeptidase NEDP1 is heterologously expressed to expose the N-degron. By fusing the NEDD8-degron tag onto GFP, luciferase, and the pleiotropic regulator RNase J2, we demonstrate that the N-terminal proteolysis approach exhibits far superior performance compared to the classic transcriptional depletion approach and is similarly applicable for the study of highly toxic mutations.

Regulated Proteolysis in Bacteria: Caulobacter

Protein degradation is essential for all living things. Bacteria use energy-dependent proteases to control protein destruction in a highly specific manner. Recognition of substrates is determined by the inherent specificity of the proteases and through adaptor proteins that alter the spectrum of substrates. In the α-proteobacterium Caulobacter crescentus, regulated protein degradation is required for stress responses, developmental transitions, and cell cycle progression. In this review, we describe recent progress in our understanding of the regulated and stress-responsive protein degradation pathways in Caulobacter. We discuss how organization of highly specific adaptors into functional hierarchies drives destruction of proteins during the bacterial cell cycle. Because all cells must balance the need for degradation of many true substrates with the toxic consequences of nonspecific protein destruction, principles found in one system likely generalize to others.

tmRNA-mediated trans-translation as the major ribosome rescue system in a bacterial cell

Transfer messenger RNA (tmRNA; also known as 10Sa RNA or SsrA RNA) is a small RNA molecule that is conserved among bacteria. It has structural and functional similarities to tRNA: it has an upper half of the tRNA-like structure, its 5’ end is processed by RNase P, it has typical tRNA-specific base modifications, it is aminoacylated with alanine, it binds to EF-Tu after aminoacylation and it enters the ribosome with EF-Tu and GTP. However, tmRNA lacks an anticodon, and instead it has a coding sequence for a short peptide called tag-peptide. An elaborate interplay of actions of tmRNA as both tRNA and mRNA with the help of a tmRNA-binding protein, SmpB, facilitates trans-translation, which produces a single polypeptide from two mRNA molecules. Initially alanyl-tmRNA in complex with EF-Tu and SmpB enters the vacant A-site of the stalled ribosome like aminoacyl-tRNA but without a codon–anticodon interaction, and subsequently truncated mRNA is replaced with the tag-encoding region of tmRNA. During these processes, not only tmRNA but also SmpB structurally and functionally mimics both tRNA and mRNA. Thus trans-translation rescues the stalled ribosome, thereby allowing recycling of the ribosome. Since the tag-peptide serves as a target of AAA⁺ proteases, the trans-translation products are preferentially degraded so that they do not accumulate in the cell. Although alternative rescue systems have recently been revealed, trans-translation is the only system that universally exists in bacteria. Furthermore, it is unique in that it employs a small RNA and that it prevents accumulation of non-functional proteins from truncated mRNA in the cell. It might play the major role in rescuing the stalled translation in the bacterial cell.

Regulated Expression Systems for Mycobacteria and Their Applications

For bacterial model organisms like Escherichia coli and Bacillus subtilis genetic tools to experimentally manipulate the activity of individual genes existed for decades. But for genetically less tractable yet medically important bacteria such as M. tuberculosis such tools have rarely been available. More recently several groups developed genetic switches that function efficiently in M. tuberculosis and other mycobacteria. Together these systems utilize six different transcription factors, eight different regulated promoters, and three different regulatory principles. Here we describe their design features, review their main applications, and discuss advantages and disadvantages of regulating transcription, translation, or protein stability for controlling gene activities in bacteria.

Integrated transcriptomic and proteomic analysis of the global response of Wolbachia to doxycycline-induced stress

