Acetylation of L12 increases interactions in the Escherichia coli ribosomal stalk complex

Biofilm-producing Escherichia coli O104:H4 overcomes bile salts toxicity by expressing virulence and resistance proteins

We investigated bile salts' ability to induce phenotypic changes in biofilm production and protein expression of pathogenic Escherichia coli strains. For this purpose, 82 pathogenic E. coli strains isolated from humans (n = 70), and animals (n = 12), were examined for their ability to form biofilms in the presence or absence of bile salts. We also identified bacterial proteins expressed in response to bile salts using sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-electrophoresis) and liquid chromatography-mass spectrometry (LC-MS/MS). Lastly, we evaluated the ability of these strains to adhere to Caco-2 epithelial cells in the presence of bile salts. Regarding biofilm formation, two strains isolated from an outbreak in Republic of Georgia in 2009 were the only ones that showed a high and moderate capacity to form biofilm in the presence of bile salts. Further, we observed that those isolates, when in the presence of bile salts, expressed different proteins identified as outer membrane proteins (i.e. OmpC), and resistance to adverse growth conditions (i.e. F0F1, HN-S, and L7/L12). We also found that these isolates exhibited high adhesion to epithelial cells in the presence of bile salts. Together, these results contribute to the phenotypic characterization of E. coli O104: H4 strains.

Functional and structural characterisation of RimL from Bacillus cereus, a new Nα-acetyltransferase of ribosomal proteins that was wrongly assigned as an aminoglycosyltransferase

Enzymes of the GNAT (GCN5-relate N-acetyltransferases) superfamily are important regulators of cell growth and development. They are functionally diverse and share low amino acid sequence identity, making functional annotation difficult. In this study, we report the function and structure of a new ribosomal enzyme, Nα-acetyl transferase from Bacillus cereus (RimLBC), a protein that was previously wrongly annotated as an aminoglycosyltransferase. Firstly, extensive comparative amino acid sequence analyses suggested RimLBC belongs to a cluster of proteins mediating acetylation of the ribosomal protein L7/L12. To assess if this was the case, several well established substrates of aminoglycosyltransferases were screened. The results of these studies did not support an aminoglycoside acetylating function for RimLBC. To gain further insight into RimLBC biological role, a series of studies that included MALDI-TOF, isothermal titration calorimetry, NMR, X-ray protein crystallography, and site-directed mutagenesis confirmed RimLBC affinity for Acetyl-CoA and that the ribosomal protein L7/L12 is a substrate of RimLBC. Last, we advance a mechanistic model of RimLBC mode of recognition of its protein substrates. Taken together, our studies confirmed RimLBC as a new ribosomal Nα-acetyltransferase and provide structural and functional insights into substrate recognition by Nα-acetyltransferases and protein acetylation in bacteria.

Coping with stress: How bacteria fine-tune protein synthesis and protein transport

Maintaining a functional proteome under different environmental conditions is challenging for every organism, in particular for unicellular organisms, such as bacteria. In order to cope with changing environments and stress conditions, bacteria depend on strictly coordinated proteostasis networks that control protein production, folding, trafficking, and degradation. Regulation of ribosome biogenesis and protein synthesis are cornerstones of this cellular adaptation in all domains of life, which is rationalized by the high energy demand of both processes and the increased resistance of translationally silent cells against internal or external poisons. Reduced protein synthesis ultimately also reduces the substrate load for protein transport systems, which are required for maintaining the periplasmic, inner, and outer membrane subproteomes. Consequences of impaired protein transport have been analyzed in several studies and generally induce a multifaceted response that includes the upregulation of chaperones and proteases and the simultaneous downregulation of protein synthesis. In contrast, generally less is known on how bacteria adjust the protein targeting and transport machineries to reduced protein synthesis, e.g., when cells encounter stress conditions or face nutrient deprivation. In the current review, which is mainly focused on studies using Escherichia coli as a model organism, we summarize basic concepts on how ribosome biogenesis and activity are regulated under stress conditions. In addition, we highlight some recent developments on how stress conditions directly impair protein targeting to the bacterial membrane. Finally, we describe mechanisms that allow bacteria to maintain the transport of stress-responsive proteins under conditions when the canonical protein targeting pathways are impaired.

System-wide analyses reveal essential roles of N-terminal protein modification in bacterial membrane integrity

The removal of the N-terminal formyl group on nascent proteins by peptide deformylase (PDF) is the most prevalent protein modification in bacteria. PDF is a critical target of antibiotic development; however, its role in bacterial physiology remains a long-standing question. This work used the time-resolved analyses of the Escherichia coli translatome and proteome to investigate the consequences of PDF inhibition. Loss of PDF activity rapidly induces cellular stress responses, especially those associated with protein misfolding and membrane defects, followed by a global down-regulation of metabolic pathways. Rapid membrane hyperpolarization and impaired membrane integrity were observed shortly after PDF inhibition, suggesting that the plasma membrane disruption is the most immediate and primary consequence of formyl group retention on nascent proteins. This work resolves the physiological function of a ubiquitous protein modification and uncovers its crucial role in maintaining the structure and function of the bacterial membrane.

•

PDF inhibition induces membrane defects and metabolic imbalance

•

Deformylation is involved in nascent protein folding

•

Membrane is the earliest and primary target of N-formylation on nascent proteins

•

PDF activity is essential for redox homeostasis in bacteria

Native top-down mass spectrometry for higher-order structural characterization of proteins and complexes

Progress in structural biology research has led to a high demand for powerful and yet complementary analytical tools for structural characterization of proteins and protein complexes. This demand has significantly increased interest in native mass spectrometry (nMS), particularly native top-down mass spectrometry (nTDMS) in the past decade. This review highlights recent advances in nTDMS for structural research of biological assemblies, with a particular focus on the extra multi-layers of information enabled by TDMS. We include a short introduction of sample preparation and ionization to nMS, tandem fragmentation techniques as well as mass analyzers and software/analysis pipelines used for nTDMS. We highlight unique structural information offered by nTDMS and examples of its broad range of applications in proteins, protein-ligand interactions (metal, cofactor/drug, DNA/RNA, and protein), therapeutic antibodies and antigen-antibody complexes, membrane proteins, macromolecular machineries (ribosome, nucleosome, proteosome, and viruses), to endogenous protein complexes. The challenges, potential, along with perspectives of nTDMS methods for the analysis of proteins and protein assemblies in recombinant and biological samples are discussed.

