Suparna Sanyal

Protein Folding Activity of Ribosomal RNA Is a Selective Target of Two Unrelated Antiprion Drugs

Background 6-Aminophenanthridine (6AP) and Guanabenz (GA, a drug currently in use for the treatment of hypertension) were isolated as antiprion drugs using a yeast-based assay. These structurally unrelated molecules are also active against mammalian prion in several cell-based assays and in vivo in a mouse model for prion-based diseases. Methodology/Principal Findings Here we report the identification of cellular targets of these drugs. Using affinity chromatography matrices for both drugs, we demonstrate an RNA-dependent interaction of 6AP and GA with the ribosome. These specific interactions have no effect on the peptidyl transferase activity of the ribosome or on global translation. In contrast, 6AP and GA specifically inhibit the ribosomal RNA-mediated protein folding activity of the ribosome. Conclusion/Significance 6AP and GA are therefore the first compounds to selectively inhibit the protein folding activity of the ribosome. They thus constitute precious tools to study the yet largely unexplored biological role of this protein folding activity.

A single-step method for purification of active His-tagged ribosomes from a genetically engineered Escherichia coli

With the rapid development of the ribosome field in recent years a quick, simple and high-throughput method for purification of the bacterial ribosome is in demand. We have designed a new strain of Escherichia coli (JE28) by an in-frame fusion of a nucleotide sequence encoding a hexa-histidine affinity tag at the 3′-end of the single copy rplL gene (encoding the ribosomal protein L12) at the chromosomal site of the wild-type strain MG1655. As a result, JE28 produces a homogeneous population of ribosomes (His)6-tagged at the C-termini of all four L12 proteins. Furthermore, we have developed a single-step, high-throughput method for purification of tetra-(His)6-tagged 70S ribosomes from this strain using affinity chromatography. These ribosomes, when compared with the conventionally purified ones in sucrose gradient centrifugation, 2D-gel, dipeptide formation and a full-length protein synthesis assay showed higher yield and activity. We further describe how this method can be adapted for purification of ribosomal subunits and mutant ribosomes. These methodologies could, in principle, also be used to purify any functional multimeric complex from the bacterial cell.

Structural and Functional Insights into the Mode of Action of a Universally Conserved Obg GTPase

Kinetics and cryo-electronmicroscopy data provide insights into GTPase ObgE’s role as a ribosome anti-association factor that is modulated by nutrient availability, coupling growth control to ribosome biosynthesis and protein translation.

Recognition of aminoacyl-tRNA: a common molecular mechanism revealed by cryo-EM.

The accuracy of ribosomal translation is achieved by an initial selection and a proofreading step, mediated by EF‐Tu, which forms a ternary complex with aminoacyl(aa)‐tRNA. To study the binding modes of different aa‐tRNAs, we compared cryo‐EM maps of the kirromycin‐stalled ribosome bound with ternary complexes containing Phe‐tRNAPhe, Trp‐tRNATrp, or Leu‐tRNALeuI. The three maps suggest a common binding manner of cognate aa‐tRNAs in their specific binding with both the ribosome and EF‐Tu. All three aa‐tRNAs have the same ‘loaded spring’ conformation with a kink and twist between the D‐stem and anticodon stem. The three complexes are similarly integrated in an interaction network, extending from the anticodon loop through h44 and protein S12 to the EF‐Tu‐binding CCA end of aa‐tRNA, proposed to signal cognate codon–anticodon interaction to the GTPase centre and tune the accuracy of aa‐tRNA selection.

Role of the ribosome in protein folding

In all organisms, the ribosome synthesizes and folds full length polypeptide chains into active three-dimensional conformations. The nascent protein goes through two major interactions, first with the ribosome which synthesizes the polypeptide chain and holds it for a considerable length of time, and then with the chaperones. Some of the chaperones are found in solution as well as associated to the ribosome. A number of in vitro and in vivo experiments revealed that the nascent protein folds through specific interactions of some amino acids with the nucleotides in the peptidyl transferase center (PTC) in the large ribosomal subunit. The mechanism of this folding differs from self-folding. In this article, we highlight the folding of nascent proteins on the ribosome and the influence of chaperones etc. on protein folding.