The bacterium Wolbachia (order Rickettsiales), representing perhaps the most abundant vertically transmitted microbe worldwide, infects arthropods and filarial nematodes. In arthropods, Wolbachia can induce reproductive alterations and interfere with the transmission of several arthropod-borne pathogens. In addition, Wolbachia is an obligate mutualist of the filarial parasites that cause lymphatic filariasis and onchocerciasis in the tropics. Targeting Wolbachia with tetracycline antibiotics leads to sterilisation and ultimately death of adult filariae. However, several weeks of treatment are required, restricting the implementation of this control strategy. To date, the response of Wolbachia to stress has not been investigated, and almost nothing is known about global regulation of gene expression in this organism. We exposed an arthropod Wolbachia strain to doxycycline in vitro, and analysed differential expression by directional RNA-seq and label-free, quantitative proteomics. We found that Wolbachia responded not only by modulating expression of the translation machinery, but also by upregulating nucleotide synthesis and energy metabolism, while downregulating outer membrane proteins. Moreover, Wolbachia increased the expression of a key component of the twin-arginine translocase (tatA) and a phosphate ABC transporter ATPase (PstB); the latter is associated with decreased susceptibility to antimicrobials in free-living bacteria. Finally, the downregulation of 6S RNA during translational inhibition suggests that this small RNA is involved in growth rate control. Despite its highly reduced genome, Wolbachia shows a surprising ability to regulate gene expression during exposure to a potent stressor. Our findings have general relevance for the chemotherapy of obligate intracellular bacteria and the mechanistic basis of persistence in the Rickettsiales.

Machines of destruction - AAA+ proteases and the adaptors that control them

Bacteria are frequently exposed to changes in environmental conditions, such as fluctuations in temperature, pH or the availability of nutrients. These assaults can be detrimental to cell as they often result in a proteotoxic stress, which can cause the accumulation of unfolded proteins. In order to restore a productive folding environment in the cell, bacteria have evolved a network of proteins, known as the protein quality control (PQC) network, which is composed of both chaperones and AAA+ proteases. These AAA+ proteases form a major part of this PQC network, as they are responsible for the removal of unwanted and damaged proteins. They also play an important role in the turnover of specific regulatory or tagged proteins. In this review, we describe the general features of an AAA+ protease, and using two of the best-characterised AAA+ proteases in Escherichia coli (ClpAP and ClpXP) as a model for all AAA+ proteases, we provide a detailed mechanistic description of how these machines work. Specifically, the review examines the physiological role of these machines, as well as the substrates and the adaptor proteins that modulate their substrate specificity.

An essential tyrosine phosphatase homolog regulates cell separation, outer membrane integrity, and morphology in Caulobacter crescentus

Although reversible phosphorylation on tyrosine residues regulates the activity of many eukaryotic proteins, there are few examples of this type of regulation in bacteria. We have identified the first essential tyrosine phosphatase homolog in a bacterium, Caulobacter crescentus CtpA. ctpA mutants with altered active-site residues are nonviable, and depletion of CtpA yields chains of cells with blebbed outer membranes, linked by unresolved peptidoglycan. CtpA overexpression reduces cell curvature in a manner similar to deleting the intermediate filament protein crescentin, but it does not disrupt crescentin localization or membrane attachment. Although it has no obvious signal sequence or transmembrane-spanning domains, CtpA associates with the Caulobacter inner membrane. Immunolocalization experiments suggest that CtpA accumulates at the division site during the last quarter of the cell cycle. We propose that CtpA dephosphorylates one or more proteins involved in peptidoglycan biosynthesis or remodeling, which in turn affect cell separation, cell envelope integrity, and vibrioid morphology.

Protein inactivation in mycobacteria by controlled proteolysis and its application to deplete the beta subunit of RNA polymerase

Using a component of the Escherichia coli protein degradation machinery, we have established a system to regulate protein stability in mycobacteria. A protein tag derived from the E. coli SsrA degradation signal did not affect several reporter proteins in wild-type Mycobacterium smegmatis or Mycobacterium tuberculosis. Expression of the adaptor protein SspB, which recognizes this modified tag and helps deliver tagged proteins to the protease ClpXP, strongly decreased the activities and protein levels of different reporters. This inactivation did not occur when the function of ClpX was inhibited. Using this system, we constructed a conditional M. smegmatis knockdown mutant in which addition of anhydrotetracycline (atc) caused depletion of the beta subunit of RNA polymerase, RpoB. The impact of atc on this mutant was dose-dependent. Very low amounts of atc did not prevent growth but increased sensitivity to an antibiotic that inactivates RpoB. Intermediate amounts of RpoB knockdown resulted in bacteriostasis and a more substantial depletion led to a decrease in viability by up to 99%. These studies identify SspB-mediated proteolysis as an efficient approach to conditionally inactivate essential proteins in mycobacteria. They further demonstrate that depletion of RpoB by ∼93% is sufficient to cause death of M. smegmatis.

