Multimodal binding and inhibition of bacterial ribosomes by the antimicrobial peptides Api137 and Api88.

Proline-rich antimicrobial peptides (PrAMPs) inhibit bacterial protein biosynthesis by binding to the polypeptide exit tunnel (PET) near the peptidyl transferase center. Api137, an optimized derivative of honeybee PrAMP apidaecin, inhibits protein expression by trapping release factors (RFs), which interact with stop codons on ribosomes to terminate translation. This study uses cryo-EM, functional assays and molecular dynamic (MD) simulations to show that Api137 additionally occupies a second binding site near the exit of the PET and can repress translation independently of RF-trapping. Api88, a C-terminally amidated (-CONH2) analog of Api137 (-COOH), binds to the same sites, occupies a third binding pocket and interferes with the translation process presumably without RF-trapping. In conclusion, apidaecin-derived PrAMPs inhibit bacterial ribosomes by multimodal mechanisms caused by minor structural changes and thus represent a promising pool for drug development efforts.

Critical steps in the assembly process of the bacterial 50S ribosomal subunit.

During assembly, ribosomal particles in bacteria fold according to energy landscapes comprised of multiple parallel pathways. Cryo-electron microscopy studies have identified a critical maturation step that occurs during the late assembly stages of the 50S subunit in Bacillus subtilis. This step acts as a point of convergency for all the parallel assembly pathways of the subunit, where an assembly intermediate accumulates in a 'locked' state, causing maturation to pause. Assembly factors then act on this critical step to 'unlock' the last maturation steps involving the functional sites. Without these factors, the 50S subunit fails to complete its assembly, causing cells to die due to a lack of functional ribosomes to synthesize proteins. In this review, we analyze these findings in B. subtilis and examine other cryo-EM studies that have visualized assembly intermediates in different bacterial species, to determine if convergency points in the ribosome assembly process are a common theme among bacteria. There are still gaps in our knowledge, as these methodologies have not yet been applied to diverse species. However, identifying and characterizing these convergency points can reveal how different bacterial species implement unique mechanisms to regulate critical steps in the ribosome assembly process.

Decoding a ribosome uncertainty.

The ribosome is among the most ancient macromolecular complexes. Throughout evolution, the function of the ribosome has remained essential and conserved: the decoding of an mRNA template with tRNA-linked amino acids, to synthesize a protein. In a recent study, Holm et al. capture evolutionary distinctions in the structure and kinetics of 'mRNA decoding' by the human ribosome.

Cryo-EM captures early ribosome assembly in action.

Ribosome biogenesis is a fundamental multi-step cellular process in all domains of life that involves the production, processing, folding, and modification of ribosomal RNAs (rRNAs) and ribosomal proteins. To obtain insights into the still unexplored early assembly phase of the bacterial 50S subunit, we exploited a minimal in vitro reconstitution system using purified ribosomal components and scalable reaction conditions. Time-limited assembly assays combined with cryo-EM analysis visualizes the structurally complex assembly pathway starting with a particle consisting of ordered density for only ~500 nucleotides of 23S rRNA domain I and three ribosomal proteins. In addition, our structural analysis reveals that early 50S assembly occurs in a domain-wise fashion, while late 50S assembly proceeds incrementally. Furthermore, we find that both ribosomal proteins and folded rRNA helices, occupying surface exposed regions on pre-50S particles, induce, or stabilize rRNA folds within adjacent regions, thereby creating cooperativity.

Snapshots of native pre-50S ribosomes reveal a biogenesis factor network and evolutionary specialization.

Ribosome biogenesis is a fundamental multi-step cellular process that culminates in the formation of ribosomal subunits, whose production and modification are regulated by numerous biogenesis factors. In this study, we analyze physiologic prokaryotic ribosome biogenesis by isolating bona fide pre-50S subunits from an Escherichia coli strain with the biogenesis factor ObgE, affinity tagged at its native gene locus. Our integrative structural approach reveals a network of interacting biogenesis factors consisting of YjgA, RluD, RsfS, and ObgE on the immature pre-50S subunit. In addition, our study provides mechanistic insight into how the GTPase ObgE, in concert with other biogenesis factors, facilitates the maturation of the 50S functional core and reveals both conserved and divergent evolutionary features of ribosome biogenesis between prokaryotes and eukaryotes.