Ribosomal protein S18 acetyltransferase RimI is responsible for the acetylation of elongation factor Tu

N-terminal acetylation is widespread in the eukaryotic proteome but in bacteria is restricted to a small number of proteins mainly involved in translation. It was long known that elongation factor Tu (EF-Tu) is N-terminally acetylated, whereas the enzyme responsible for this process was unclear. Here, we report that RimI acetyltransferase, known to modify ribosomal protein S18, is likewise responsible for N-acetylation of the EF-Tu. With the help of inducible tufA expression plasmid, we demonstrated that the acetylation does not alter the stability of EF-Tu. Binding of aminoacyl tRNA to the recombinant EF-Tu in vitro was found to be unaffected by the acetylation. At the same time, with the help of fast kinetics methods, we demonstrate that an acetylated variant of EF-Tu more efficiently accelerates A-site occupation by aminoacyl-tRNA, thus increasing the efficiency of in vitro translation. Finally, we show that a strain devoid of RimI has a reduced growth rate, expanded to an evolutionary timescale, and might potentially promote conservation of the acetylation mechanism of S18 and EF-Tu. This study increased our understanding of the modification of bacterial translation apparatus.

Specialised ribosomes as versatile regulators of gene expression

The ribosome has long been thought to be a homogeneous cellular machine that constitutively and globally synthesises proteins from mRNA. However, recent studies have revealed that ribosomes are highly heterogeneous, dynamic macromolecular complexes with specialised roles in translational regulation in many organisms across the kingdoms. In this review, we summarise the current understanding of ribosome heterogeneity and the specialised functions of heterogeneous ribosomes. We also discuss specialised translation systems that utilise orthogonal ribosomes.

Post-translational Lysine Ac(et)ylation in Bacteria: A Biochemical, Structural, and Synthetic Biological Perspective

Ac(et)ylation is a post-translational modification present in all domains of life. First identified in mammals in histones to regulate RNA synthesis, today it is known that is regulates fundamental cellular processes also in bacteria: transcription, translation, metabolism, cell motility. Ac(et)ylation can occur at the ε-amino group of lysine side chains or at the α-amino group of a protein. Furthermore small molecules such as polyamines and antibiotics can be acetylated and deacetylated enzymatically at amino groups. While much research focused on N-(ε)-ac(et)ylation of lysine side chains, much less is known about the occurrence, the regulation and the physiological roles on N-(α)-ac(et)ylation of protein amino termini in bacteria. Lysine ac(et)ylation was shown to affect protein function by various mechanisms ranging from quenching of the positive charge, increasing the lysine side chains’ size affecting the protein surface complementarity, increasing the hydrophobicity and by interfering with other post-translational modifications. While N-(ε)-lysine ac(et)ylation was shown to be reversible, dynamically regulated by lysine acetyltransferases and lysine deacetylases, for N-(α)-ac(et)ylation only N-terminal acetyltransferases were identified and so far no deacetylases were discovered neither in bacteria nor in mammals. To this end, N-terminal ac(et)ylation is regarded as being irreversible. Besides enzymatic ac(et)ylation, recent data showed that ac(et)ylation of lysine side chains and of the proteins N-termini can also occur non-enzymatically by the high-energy molecules acetyl-coenzyme A and acetyl-phosphate. Acetyl-phosphate is supposed to be the key molecule that drives non-enzymatic ac(et)ylation in bacteria. Non-enzymatic ac(et)ylation can occur site-specifically with both, the protein primary sequence and the three dimensional structure affecting its efficiency. Ac(et)ylation is tightly controlled by the cellular metabolic state as acetyltransferases use ac(et)yl-CoA as donor molecule for the ac(et)ylation and sirtuin deacetylases use NAD⁺ as co-substrate for the deac(et)ylation. Moreover, the accumulation of ac(et)yl-CoA and acetyl-phosphate is dependent on the cellular metabolic state. This constitutes a feedback control mechanism as activities of many metabolic enzymes were shown to be regulated by lysine ac(et)ylation. Our knowledge on lysine ac(et)ylation significantly increased in the last decade predominantly due to the huge methodological advances that were made in fields such as mass-spectrometry, structural biology and synthetic biology. This also includes the identification of additional acylations occurring on lysine side chains with supposedly different regulatory potential. This review highlights recent advances in the research field. Our knowledge on enzymatic regulation of lysine ac(et)ylation will be summarized with a special focus on structural and mechanistic characterization of the enzymes, the mechanisms underlying non-enzymatic/chemical ac(et)ylation are explained, recent technological progress in the field are presented and selected examples highlighting the important physiological roles of lysine ac(et)ylation are summarized.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

Mass Analysis of Macro-molecular Analytes via Multiply-Charged Ion Attachment

A novel gas-phase charge and mass manipulation approach is demonstrated to facilitate the mass measurement of high mass complexes within the context of native mass spectrometry. Electrospray ionization applied to solutions generated under native or near-native conditions has been demonstrated to be capable of preserving biologically relevant complexes into the gas phase as multiply charged ions suitable for mass spectrometric analysis. However, charge state distributions tend to be narrow and extensive salt adduction, heterogeneity, and so on tend to lead to significantly broadened peaks. These issues can compromise mass measurement of high mass bio-complexes, particularly when charge states are not clearly resolved. In this work, we show that the attachment of high mass ions of known mass and charge to populations of ions of interest can lead to well-separated signals that can yield confident charge state and mass assignments from otherwise poorly resolved signals.

Acetylation of translation machinery affected protein translation in E. coli

Reversible lysine acetylation (RLA) of translation machinery components, such as ribosomal proteins (RPs) and translation factors (TFs), was identified in many microorganisms, while knowledge of its function and effect on translation remains limited. Herein, we show that translation machinery is regulated by acetylation. Using the cell-free translation system of E. coli, we found that AcP-driven acetylation significantly reduced the relative translation rate, and deacetylation partially restored the translation activity. Hyperacetylation caused by intracellular AcP accumulation or carbon/nitrogen fluctuation (carbon overflow or nitrogen limitation) modulated protein translation in vivo. These results uncovered a critical role of acetylation in translation regulation and indicated that carbon/nitrogen imbalance induced acetylation of ribosome in E. coli and dynamically affected translation rate via a global, uniform manner. KEY POINTS: • Acetylation of translation machinery directly regulated global translation. • K618 of EF-G, K411, and K464 of S1 are the key points influencing translation rate. • Carbon/nitrogen imbalance triggers AcP-dependent acetylation.