Study of the functionality of the Helicobacter pylori trans-translation components SmpB and SsrA in an heterologous system

BACKGROUND

Trans-translation is a ubiquitous bacterial quality control-mechanism for both transcription and translation. With its two major partners, SsrA a small stable RNA and the SmpB protein, it promotes the release of ribosomes stalled on defective mRNAs and directs the corresponding truncated proteins to degradation pathways. We have recently shown that trans-translation is an essential function in the gastric pathogen Helicobacter pylori. Our results suggested that some properties of the H. pylori trans-translation machinery distinguishes it from the well known system in E. coli. Therefore, we decided to test the functionality of the SmpB and SsrA molecules of H. pylori in the E. coli heterologous system using two established phenotypic tests.

RESULTS

H. pylori SmpB protein was found to successfully restore the E. coli DeltasmpB mutant growth defect and its capacity to propagate lambdaimmP22 phage. We showed that in E. coli, H. pylori SsrA (Hp-SsrA) was stably expressed and maturated and that this molecule could restore wild type growth to the E. coli DeltassrA mutant. Hp-SsrA mutants affected in the ribosome rescue function were not able to restore normal growth to E. coli DeltassrA supporting a major role of ribosome rescue in this phenotype. Surprisingly, Hp-SsrA did not restore the phage lambdaimmP22 propagation capacity to the E. coli DeltassrA mutant.

CONCLUSIONS

These data suggest an additional role of the tag sequence that presents specific features in Hp-SsrA. Our interpretation is that a secondary role of protein tagging in phage propagation is revealed by heterologous complementation because ribosome rescue is less efficient. In conclusion, tmRNAs present in all eubacteria, have coevolved with the translational machinery of their host and possess specific determinants that can be revealed by heterologous complementation studies.

Versatile modes of peptide recognition by the ClpX N domain mediate alternative adaptor-binding specificities in different bacterial species

ClpXP, an AAA+ protease, plays key roles in protein-quality control and many regulatory processes in bacteria. The N-terminal domain of the ClpX component of ClpXP is involved in recognition of many protein substrates, either directly or by binding the SspB adaptor protein, which delivers specific classes of substrates for degradation. Despite very limited sequence homology between the E. coli and C. crescentus SspB orthologs, each of these adaptors can deliver substrates to the ClpXP enzyme from the other bacterial species. We show that the ClpX N domain recognizes different sequence determinants in the ClpX-binding (XB) peptides of C. crescentus SspBalpha and E. coli SspB. The C. crescentus XB determinants span 10 residues and involve interactions with multiple side chains, whereas the E. coli XB determinants span half as many residues with only a few important side chain contacts. These results demonstrate that the N domain of ClpX functions as a highly versatile platform for peptide recognition, allowing the emergence during evolution of alternative adaptor-binding specificities. Our results also reveal highly conserved residues in the XB peptides of both E. coli SspB and C. crescentus SspBalpha that play no detectable role in ClpX-binding or substrate delivery.

ClpXP degrades SsrA-tagged proteins in Streptococcus pneumoniae

Bacterial proteins that are abnormally truncated due to incomplete mRNA or the presence of rare codons are extended by an SsrA tag during ribosome rescue in a trans-translation process important for maintaining protein quality. In Escherichia coli, the SsrA-tagged proteins become the target of the Tsp, Lon, FtsH, ClpXP, and ClpAP proteases. Here we show that degradation of model SsrA-tagged proteins in Streptococcus pneumoniae depends primarily or exclusively on ClpXP in vivo. In addition, we show the E. coli SsrA tag is also a target of S. pneumoniae ClpXP in vivo, even though the N-terminal portions of the tags differ significantly between the two species, suggesting there may be no adaptor protein for SsrA in S. pneumoniae.