Multifaceted Stoichiometry Control of Bacterial Operons Revealed by Deep Proteome Quantification.

More than half of the protein-coding genes in bacteria are organized in polycistronic operons composed of two or more genes. It remains under debate whether the operon organization maintains the stoichiometric expression of the genes within an operon. In this study, we performed a label-free data-independent acquisition hyper reaction monitoring mass-spectrometry (HRM-MS) experiment to quantify the Escherichia coli proteome in exponential phase and quantified 93.6% of the cytosolic proteins, covering 67.9% and 56.0% of the translating polycistronic operons in BW25113 and MG1655 strains, respectively. We found that the translational regulation contributes largely to the proteome complexity: the shorter operons tend to be more tightly controlled for stoichiometry than longer operons; the operons which mainly code for complexes is more tightly controlled for stoichiometry than the operons which mainly code for metabolic pathways. The gene interval (distance between adjacent genes in one operon) may serve as a regulatory factor for stoichiometry. The catalytic efficiency might be a driving force for differential expression of enzymes encoded in one operon. These results illustrated the multifaceted nature of the operon regulation: the operon unified transcriptional level and gene-specific translational level. This multi-level regulation benefits the host by optimizing the efficiency of the productivity of metabolic pathways and maintenance of different types of protein complexes.

Translatomics: The Global View of Translation.

In all kingdoms of life, proteins are synthesized by ribosomes in a process referred to as translation. The amplitude of translational regulation exceeds the sum of transcription, mRNA degradation and protein degradation. Therefore, it is essential to investigate translation in a global scale. Like the other "omics"-methods, translatomics investigates the totality of the components in the translation process, including but not limited to translating mRNAs, ribosomes, tRNAs, regulatory RNAs and nascent polypeptide chains. Technical advances in recent years have brought breakthroughs in the investigation of these components at global scale, both for their composition and dynamics. These methods have been applied in a rapidly increasing number of studies to reveal multifaceted aspects of translation control. The process of translation is not restricted to the conversion of mRNA coding sequences into polypeptide chains, it also controls the composition of the proteome in a delicate and responsive way. Therefore, translatomics has extended its unique and innovative power to many fields including proteomics, cancer research, bacterial stress response, biological rhythmicity and plant biology. Rational design in translation can enhance recombinant protein production for thousands of times. This brief review summarizes the main state-of-the-art methods of translatomics, highlights recent discoveries made in this field and introduces applications of translatomics on basic biological and biomedical research.

Structural Visualization of the Formation and Activation of the 50S Ribosomal Subunit during In Vitro Reconstitution.

The assembly of ribosomal subunits is an essential prerequisite for protein biosynthesis in all domains of life. Although biochemical and biophysical approaches have advanced our understanding of ribosome assembly, our mechanistic comprehension of this process is still limited. Here, we perform an in vitro reconstitution of the Escherichia coli 50S ribosomal subunit. Late reconstitution products were subjected to high-resolution cryo-electron microscopy and multiparticle refinement analysis to reconstruct five distinct precursors of the 50S subunit with 4.3-3.8 Å resolution. These assembly intermediates define a progressive maturation pathway culminating in a late assembly particle, whose structure is more than 96% identical to a mature 50S subunit. Our structures monitor the formation and stabilization of structural elements in a nascent particle in unprecedented detail and identify the maturation of the rRNA-based peptidyl transferase center as the final critical step along the 50S assembly pathway.

Two Antagonistic MALT1 Auto-Cleavage Mechanisms Reveal a Role for TRAF6 to Unleash MALT1 Activation.