Native mass spectrometry-A valuable tool in structural biology

Proteins and the complexes they form with their ligands are the players of cellular action. Their function is directly linked with their structure making the structural analysis of protein-ligand complexes essential. Classical techniques of structural biology include X-ray crystallography, nuclear magnetic resonance spectroscopy and recently distinguished cryo-electron microscopy. However, protein-ligand complexes are often dynamic and heterogeneous and consequently challenging for these techniques. Alternative approaches are therefore needed and gained importance during the last decades. One alternative is native mass spectrometry, which is the analysis of intact protein complexes in the gas phase. To achieve this, sample preparation and instrument conditions have to be optimised. Native mass spectrometry then reveals stoichiometry, protein interactions and topology of protein assemblies. Advanced techniques such as ion mobility and high-resolution mass spectrometry further add to the range of applications and deliver information on shape and microheterogeneity of the complexes. In this tutorial, we explain the basics of native mass spectrometry including sample requirements, instrument modifications and interpretation of native mass spectra. We further discuss the developments of native mass spectrometry and provide example spectra and applications.

Oligoglutamylation of E. coli ribosomal protein S6 is under growth phase control

Ribosomal protein S6 in Escherichia coli is modified by ATP-dependent glutamate ligase RimK. Up to four glutamate residues are added to the C-terminus of S6 protein. In this work we demonstrated that unlike the majority of ribosome modifications in E. coli, oligoglutamylation of S6 protein is regulated and happens only in the stationary phase of bacterial culture. Only S6 protein incorporated into assembled small ribosomal subunits, but not newly made free S6 protein is a substrate for RimK protein. Overexpression of the rimK gene leads to the modification of S6 protein even in the exponential phase of bacterial culture. Thus, it is unlikely that any stationary phase specific factor is needed for the modification. We propose a model that S6 modification is regulated solely via the rate of ribosome biosynthesis at limiting concentration of RimK enzyme.

Biophysical and functional characterizations of recombinant RimI acetyltransferase from Mycobacterium tuberculosis

Nα-acetylation is a universal protein modification related to a wide range of physiological processes in eukaryotes and prokaryotes. RimI, an Nα-acetyltransferase in Mycobacterium tuberculosis, is responsible for the acetylation of the α-amino group of the N-terminal residue in the ribosomal protein S18. Despite growing evidence that protein acetylation may be correlated with the pathogenesis of tuberculosis, no structural information is yet available for mechanistically understanding the MtRimI acetylation. To enable structural studies for MtRimI, we constructed a serial of recombinant MtRimI proteins and assessed their biochemical properties. We then chose an optimal construct MtRimIC21A4-153 and expressed and purified the truncated high-quality protein for further biophysical and functional characterizations. The 2D 1H-15N heteronuclear single quantum coherence spectrum of MtRimIC21A4-153 exhibits wider chemical shift dispersion and favorable peak isolation, indicating that MtRimIC21A4-153 is amendable for further structural determination. Moreover, bio-layer interferometry experiments showed that MtRimIC21A4-153 possessed similar micromolar affinity to full-length MtRimI for binding the hexapeptide substrate Ala-Arg-Tyr-Phe-Arg-Arg. Enzyme kinetic assays also exhibited that MtRimIC21A4-153 had almost identical enzymatic activity to MtRimI, indicating insignificant influence of the recombinant variations on enzymatic functions. Furthermore, binding sites of the peptide were predicted by molecular docking approach, suggesting that this substrate binds to MtRimI primarily through electrostatic and hydrogen bonding interactions. Our results lay a foundation for the further structural determination and dynamics detection of MtRimI.

Exceptionally versatile – arginine in bacterial post-translational protein modifications

Post-translational modifications (PTM) are the evolutionary solution to challenge and extend the boundaries of genetically predetermined proteomic diversity. As PTMs are highly dynamic, they also hold an enormous regulatory potential. It is therefore not surprising that out of the 20 proteinogenic amino acids, 15 can be post-translationally modified. Even the relatively inert guanidino group of arginine is subject to a multitude of mostly enzyme mediated chemical changes. The resulting alterations can have a major influence on protein function. In this review, we will discuss how bacteria control their cellular processes and develop pathogenicity based on post-translational protein-arginine modifications.

Bacillus megaterium adapts to acid stress condition through a network of genes: Insight from a genome-wide transcriptome analysis

RNA-seq analysis of B. megaterium exposed to pH 7.0 and pH 4.5 showed differential expression of 207 genes related to several processes. Among the 207 genes, 11 genes displayed increased transcription exclusively in pH 4.5. Exposure to pH 4.5 induced the expression of genes related to maintenance of cell integrity, pH homeostasis, alternative energy generation and modification of metabolic processes. Metabolic processes like pentose phosphate pathway, fatty acid biosynthesis, cysteine and methionine metabolism and synthesis of arginine and proline were remodeled during acid stress. Genes associated with oxidative stress and osmotic stress were up-regulated at pH 4.5 indicating a link between acid stress and other stresses. Acid stress also induced expression of genes that encoded general stress-responsive proteins as well as several hypothetical proteins. Our study indicates that a network of genes aid B. megaterium G18 to adapt and survive in acid stress condition.

Transcription-translation coupling: direct interactions of RNA polymerase with ribosomes and ribosomal subunits

In prokaryotes, RNA polymerase and ribosomes can bind concurrently to the same RNA transcript, leading to the functional coupling of transcription and translation. The interactions between RNA polymerase and ribosomes are crucial for the coordination of transcription with translation. Here, we report that RNA polymerase directly binds ribosomes and isolated large and small ribosomal subunits. RNA polymerase and ribosomes form a one-to-one complex with a micromolar dissociation constant. The formation of the complex is modulated by the conformational and functional states of RNA polymerase and the ribosome. The binding interface on the large ribosomal subunit is buried by the small subunit during protein synthesis, whereas that on the small subunit remains solvent-accessible. The RNA polymerase binding site on the ribosome includes that of the isolated small ribosomal subunit. This direct interaction between RNA polymerase and ribosomes may contribute to the coupling of transcription to translation.

From molecular chaperones to membrane motors: through the lens of a mass spectrometrist

Twenty-five years ago, we obtained our first mass spectra of molecular chaperones in complex with protein ligands and entered a new field of gas-phase structural biology. It is perhaps now time to pause and reflect, and to ask how many of our initial structure predictions and models derived from mass spectrometry (MS) datasets were correct. With recent advances in structure determination, many of the most challenging complexes that we studied over the years have become tractable by other structural biology approaches enabling such comparisons to be made. Moreover, in the light of powerful new electron microscopy methods, what role is there now for MS? In considering these questions, I will give my personal view on progress and problems as well as my predictions for future directions.