Biology of trans-translation

The trans-translation mechanism is a key component of multiple quality control pathways in bacteria that ensure proteins are synthesized with high fidelity in spite of challenges such as transcription errors, mRNA damage, and translational frameshifting. trans-Translation is performed by a ribonucleoprotein complex composed of tmRNA, a specialized RNA with properties of both a tRNA and an mRNA, and the small protein SmpB. tmRNA-SmpB interacts with translational complexes stalled at the 3' end of an mRNA to release the stalled ribosomes and target the nascent polypeptides and mRNAs for degradation. In addition to quality control pathways, some genetic regulatory circuits use trans-translation to control gene expression. Diverse bacteria require trans-translation when they execute large changes in their genetic programs, including responding to stress, pathogenesis, and differentiation.

Trans-translation in Helicobacter pylori: essentiality of ribosome rescue and requirement of protein tagging for stress resistance and competence

BACKGROUND

The ubiquitous bacterial trans-translation is one of the most studied quality control mechanisms. Trans-translation requires two specific factors, a small RNA SsrA (tmRNA) and a protein co-factor SmpB, to promote the release of ribosomes stalled on defective mRNAs and to add a specific tag sequence to aberrant polypeptides to direct them to degradation pathways. Helicobacter pylori is a pathogen persistently colonizing a hostile niche, the stomach of humans.

PRINCIPAL FINDINGS

We investigated the role of trans-translation in this bacterium well fitted to resist stressful conditions and found that both smpB and ssrA were essential genes. Five mutant versions of ssrA were generated in H. pylori in order to investigate the function of trans-translation in this organism. Mutation of the resume codon that allows the switch of template of the ribosome required for its release was essential in vivo, however a mutant in which this codon was followed by stop codons interrupting the tag sequence was viable. Therefore one round of translation is sufficient to promote the rescue of stalled ribosomes. A mutant expressing a truncated SsrA tag was viable in H. pylori, but affected in competence and tolerance to both oxidative and antibiotic stresses. This demonstrates that control of protein degradation through trans-translation is by itself central in the management of stress conditions and of competence and supports a regulatory role of trans-translation-dependent protein tagging. In addition, the expression of smpB and ssrA was found to be induced upon acid exposure of H. pylori.

CONCLUSIONS

We conclude to a central role of trans-translation in H. pylori both for ribosome rescue possibly due to more severe stalling and for protein degradation to recover from stress conditions frequently encountered in the gastric environment. Finally, the essential trans-translation machinery of H. pylori is an excellent specific target for the development of novel antibiotics.

Evolution of the ssrA degradation tag in Mycoplasma: specificity switch to a different protease

Stalled ribosomes in bacteria are rescued by the tmRNA system. In this process, the nascent polypeptide is modified by the addition of a short C-terminal sequence called the ssrA tag, which is encoded by tmRNA and allows normal termination and release of ribosomal subunits. In most bacteria, ssrA-tagged proteins are degraded by the AAA+ protease, ClpXP. However, in bacterial species of the genus Mycoplasma, genes for ClpXP and many other proteins were lost through reductive evolution. Interestingly, Mycoplasma ssrA tag sequences are very different from the tags in other bacteria. We report that ssrA-tagged proteins in Mesoplasma florum, a Mycoplasma species, are efficiently recognized and degraded by the AAA+ Lon protease. Thus, retaining degradation of ssrA-tagged translation products was apparently important enough during speciation of Mycoplasma to drive adaptation of the ssrA tag to a different protease. These results emphasize the importance of coupling proteolysis with tmRNA-mediated tagging and ribosome rescue.

Inducible protein degradation in Bacillus subtilis using heterologous peptide tags and adaptor proteins to target substrates to the protease ClpXP

The ability to manipulate protein levels is useful for dissecting regulatory pathways, elucidating gene function and constructing synthetic biological circuits. We engineered an inducible protein degradation system for use in Bacillus subtilis based on Escherichia coli and Caulobacter crescentusssrA tags and SspB adaptors that deliver proteins to ClpXP for proteolysis. In this system, modified ssrA degradation tags are fused onto the 3' end of the genes of interest. Unlike wild-type ssrA, these modified tags require the adaptor protein SspB to target tagged proteins for proteolysis. In the absence of SspB, the tagged proteins accumulate to near physiological levels. By inducing SspB expression from a regulated promoter, the tagged substrates are rapidly delivered to the B. subtilis ClpXP protease for degradation. We used this system to degrade the reporter GFP and several native B. subtilis proteins, including, the transcription factor ComA, two sporulation kinases (KinA, KinB) and the sporulation and chromosome partitioning protein Spo0J. We also used modified E. coli and C. crescentus ssrA tags to independently control the degradation of two different proteins in the same cell. These tools will be useful for studying biological processes in B. subtilis and can potentially be modified for use in other bacteria.