The paracaspase MALT1 has arginine-directed proteolytic activity triggered by engagement of immune receptors. Recruitment of MALT1 into activation complexes is required for MALT1 proteolytic function. Here, co-expression of MALT1 in HEK293 cells, either with activated CARD11 and BCL10 or with TRAF6, was used to explore the mechanism of MALT1 activation at the molecular level. This work identified a prominent self-cleavage site of MALT1 isoform A (MALT1A) at R781 (R770 in MALT1B) and revealed that TRAF6 can activate MALT1 independently of the CBM. Intramolecular cleavage at R781/R770 removes a C-terminal TRAF6-binding site in both MALT1 isoforms, leaving MALT1B devoid of the two key interaction sites with TRAF6. A previously identified auto-proteolysis site of MALT1 at R149 leads to deletion of the death-domain, thereby abolishing interaction with BCL10. By using MALT1 isoforms and cleaved fragments thereof, as well as TRAF6 WT and mutant forms, this work shows that TRAF6 induces N-terminal auto-proteolytic cleavage of MALT1 at R149 and accelerates MALT1 protein turnover. The MALT1 fragment generated by N-terminal self-cleavage at R149 was labile and displayed enhanced signaling properties that required an intact K644 residue, previously shown to be a site for mono-ubiquitination of MALT1. Conversely, C-terminal self-cleavage at R781/R770 hampered the ability for self-cleavage at R149 and stabilized MALT1 by hindering interaction with TRAF6. C-terminal self-cleavage had limited impact on MALT1A but severely reduced MALT1B proteolytic and signaling functions. It also abrogated NF-κB activation by N-terminally cleaved MALT1A. Altogether, this study provides further insights into mechanisms that regulate the scaffolding and activation cycle of MALT1. It also emphasizes the reduced functional capacity of MALT1B as compared to MALT1A.

Ribosome Assembly as Antimicrobial Target.

Many antibiotics target the ribosome and interfere with its translation cycle. Since translation is the source of all cellular proteins including ribosomal proteins, protein synthesis and ribosome assembly are interdependent. As a consequence, the activity of translation inhibitors might indirectly cause defective ribosome assembly. Due to the difficulty in distinguishing between direct and indirect effects, and because assembly is probably a target in its own right, concepts are needed to identify small molecules that directly inhibit ribosome assembly. Here, we summarize the basic facts of ribosome targeting antibiotics. Furthermore, we present an in vivo screening strategy that focuses on ribosome assembly by a direct fluorescence based read-out that aims to identify and characterize small molecules acting as primary assembly inhibitors.

Fluorescence-based monitoring of ribosome assembly landscapes.

BACKGROUND: Ribosomes and functional complexes of them have been analyzed at the atomic level. Far less is known about the dynamic assembly and degradation events that define the half-life of ribosomes and guarantee their quality control. RESULTS: We developed a system that allows visualization of intact ribosomal subunits and assembly intermediates (i.e. assembly landscapes) by convenient fluorescence-based analysis. To this end, we labeled the early assembly ribosomal proteins L1 and S15 with the fluorescent proteins mAzami green and mCherry, respectively, using chromosomal gene insertion. The reporter strain harbors fluorescently labeled ribosomal subunits that operate wild type-like, as shown by biochemical and growth assays. Using genetic and chemical perturbations by depleting genes encoding the ribosomal proteins L3 and S17, respectively, or using ribosome-targeting antibiotics, we provoked ribosomal subunit assembly defects. These defects were readily identified by fluorometric analysis after sucrose density centrifugation in unprecedented resolution. CONCLUSION: This strategy is useful to monitor and characterize subunit specific assembly defects caused by ribosome-targeting drugs that are currently used and to characterize new molecules that affect ribosome assembly and thereby constitute new classes of antibacterial agents.

Validation of a fluorescence-based screening concept to identify ribosome assembly defects in Escherichia coli.

While the structure of mature ribosomes is analyzed in atomic detail considerably less is known about their assembly process in living cells. This is mainly due to technical and conceptual hurdles. To analyze ribosome assembly in vivo, we designed and engineered an Escherichiacoli strain--using chromosomal gene knock-in techniques--that harbors large and small ribosomal subunits labeled with the fluorescent proteins EGFP and mCherry, respectively. A thorough characterization of this reporter strain revealed that its growth properties and translation apparatus were wild-type like. Alterations in the ratio of EGFP over mCherry fluorescence are supposed to indicate ribosome assembly defects. To provide proof of principle, subunit specific assembly defects were provoked and could be identified by both manual and fully automated fluorometric in vivo assays. This is to our knowledge the first methodology that directly detects ribosome assembly defects in vivo in a high-throughput compatible format. Screening of knock-out collections and small molecule libraries will allow identification of new ribosome assembly factors and possible inhibitors.

Structural determinants of MALT1 protease activity.