Investigation of Structure of the Ribosomal L12/P Stalk

This review contains recent data on the structure of the functionally important ribosomal domain, L12/P stalk, of the large ribosomal subunit. It is the most mobile site of the ribosome; it has been found in ribosomes of all living cells, and it is involved in the interaction between ribosomes and translation factors. The difference between the structures of the ribosomal proteins forming this protuberance (despite their general resemblance) determines the specificity of interaction between eukaryotic and prokaryotic ribosomes and the respective protein factors of translation. In this review, works on the structures of ribosomal proteins forming the L12/P-stalk in bacteria, archaea, and eukaryotes and data on structural aspects of interactions between these proteins and rRNA are described in detail.

Structure of the 70S ribosome from human pathogen Staphylococcus aureus

Comparative structural studies of ribosomes from various organisms keep offering exciting insights on how species-specific or environment-related structural features of ribosomes may impact translation specificity and its regulation. Although the importance of such features may be less obvious within more closely related organisms, their existence could account for vital yet species-specific mechanisms of translation regulation that would involve stalling, cell survival and antibiotic resistance. Here, we present the first full 70S ribosome structure from Staphylococcus aureus, a Gram-positive pathogenic bacterium, solved by cryo-electron microscopy. Comparative analysis with other known bacterial ribosomes pinpoints several unique features specific to S. aureus around a conserved core, at both the protein and the RNA levels. Our work provides the structural basis for the many studies aiming at understanding translation regulation in S. aureus and for designing drugs against this often multi-resistant pathogen.

The world of protein acetylation

Acetylation is one of the major post-translational protein modifications in the cell, with manifold effects on the protein level as well as on the metabolome level. The acetyl group, donated by the metabolite acetyl-coenzyme A, can be co- or post-translationally attached to either the α-amino group of the N-terminus of proteins or to the ε-amino group of lysine residues. These reactions are catalyzed by various N-terminal and lysine acetyltransferases. In case of lysine acetylation, the reaction is enzymatically reversible via tightly regulated and metabolism-dependent mechanisms. The interplay between acetylation and deacetylation is crucial for many important cellular processes. In recent years, our understanding of protein acetylation has increased significantly by global proteomics analyses and in depth functional studies. This review gives a general overview of protein acetylation and the respective acetyltransferases, and focuses on the regulation of metabolic processes and physiological consequences that come along with protein acetylation.

Insights into the Stress Response Triggered by Kasugamycin in Escherichia coli

The bacteriostatic aminoglycoside antibiotic kasugamycin inhibits protein synthesis at an initial step without affecting translation elongation. It binds to the mRNA track of the ribosome and prevents formation of the translation initiation complex on canonical mRNAs. In contrast, translation of leaderless mRNAs continues in the presence of the drug in vivo. Previously, we have shown that kasugamycin treatment in E. coli stimulates the formation of protein-depleted ribosomes that are selective for leaderless mRNAs. Here, we provide evidence that prolonged kasugamycin treatment leads to selective synthesis of specific proteins. Our studies indicate that leaderless and short-leadered mRNAs are generated by different molecular mechanisms including alternative transcription and RNA processing. Moreover, we provide evidence for ribosome heterogeneity in response to kasugamycin treatment by alteration of the modification status of the stalk proteins bL7/L12.

Proteome-wide analysis of the amino terminal status of Escherichia coli proteins at the steady-state and upon deformylation inhibition

A proteome wide analysis was performed in Escherichia coli to identify the impact on protein N-termini of actinonin, an antibiotic specifically inhibiting peptide deformylase (PDF). A strategy and tool suite (SILProNaQ) was employed to provide large-scale quantitation of N-terminal modifications. In control conditions, more than 1000 unique N-termini were identified with 56% showing initiator methionine removal. Additional modifications corresponded to partial or complete Nα-acetylation (10%) and N-formyl retention (5%). Among the proteins undergoing these N-terminal modifications, 140 unique N-termini from translocated membrane proteins were highlighted. The very early time-course impact of actinonin was followed after addition of bacteriostatic concentrations of the drug. Under these conditions, 26% of all proteins did not undergo deformylation any longer after 10 min, a value reaching more than 60% of all characterized proteins after 40 min of treatment. The N-formylation ratio measured on individual proteins increased with the same trend. Upon early PDF inhibition, two major categories of proteins retained their N-formyl group: a large number of inner membrane proteins and many proteins involved in protein synthesis including factors assisting the nascent chains in early cotranslational events. All MS data have been deposited in the ProteomeXchange with identifiers PXD001979, PXD002012 and PXD001983 (http://proteomecentral.proteomexchange.org/dataset/PXD001979, http://proteomecentral.proteomexchange.org/dataset/PXD002012 and http://proteomecentral.proteomexchange.org/dataset/PXD001983).

Bacterial GCN5-Related N-Acetyltransferases: From Resistance to Regulation

The GCN5-related N-acetyltransferases family (GNAT) is an important family of proteins that includes more than 100000 members among eukaryotes and prokaryotes. Acetylation appears as a major regulatory post-translational modification and is as widespread as phosphorylation. N-Acetyltransferases transfer an acetyl group from acetyl-CoA to a large array of substrates, from small molecules such as aminoglycoside antibiotics to macromolecules. Acetylation of proteins can occur at two different positions, either at the amino-terminal end (αN-acetylation) or at the ε-amino group (εN-acetylation) of an internal lysine residue. GNAT members have been classified into different groups on the basis of their substrate specificity, and in spite of a very low primary sequence identity, GNAT proteins display a common and conserved fold. This Current Topic reviews the different classes of bacterial GNAT proteins, their functions, their structural characteristics, and their mechanism of action.

Multiple Conformations of the Loop Region Confers Heat-Resistance on SsArd1, a Thermophilic NatA

Structural comparison indicates that the loop region between β3 and β4 of SsArd1 is extended relative to the corresponding region in mesophilic Nats, and forms a plastic hydrogen-bond network mainly at two serine residues. Strikingly, two single-point mutants showed ∼3 °C decrease in melting temperature, and two other variants showed ∼7 °C decrease; this correlated with significantly reduced enzymatic activity. To our knowledge, this is the first discovery of a loop region capable of remarkably improving protein thermostability. This provides a novel route to engineer heat-resistant proteins.