A degradation signal recognition in prokaryotes

The degradation of ssrA-tagged substrates in prokaryotes is conducted by a subset of ATP-dependent proteases, including ClpXP complex. More than 630 sequences of ssrA have been identified from 514 species, and are conserved in a wide range of prokaryotes. SspB protein markedly stimulates the degradation of these ssrA-tagged substrates by the ClpXP proteolytic machine. The dimeric SspB protein is composed of a compact ssrA-binding domain, which has a dimerization surface and a flexible C-terminal tail with a ClpX-binding motif at its very end. Since SspB is an adaptor protein for the ClpXP complex, designed mutagenesis, fluorescence spectroscopy, biochemistry and X-ray crystallography have been used to investigate the mechanism of delivery of ssrA-tagged proteins. In this paper the structural basis of ssrA-tag recognition by ClpX and SspB, as well as SspB-tail recognition by ZBD, is described.

The tmRNA system for translational surveillance and ribosome rescue

The tmRNA system performs translational surveillance and ribosome rescue in all eubacteria and some eukaryotic organelles. This system intervenes when ribosomes read to the 3' end of an mRNA or pause at internal codons with subsequent mRNA cleavage. A complex of alanyl-tmRNA (which functions as a tRNA and mRNA), SmpB protein, and EF-TucGTP binds stalled ribosomes, the nascent polypeptide is transferred to the alanine on tmRNA, and translation switches from the original message to a short tmRNA open reading frame (ORF) that encodes a degradation tag. Translation of the ORF and normal termination releases the tagged polypeptide for degradation and permits disassembly and recycling of ribosomal subunits for new rounds of protein synthesis. Structural and biochemical studies suggest mechanisms that keep tmRNA from interrupting normal translation and target ribosomes stalled with very short 3' mRNA extensions. Additional biological roles of tmRNA include stress management and the regulation of transcriptional circuits.

Discovery of antibacterial cyclic peptides that inhibit the ClpXP protease

A method to rapidly screen libraries of cyclic peptides in vivo for molecules with biological activity has been developed and used to isolate cyclic peptide inhibitors of the ClpXP protease. Fluorescence activated cell sorting was used in conjunction with a fluorescent reporter to isolate cyclic peptides that inhibit the proteolysis of tmRNA-tagged proteins in Escherichia coli. Inhibitors shared little sequence similarity and interfered with unexpected steps in the ClpXP mechanism in vitro. One cyclic peptide, IXP1, inhibited the degradation of unrelated ClpXP substrates and has bactericidal activity when added to growing cultures of Caulobacter crescentus, a model organism that requires ClpXP activity for viability. The screen used here could be adapted to identify cyclic peptide inhibitors of any enzyme that can be expressed in E. coli in conjunction with a fluorescent reporter.

Direct and adaptor-mediated substrate recognition by an essential AAA+ protease

Regulated proteolysis is required to execute many cellular programs. In Caulobacter crescentus, timely degradation of the master regulator CtrA by ClpXP protease is essential for cell-cycle progression and requires the colocalization of CtrA and RcdA. Here, we establish a biochemical framework to understand regulated proteolysis in C. crescentus and show that RcdA is not an adaptor for CtrA degradation. CtrA is rapidly degraded without RcdA and is recognized with an affinity comparable with the best ClpXP substrates. In contrast, SspBalpha, the alpha-proteobacterial homolog of SspB, functions as an adaptor to enhance degradation of specific substrates. Cargo-free SspBalpha is also itself a substrate of ClpXP-mediated proteolysis. Thus, our analysis (i) reveals the consequences of both direct and adaptor-stimulated recognition in mediating substrate specificity in vitro, (ii) reveals a potential regulatory role of controlled adaptor stability, and (iii) suggests that cell-cycle regulation of CtrA stability depends on repression of its intrinsic degradation rather than adaptor-mediated enhancement.