The formation of the CBM (CARD11-BCL10-MALT1) complex is pivotal for antigen-receptor-mediated activation of the transcription factor NF-κB. Signaling is dependent on MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1), which not only acts as a scaffolding protein but also possesses proteolytic activity mediated by its caspase-like domain. It remained unclear how the CBM activates MALT1. Here, we provide biochemical and structural evidence that MALT1 activation is dependent on its dimerization and show that mutations at the dimer interface abrogate activity in cells. The unliganded protease presents itself in a dimeric yet inactive state and undergoes substantial conformational changes upon substrate binding. These structural changes also affect the conformation of the C-terminal Ig-like domain, a domain that is required for MALT1 activity. Binding to the active site is coupled to a relative movement of caspase and Ig-like domains. MALT1 binding partners thus may have the potential of tuning MALT1 protease activity without binding directly to the caspase domain.

CHIP participates in protein triage decisions by preferentially ubiquitinating Hsp70-bound substrates.

The E3 ubiquitin ligase CHIP (C-terminus of Hsc70-interacting protein) is believed to be a central player in the cellular triage decision, as it links the molecular chaperones Hsp70/Hsc70 and Hsp90 to the ubiquitin proteasomal degradation pathway. To better understand the decision process, we determined the affinity of CHIP for Hsp70 and Hsp90 using isothermal titration calorimetry. We analyzed the influence of CHIP on the ATPase cycles of both chaperones in the presence of co-chaperones and a substrate, and determined the ubiquitination efficacy of CHIP in the presence of the chaperones. We found that CHIP has a sixfold higher affinity for Hsp90 compared with Hsc70. CHIP had no influence on ADP dissociation or ATP association, but reduced the Hsp70 cochaperone Hdj1-stimulated single-turnover ATPase rates of Hsc70 and Hsp70. CHIP did not influence the ATPase cycle of Hsp90 in the absence of co-chaperones or in the presence of the Hsp90 cochaperones Aha1 or p23. Polyubiquitination of heat-denatured luciferase and the native substrate p53 was much more efficient in the presence of Hsc70 and Hdj1 than in the presence of Hsp90, indicating that CHIP preferentially ubiquitinates Hsp70-bound substrates.

Insights into the conformational dynamics of the E3 ubiquitin ligase CHIP in complex with chaperones and E2 enzymes.

The dimeric E3 ubiquitin ligase CHIP binds with its tetratricopeptide repeat (TPR) domain the C-terminus of molecular chaperones Hsp70 and Hsp90 and with its U-box region E2 ubiquitin-conjugating enzymes. By ubiquitinating chaperone-bound polypeptides, CHIP thus links the chaperone machinery to the proteasomal degradation pathway. The molecular mechanism of how CHIP discriminates between folding and destruction of chaperone substrates is not yet understood. Two recently published crystal structures of mouse and zebrafish CHIP truncation constructs differ substantially, showing either an asymmetric assembly or a symmetric assembly with a highly ordered middle domain. To characterize the conformational properties of the intact full-length protein in solution, we performed amide hydrogen exchange mass spectrometry (HX-MS) with human CHIP. In addition, we monitored conformational changes in CHIP upon binding of Hsp70, Hsp90, and their respective C-terminal EEVD peptides, and in complex with the different E2 ubiquitin-conjugating enzymes UbcH5a and Ubc13. Solution HX-MS data suggest a symmetric dimer assembly with highly flexible parts in the middle domain contrasting both the asymmetric and the symmetric crystal structure. CHIP exhibited an extraordinary flexibility with a largely unprotected N-terminal TPR domain. Formation of a complex with intact Hsp70 and Hsp90 or their respective C-terminal octapeptides induced folding of the TPR domain to a defined, highly stabilized structure with protected amide hydrogens. Interaction of CHIP with two different E2 ubiquitin-conjugating enzymes, UbcH5a and Ubc13, had distinct effects on the conformational dynamics of CHIP, suggesting different roles of the CHIP-E2 interaction in the ubiquitination of substrates and interaction with chaperones.

A conserved cysteine motif essential for ceramide kinase function.