The biological functions of Naa10 - From amino-terminal acetylation to human disease

N-terminal acetylation (NTA) is one of the most abundant protein modifications known, and the N-terminal acetyltransferase (NAT) machinery is conserved throughout all Eukarya. Over the past 50 years, the function of NTA has begun to be slowly elucidated, and this includes the modulation of protein-protein interaction, protein-stability, protein function, and protein targeting to specific cellular compartments. Many of these functions have been studied in the context of Naa10/NatA; however, we are only starting to really understand the full complexity of this picture. Roughly, about 40% of all human proteins are substrates of Naa10 and the impact of this modification has only been studied for a few of them. Besides acting as a NAT in the NatA complex, recently other functions have been linked to Naa10, including post-translational NTA, lysine acetylation, and NAT/KAT-independent functions. Also, recent publications have linked mutations in Naa10 to various diseases, emphasizing the importance of Naa10 research in humans. The recent design and synthesis of the first bisubstrate inhibitors that potently and selectively inhibit the NatA/Naa10 complex, monomeric Naa10, and hNaa50 further increases the toolset to analyze Naa10 function.

Zinc regulates a switch between primary and alternative S18 ribosomal proteins in Mycobacterium tuberculosis

The Mycobacterium tuberculosis genome encodes five putative 'alternative' ribosomal proteins whose expression is repressed at high Zn(2+) concentration. Each alternative protein has a primary homologue that is predicted to bind Zn(2+). We hypothesized that zinc triggers a switch between these paired homologous proteins and therefore chose one of these pairs, S18-1/S18-2, to study mechanisms of the predicted competition for their incorporation into ribosomes. Our data show that Zn(2+)-depletion causes accumulation of both S18-2 mRNA and protein. In contrast, S18-1 mRNA levels are unchanged to slightly elevated under Zn(2+)-limited conditions. However, the amount of S18-1 protein is markedly decreased. We further demonstrate that both S18 proteins interact with ribosomal protein S6, a committed step in ribosome biogenesis. Zn(2+) is absolutely required for the S18-1/S6 interaction while it is dispensable for S18-2/S6 dimer formation. These data suggest a model in which S18-1 is the dominant ribosome constituent in high zinc conditions, e.g. inside of phagosomes, but that it can be replaced by S18-2 when zinc is deficient, e.g. in the extracellular milieu. Consequently, Zn(2+)-depletion may serve as a signal for building alternative ribosomes when M. tuberculosis is released from macrophages, to allow survival in the extracellular environment.

Structural basis for the interaction of protein S1 with the Escherichia coli ribosome

In Gram-negative bacteria, the multi-domain protein S1 is essential for translation initiation, as it recruits the mRNA and facilitates its localization in the decoding centre. In sharp contrast to its functional importance, S1 is still lacking from the high-resolution structures available for Escherichia coli and Thermus thermophilus ribosomes and thus the molecular mechanism governing the S1-ribosome interaction has still remained elusive. Here, we present the structure of the N-terminal S1 domain D1 when bound to the ribosome at atomic resolution by using a combination of NMR, X-ray crystallography and cryo-electron microscopy. Together with biochemical assays, the structure reveals that S1 is anchored to the ribosome primarily via a stabilizing π-stacking interaction within the short but conserved N-terminal segment that is flexibly connected to domain D1. This interaction is further stabilized by salt bridges involving the zinc binding pocket of protein S2. Overall, this work provides one hitherto enigmatic piece in the 'ribosome puzzle', namely the detailed molecular insight into the topology of the S1-ribosome interface. Moreover, our data suggest novel mechanisms that have the potential to modulate protein synthesis in response to environmental cues by changing the affinity of S1 for the ribosome.

Heterogeneity of the translational machinery: Variations on a common theme

In all organisms the universal process of protein synthesis is performed by the ribosome, a complex multi-component assembly composed of RNA and protein elements. Although ribosome heterogeneity was observed already more than 40 years ago, the ribosome is still traditionally viewed as an unchangeable entity that has to be equipped with all ribosomal components and translation factors in order to precisely accomplish all steps in protein synthesis. In the recent years this concept was challenged by several studies highlighting a broad variation in the composition of the translational machinery in response to environmental signals, which leads to its adaptation and functional specialization. Here, we summarize recent reports on the variability of the protein synthesis apparatus in diverse organisms and discuss the multiple mechanisms and possibilities that can lead to functional ribosome heterogeneity. Collectively, these results indicate that all cells are equipped with a remarkable toolbox to fine tune gene expression at the level of translation and emphasize the physiological importance of ribosome heterogeneity for the immediate implementation of environmental information.

The RimL transacetylase provides resistance to translation inhibitor microcin C

Peptide-nucleotide antibiotic microcin C (McC) is produced by some Escherichia coli strains. Inside a sensitive cell, McC is processed, releasing a nonhydrolyzable analog of aspartyl-adenylate, which inhibits aspartyl-tRNA synthetase. The product of mccE, a gene from the plasmid-borne McC biosynthetic cluster, acetylates processed McC, converting it into a nontoxic compound. MccE is homologous to chromosomally encoded acetyltransferases RimI, RimJ, and RimL, which acetylate, correspondingly, the N termini of ribosomal proteins S18, S5, and L12. Here, we show that E. coli RimL, but not other Rim acetyltransferases, provides a basal level of resistance to McC and various toxic nonhydrolyzable aminoacyl adenylates. RimL acts by acetylating processed McC, which along with ribosomal protein L12 should be considered a natural RimL substrate. When overproduced, RimL also makes cells resistant to albomycin, an antibiotic that upon intracellular processing gives rise to a seryl-thioribosyl pyrimidine that targets seryl-tRNA synthetase. We further show that E. coli YhhY, a protein related to Rim acetyltransferases but without a known function, is also able to detoxify several nonhydrolyzable aminoacyl adenylates but not processed McC. We propose that RimL and YhhY protect bacteria from various toxic aminoacyl nucleotides, either exogenous or those generated inside the cell during normal metabolism.

The acidic ribosomal protein P2 from Euplotes octocarinatus is phosphorylated at its N-terminal domain

The eukaryotic acid ribosomal P0, P1, and P2 proteins share a conserved flexible C-terminal tail that is rich in acidic residues, which are involved in the interaction with elongation factor 2 during protein synthesis. Our previous work suggested that the acidic ribosomal P proteins from Euplotes octocarinatus have a special C-terminal domain. To further understand this characteristic feature, both P2 and elongation factor 2 from E. octocarinatus were overexpressed, for the first time, in Escherichia coli in this study. GST pull-down assay indicated that P2 protein from E. octocarinatus (EoP2) interacted specifically with the N-terminal domain of elongation factor 2 from E. octocarinatus (EoEF-2) in vitro. The interacting part of EoP2 is in the C-terminal domains, consistent with the observation in other organisms. Phosphorylation of the recombinant EoP2 was performed in vitro using multiple methods such as (31)P-NMR spectroscopy, native PAGE, and Phos-tag(TM) SDS-PAGE. Results showed that ribosomal protein EoP2 was phosphorylated by casein kinase II at serine 21 located at the N terminus. This phosphorylation site identified in EoP2 is quite different from that of P2 from other organisms, in which the phosphorylation site is located in the conserved C-terminal region.