Ceramide kinase (CerK) is a sphingolipid metabolizing enzyme very sensitive to oxidation; however, the determinants are unknown. We show here that the thiol-modifying agent N-ethyl-maleimide abrogates CerK activity in vitro and in a cell based assay, implying that important cysteine residues are accessible in purified as well as endogenous CerK. We replaced every 22 residues in human CerK, by an alanine, and measured activity in the resulting mutant proteins. This led to identification of a cluster of cysteines, C(347)XXXC(351)XXC(354), essential for CerK function. These findings are discussed based on homology modeling of the catalytic domain of CerK.

The Drosophila mitotic inhibitor Frühstart specifically binds to the hydrophobic patch of cyclins.

The hydrophobic patch of cyclins interacts with cyclin-dependent kinase (Cdk) substrates and p27-type Cdk inhibitors. Although this interaction is assumed to contribute to the specificity of different Cdk-Cyclin complexes, its role in specific steps of the cell cycle has not been demonstrated. Here, we show that in Drosophila the mitotic inhibitor Frühstart (Frs) binds specifically and with high affinity to the hydrophobic patch of cyclins. In contrast to p27-type Cdk inhibitors, Frs does not form a stable interaction with the catalytic centre of Cdk and allows phosphorylation of generic model substrates, such as histone H1. Consistent with a 2.5 times stronger binding to CycA than to CycE in vitro, ectopic expression of frs induces endocycles, in a manner similar to that reported previously for downregulation of CycA or Cdk1. We propose that binding of Frs to cyclins blocks the hydrophobic patch to interfere with Cdk1 substrate recognition.

A conserved motif is prerequisite for the interaction of NAC with ribosomal protein L23 and nascent chains.

In eukaryotes, newly synthesized proteins interact co-translationally with a multitude of different ribosome-bound factors and chaperones including the conserved heterodimeric nascent polypeptide-associated complex (NAC) and a Hsp40/70-based chaperone system. These factors are thought to play an important role in protein folding and targeting, yet their specific ribosomal localizations, which are prerequisite for their functions, remain elusive. This study describes the ribosomal localization of NAC and the molecular details by which NAC is able to contact the ribosome and gain access to nascent polypeptides. We identified a conserved RRK(X)nKK ribosome binding motif within the beta-subunit of NAC that is essential for the entire NAC complex to attach to ribosomes and allow for its interaction with nascent polypeptide chains. The motif localizes within a potential loop region between two predicted alpha-helices in the N terminus of betaNAC. This N-terminal betaNAC ribosome-binding domain was completely portable and sufficient to target an otherwise cytosolic protein to the ribosome. NAC modified with a UV-activatable cross-linker within its ribosome binding motif specifically cross-linked to L23 ribosomal protein family members at the exit site of the ribosome, providing the first evidence of NAC-L23 interaction in the context of the ribosome. Mutations of L23 reduced NAC ribosome binding in vivo and in vitro, whereas other eukaryotic ribosome-associated factors such as the Hsp70/40 chaperones Ssb or Zuotin were unaffected. We conclude that NAC employs a conserved ribosome binding domain to position itself on the L23 ribosomal protein adjacent to the nascent polypeptide exit site.

Dimerization of the human E3 ligase CHIP via a coiled-coil domain is essential for its activity.

The Hsp70-interacting E3-ubiquitin ligase CHIP has been implicated in the decision as to whether a target protein enters the refolding or the degradation pathway. To further characterize the activity of CHIP we purified untagged Homo sapiens and Drosophila melanogaster CHIP (hCHIP, dCHIP). In contrast to other E3-ubiquitin ligases, both hCHIP and dCHIP proteins formed homodimers at physiological concentrations. We identified a predicted coiled-coil region in a mixed charge segment of the hCHIP and dCHIP sequence and found it to be necessary and sufficient for dimer formation. A mutant of hCHIP lacking this segment (hCHIPdelta-(128-229)) was incapable of dimer formation, but the segment by itself (hCHIP-(128-229)) readily dimerized. Furthermore, we demonstrated that dimerization is a prerequisite for activity of hCHIP in the reconstituted ubiquitination assay. Control of dimerization may thus provide a mechanism for regulation of CHIP activity.

Aha, another regulator for hsp90 chaperones.

A large number of key regulators controlling homeostasis and cell fate are chaperoned by the Hsp90 folding machine. In this issue of Molecular Cell, report the discovery of a new stress-regulated cochaperone, Aha1, which accelerates the dynamics of this machine.