Mechanism and rates of exchange of L7/L12 between ribosomes and the effects of binding EF-G

The ribosomal stalk complex binds and recruits translation factors to the ribosome during protein biosynthesis. In Escherichia coli the stalk is composed of protein L10 and four copies of L7/L12. Despite the crucial role of the stalk, mechanistic details of L7/L12 subunit exchange are not established. By incubating isotopically labeled intact ribosomes with their unlabeled counterparts we monitored the exchange of the labile stalk proteins by recording mass spectra as a function of time. On the basis of kinetic analysis, we proposed a mechanism whereby exchange proceeds via L7/L12 monomers and dimers. We also compared exchange of L7/L12 from free ribosomes with exchange from ribosomes in complex with elongation factor G (EF-G), trapped in the posttranslocational state by fusidic acid. Results showed that binding of EF-G reduces the L7/L12 exchange reaction of monomers by ~27% and of dimers by ~47% compared with exchange from free ribosomes. This is consistent with a model in which binding of EF-G does not modify interactions between the L7/L12 monomers but rather one of the four monomers, and as a result one of the two dimers, become anchored to the ribosome-EF-G complex preventing their free exchange. Overall therefore our results not only provide mechanistic insight into the exchange of L7/L12 monomers and dimers and the effects of EF-G binding but also have implications for modulating stability in response to environmental and functional stimuli within the cell.

Regulation of mammalian mitochondrial translation by post-translational modifications

Mitochondria are responsible for the production of over 90% of the energy in eukaryotes through oxidative phosphorylation performed by electron transfer and ATP synthase complexes. Mitochondrial translation machinery is responsible for the synthesis of 13 essential proteins of these complexes encoded by the mitochondrial genome. Emerging data suggest that acetyl-CoA, NAD(+), and ATP are involved in regulation of this machinery through post-translational modifications of its protein components. Recent high-throughput proteomics analyses and mapping studies have provided further evidence for phosphorylation and acetylation of ribosomal proteins and translation factors. Here, we will review our current knowledge related to these modifications and their possible role(s) in the regulation of mitochondrial protein synthesis using the homology between mitochondrial and bacterial translation machineries. However, we have yet to determine the effects of phosphorylation and acetylation of translation components in mammalian mitochondrial biogenesis. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Protein N-terminal acetyltransferases: when the start matters

The majority of eukaryotic proteins are subjected to N-terminal acetylation (Nt-acetylation), catalysed by N-terminal acetyltransferases (NATs). Recently, the structure of an NAT-peptide complex was determined, and detailed proteome-wide Nt-acetylation patterns were revealed. Furthermore, Nt-acetylation just emerged as a multifunctional regulator, acting as a protein degradation signal, an inhibitor of endoplasmic reticulum (ER) translocation, and a mediator of protein complex formation. Nt-acetylation is regulated by acetyl-coenzyme A (Ac-CoA) levels, and thereby links metabolic cell states to cell death. The essentiality of NATs in humans is stressed by the recent discovery of a human hereditary lethal disease caused by a mutation in an NAT gene. Here, we discuss how these recent findings shed light on NATs as major protein regulators and key cellular players.

Global analysis of the Staphylococcus aureus response to mupirocin

In the present study, we analyzed the response of S. aureus to mupirocin, the drug of choice for nasal decolonization. Mupirocin selectively inhibits the bacterial isoleucyl-tRNA synthetase (IleRS), leading to the accumulation of uncharged isoleucyl-tRNA and eventually the synthesis of (p)ppGpp. The alarmone (p)ppGpp induces the stringent response, an important global transcriptional and translational control mechanism that allows bacteria to adapt to nutritional deprivation. To identify proteins with an altered synthesis pattern in response to mupirocin treatment, we used the highly sensitive 2-dimensional gel electrophoresis technique in combination with mass spectrometry. The results were complemented by DNA microarray, Northern blot, and metabolome analyses. Whereas expression of genes involved in nucleotide biosynthesis, DNA metabolism, energy metabolism, and translation was significantly downregulated, expression of isoleucyl-tRNA synthetase, the branched-chain amino acid pathway, and genes with functions in oxidative-stress resistance (ahpC and katA) and putative roles in stress protection (the yvyD homologue SACOL0815 and SACOL1759 and SACOL2131) and transport processes was increased. A comparison of the regulated genes to known regulons suggests the involvement of the global regulators CodY and SigB in shaping the response of S. aureus to mupirocin. Of particular interest was the induced transcription of genes encoding virulence-associated regulators (i.e., arlRS, saeRS, sarA, sarR, sarS, and sigB), as well as genes directly involved in the virulence of S. aureus (i.e., fnbA, epiE, epiG, and seb).

C7orf30 specifically associates with the large subunit of the mitochondrial ribosome and is involved in translation

In a comparative genomics study for mitochondrial ribosome-associated proteins, we identified C7orf30, the human homolog of the plant protein iojap. Gene order conservation among bacteria and the observation that iojap orthologs cannot be transferred between bacterial species predict this protein to be associated with the mitochondrial ribosome. Here, we show colocalization of C7orf30 with the large subunit of the mitochondrial ribosome using isokinetic sucrose gradient and 2D Blue Native polyacrylamide gel electrophoresis (BN-PAGE) analysis. We co-purified C7orf30 with proteins of the large subunit, and not with proteins of the small subunit, supporting interaction that is specific to the large mitoribosomal complex. Consistent with this physical association, a mitochondrial translation assay reveals negative effects of C7orf30 siRNA knock-down on mitochondrial gene expression. Based on our data we propose that C7orf30 is involved in ribosomal large subunit function. Sequencing the gene in 35 patients with impaired mitochondrial translation did not reveal disease-causing mutations in C7orf30.

Joining forces: integrating proteomics and cross-linking with the mass spectrometry of intact complexes

Protein assemblies are critical for cellular function and understanding their physical organization is the key aim of structural biology. However, applying conventional structural biology approaches is challenging for transient, dynamic, or polydisperse assemblies. There is therefore a growing demand for hybrid technologies that are able to complement classical structural biology methods and thereby broaden our arsenal for the study of these important complexes. Exciting new developments in the field of mass spectrometry and proteomics have added a new dimension to the study of protein-protein interactions and protein complex architecture. In this review, we focus on how complementary mass spectrometry-based techniques can greatly facilitate structural understanding of protein assemblies.

Bacterial ribosome requires multiple L12 dimers for efficient initiation and elongation of protein synthesis involving IF2 and EF-G

The ribosomal stalk in bacteria is composed of four or six copies of L12 proteins arranged in dimers that bind to the adjacent sites on protein L10, spanning 10 amino acids each from the L10 C-terminus. To study why multiple L12 dimers are required on the ribosome, we created a chromosomally engineered Escherichia coli strain, JE105, in which the peripheral L12 dimer binding site was deleted. Thus JE105 harbors ribosomes with only a single L12 dimer. Compared to MG1655, the parental strain with two L12 dimers, JE105 showed significant growth defect suggesting suboptimal function of the ribosomes with one L12 dimer. When tested in a cell-free reconstituted transcription–translation assay the synthesis of a full-length protein, firefly luciferase, was notably slower with JE105 70S ribosomes and 50S subunits. Further, in vitro analysis by fast kinetics revealed that single L12 dimer ribosomes from JE105 are defective in two major steps of translation, namely initiation and elongation involving translational GTPases IF2 and EF-G. Varying number of L12 dimers on the ribosome can be a mechanism in bacteria for modulating the rate of translation in response to growth condition.

Mass spectrometry: come of age for structural and dynamical biology

Over the past two decades, mass spectrometry (MS) has emerged as a bone fide approach for structural biology. MS can inform on all levels of protein organization, and enables quantitative assessments of their intrinsic dynamics. The key advantages of MS are that it is a sensitive, high-resolution separation technique with wide applicability, and thereby allows the interrogation of transient protein assemblies in the context of complex mixtures. Here we describe how molecular-level information is derived from MS experiments, and how it can be combined with spatial and dynamical restraints obtained from other structural biology approaches to allow hybrid studies of protein architecture and movements.

Protein alpha-N-acetylation studied by N-terminomics

Cotranslational protein N-terminal modifications, including proteolytic maturation such as initiator methionine excision by methionine aminopeptidases and N-terminal blocking, occur universally. Protein alpha-N-acetylation, or the transfer of the acetyl moiety of acetyl-coenzyme A to nascent protein N-termini, catalysed by multisubunit N-terminal acetyltransferase complexes, generally takes place during protein translation. Nearly all protein modifications are known to influence different protein aspects such as folding, stability, activity and localization, and several studies have indicated similar functions for protein alpha-N-acetylation. However, until recently, protein alpha-N-acetylation remained poorly explored, mainly due to the absence of targeted proteomics technologies. The recent emergence of N-terminomics technologies that allow isolation of protein N-terminal peptides, together with proteogenomics efforts combining experimental and informational content have greatly boosted the field of alpha-N-acetylation. In this review, we report on such emerging technologies as well as on breakthroughs in our understanding of protein N-terminal biology.

Bioinformatics analysis of a Saccharomyces cerevisiae N-terminal proteome provides evidence of alternative translation initiation and post-translational N-terminal acetylation

Initiation of protein translation is a well-studied fundamental process, albeit high-throughput and more comprehensive determination of the exact translation initiation sites (TIS) was only recently made possible following the introduction of positional proteomics techniques that target protein N-termini. Precise translation initiation is of crucial importance, as truncated or extended proteins might fold, function, and locate erroneously. Still, as already shown for some proteins, alternative translation initiation can also serve as a regulatory mechanism. By applying N-terminal COFRADIC (combined fractional diagonal chromatography), we here isolated N-terminal peptides of a Saccharomyces cerevisiae proteome and analyzed both annotated and alternative TIS. We analyzed this N-terminome of S. cerevisiae which resulted in the identification of 650 unique N-terminal peptides corresponding to database annotated TIS. Furthermore, 56 unique N(α)-acetylated peptides were identified that suggest alternative TIS (MS/MS-based), while MS-based evidence of N(α)-acetylation led to an additional 33 such peptides. To improve the overall sensitivity of the analysis, we also included the 5' UTR (untranslated region) in-frame translations together with the yeast protein sequences in UniProtKB/Swiss-Prot. To ensure the quality of the individual peptide identifications, peptide-to-spectrum matches were only accepted at a 99% probability threshold and were subsequently analyzed in detail by the Peptizer tool to automatically ascertain their compliance with several expert criteria. Furthermore, we have also identified 60 MS/MS-based and 117 MS-based N(α)-acetylated peptides that point to N(α)-acetylation as a post-translational modification since these peptides did not start nor were preceded (in their corresponding protein sequence) by a methionine residue. Next, we evaluated consensus sequence features of nucleic acids and amino acids across each of these groups of peptides and evaluated the results in the context of publicly available data. Taken together, we present a list of 706 annotated and alternative TIS for yeast proteins and found that under normal growth conditions alternative TIS might (co)occur in S. cerevisiae in roughly one tenth of all proteins. Furthermore, we found that the nucleic acid and amino acid features proximate to these alternative TIS favor either guanine or adenine nucleotides following the start codon or acidic amino acids following the initiator methionine. Finally, we also observed an unexpected high number of N(α)-acetylated peptides that could not be related to TIS and therefore suggest events of post-translational N(α)-acetylation.

Purification of human mitochondrial ribosomal L7/L12 stalk proteins and reconstitution of functional hybrid ribosomes in Escherichia coli

Bacterial ribosomal L7/L12 stalk is formed by L10, L11, and multiple copies of L7/L12, which plays an essential role in recruiting initiation and elongation factors during translation. The homologs of these proteins, MRPL10, MRPL11, and MRPL12, are present in human mitochondrial ribosomes. To evaluate the role of MRPL10, MRPL11, and MRPL12 in translation, we over-expressed and purified components of the human mitochondrial L7/L12 stalk proteins in Escherichia coli. Here, we designed a construct to co-express MRPL10 and MRPL12 using a duet expression system to form a functional MRPL10-MRPL12 complex. The goal is to demonstrate the homology between the mitochondrial and bacterial L7/L12 stalk proteins and to reconstitute a hybrid ribosome to be used in structural and functional studies of the mitochondrial stalk.

Neutralizing positive charges at the surface of a protein lowers its rate of amide hydrogen exchange without altering its structure or increasing its thermostability

This paper combines two techniques--mass spectrometry and protein charge ladders--to examine the relationship between the surface charge and hydrophobicity of a representative globular protein (bovine carbonic anhydrase II; BCA II) and its rate of amide hydrogen-deuterium (H/D) exchange. Mass spectrometric analysis indicated that the sequential acetylation of surface lysine-ε-NH3(+) groups--a type of modification that increases the net negative charge and hydrophobicity of the surface of BCA II without affecting its secondary or tertiary structure--resulted in a linear decrease in the aggregate rate of amide H/D exchange at pD 7.4, 15 °C. According to analysis with MS, the acetylation of each additional lysine generated between 1.4 and 0.9 additional hydrogens that are protected from H/D exchange during the 2 h exchange experiment at 15 °C, pD 7.4. NMR spectroscopy demonstrated that none of the hydrogen atoms which became protected upon acetylation were located on the side chain of the acetylated lysine residues (i.e., lys-ε-NHCOCH3) but were instead located on amide NHCO moieties in the backbone. The decrease in rate of exchange associated with acetylation paralleled a decrease in thermostability: the most slowly exchanging rungs of the charge ladder were the least thermostable (as measured by differential scanning calorimetry). This observation--that faster rates of exchange are associated with slower rates of denaturation--is contrary to the usual assumptions in protein chemistry. The fact that the rates of H/D exchange were similar for perbutyrated BCA II (e.g., [lys-ε-NHCO(CH2)2CH3]18) and peracetylated BCA II (e.g., [lys-ε-NHCOCH3]18) suggests that the electrostatic charge is more important than the hydrophobicity of surface groups in determining the rate of H/D exchange. These electrostatic effects on the kinetics of H/D exchange could complicate (or aid) the interpretation of experiments in which H/D exchange methods are used to probe the structural effects of non-isoelectric perturbations to proteins (i.e., phosphorylation, acetylation, or the binding of the protein to an oligonucleotide or to another charged ligand or protein).

Protein acetylation in archaea, bacteria, and eukaryotes

Proteins can be acetylated at the alpha-amino group of the N-terminal amino acid (methionine or the penultimate amino acid after methionine removal) or at the epsilon-amino group of internal lysines. In eukaryotes the majority of proteins are N-terminally acetylated, while this is extremely rare in bacteria. A variety of studies about N-terminal acetylation in archaea have been reported recently, and it was revealed that a considerable fraction of proteins is N-terminally acetylated in haloarchaea and Sulfolobus, while this does not seem to apply for methanogenic archaea. Many eukaryotic proteins are modified by differential internal acetylation, which is important for a variety of processes. Until very recently, only two bacterial proteins were known to be acetylation targets, but now 125 acetylation sites are known for E. coli. Knowledge about internal acetylation in archaea is extremely limited; only two target proteins are known, only one of which—Alba—was used to study differential acetylation. However, indications accumulate that the degree of internal acetylation of archaeal proteins might be underestimated, and differential acetylation has been shown to be essential for the viability of haloarchaea. Focused proteomic approaches are needed to get an overview of the extent of internal protein acetylation in archaea.

Mass spectrometry defines the stoichiometry of ribosomal stalk complexes across the phylogenetic tree

The ribosomal stalk complex plays a crucial role in delivering translation factors to the catalytic site of the ribosome. It has a very similar architecture in all cells, although the protein components in bacteria are unrelated to those in archaea and eukaryotes. Here we used mass spectrometry to investigate ribosomal stalk complexes from bacteria, eukaryotes, and archaea in situ on the ribosome. Specifically we targeted ribosomes with different optimal growth temperatures. Our results showed that for the mesophilic bacterial ribosomes we investigated the stalk complexes are exclusively pentameric or entirely heptameric in the case of thermophilic bacteria, whereas we observed only pentameric stalk complexes in eukaryotic species. We also found the surprising result that for mesophilic archaea, Methanococcus vannielii, Methanococcus maripaludis, and Methanosarcina barkeri, both pentameric and heptameric stoichiometries are present simultaneously within a population of ribosomes. Moreover the ratio of pentameric to heptameric stalk complexes changed during the course of cell growth. We consider these differences in stoichiometry within ribosomal stalk complexes in the context of convergent evolution.

B. subtilis ribosomal proteins: structural homology and post-translational modifications

Ribosomal proteins of the model gram-positive bacterium B. subtilis 168 were extensively characterized in a proteomic study. Mass spectra of the 52 proteins expected to be constitutive components of the 70S ribosome were recorded. Peptide MS/MS analysis with an average sequence coverage of 85% supported the identification of these proteins and facilitated the unambiguous assignment of post-translational modifications, including the methylation of S7, L11, and L16 and the N-terminal acetylation of S9. In addition, the high degree of structural homology between B. subtilis and other eubacterial ribosomal proteins was demonstrated through chemical labeling with S-methylthioacetimidate. One striking difference from previous characterizations of bacterial ribosomal proteins is that dozens of protein masses were found to be in error and not easily accounted for by post-translational modifications. This, in turn, led us to discover an inordinate number of sequencing errors in the reference genome of B. subtilis 168. We have found that these errors have been corrected in a recently revised version of the genome.

Comprehensive analysis of phosphorylated proteins of Escherichia coli ribosomes

Phosphorylation of bacterial ribosomal proteins has been known for decades; however, there is still very limited information available on specific locations of the phosphorylation sites in ribosomal proteins and the role they might play in protein synthesis. In this study, we have mapped the specific phosphorylation sites in 24 Escherichia coli ribosomal proteins by tandem mass spectrometry. Detection of phosphorylation was achieved by either phosphorylation specific visualization techniques, ProQ staining, and antibodies for phospho-Ser, Thr, and Tyr; or by mass spectrometry equipped with a capability to detect addition and loss of the phosphate moiety. Enrichment by immobilized metal affinity and/or strong cation exchange chromatography was used to improve the success of detection of the low abundance phosphopeptides. We found the small subunit (30S) proteins S3, S4, S5, S7, S11, S12, S13, S18, and S21 and the large subunit (50S) proteins L1, L2, L3, L5, L6, L7/L12, L13, L14, L16, L18, L19, L21, L22, L28, and L31 to be phosphorylated at one or more residues. Potential roles for each specific site in ribosome function were deduced through careful evaluation of the given phosphorylation sites in 3D-crystal structure models of ribosomes and the previous mutational studies of E. coli ribosomal proteins.