Staphylococcus aureus elongation factor G – structure and analysis of a target for fusidic acid

Fusidic acid (FA) is a bacteriostatic antibiotic that locks elongation factor G (EF‐G) on the ribosome in a post‐translocational state. It is used clinically against Gram‐positive bacteria such as pathogenic strains of Staphylococcus aureus, but no structural information has been available for EF‐G from these species. We have solved the apo crystal structure of EF‐G from S. aureus to 1.9 Å resolution. This structure shows a dramatically different overall conformation from previous structures of EF‐G, although the individual domains are highly similar. Between the different structures of free or ribosome‐bound EF‐G, domains III–V move relative to domains I–II, resulting in a displacement of the tip of domain IV relative to domain G. In S. aureus EF‐G, this displacement is about 25 Å relative to structures of Thermus thermophilus EF‐G in a direction perpendicular to that in previous observations. Part of the switch I region (residues 46–56) is ordered in a helix, and has a distinct conformation as compared with structures of EF‐Tu in the GDP and GTP states. Also, the switch II region shows a new conformation, which, as in other structures of free EF‐G, is incompatible with FA binding. We have analysed and discussed all known fusA‐based fusidic acid resistance mutations in the light of the new structure of EF‐G from S. aureus, and a recent structure of T. thermophilus EF‐G in complex with the 70S ribosome with fusidic acid [Gao YG et al. (2009) Science326, 694–699]. The mutations can be classified as affecting FA binding, EF‐G–ribosome interactions, EF‐G conformation, and EF‐G stability.

Comprehensive Analysis of Stop Codon Usage in Bacteria and Its Correlation with Release Factor Abundance

Background: Stop codon frequencies in 4684 bacterial genomes are analyzed. Results: With increasing genomic GC content, TAA% decreases, and TGA% increases reciprocally, but TAG% remains almost unchanged (∼20%). The TAG:TGA ratio matches well with the RF1:RF2 ratio. Conclusion: TAG is the minor stop codon, and expression of genes with TAG is correlated with the RF1 level. Significance: Our work establishes the correlation between stop codon frequency and RF1/RF2 abundance. We present a comprehensive analysis of stop codon usage in bacteria by analyzing over eight million coding sequences of 4684 bacterial sequences. Using a newly developed program called “stop codon counter,” the frequencies of the three classical stop codons TAA, TAG, and TGA were analyzed, and a publicly available stop codon database was built. Our analysis shows that with increasing genomic GC content the frequency of the TAA codon decreases and that of the TGA codon increases in a reciprocal manner. Interestingly, the release factor 1-specific codon TAG maintains a more or less uniform frequency (∼20%) irrespective of the GC content. The low abundance of TAG is also valid with respect to expression level of the genes ending with different stop codons. In contrast, the highly expressed genes predominantly end with TAA, ensuring termination with either of the two release factors. Using three model bacteria with different stop codon usage (Escherichia coli, Mycobacterium smegmatis, and Bacillus subtilis), we show that the frequency of TAG and TGA codons correlates well with the relative steady state amount of mRNA and protein for release factors RF1 and RF2 during exponential growth. Furthermore, using available microarray data for gene expression, we show that in both fast growing and contrasting biofilm formation conditions, the relative level of RF1 is nicely correlated with the expression level of the genes ending with TAG.

Activation of GTP hydrolysis in mRNA-tRNA translocation by elongation factor G.

Cryo-EM study reveals key molecular structural features for activation of guanosine triphosphate cleavage by EF-G during translocation. During protein synthesis, elongation of the polypeptide chain by each amino acid is followed by a translocation step in which mRNA and transfer RNA (tRNA) are advanced by one codon. This crucial step is catalyzed by elongation factor G (EF-G), a guanosine triphosphatase (GTPase), and accompanied by a rotation between the two ribosomal subunits. A mutant of EF-G, H91A, renders the factor impaired in guanosine triphosphate (GTP) hydrolysis and thereby stabilizes it on the ribosome. We use cryogenic electron microscopy (cryo-EM) at near-atomic resolution to investigate two complexes formed by EF-G H91A in its GTP state with the ribosome, distinguished by the presence or absence of the intersubunit rotation. Comparison of these two structures argues in favor of a direct role of the conserved histidine in the switch II loop of EF-G in GTPase activation, and explains why GTP hydrolysis cannot proceed with EF-G bound to the unrotated form of the ribosome.

The Role of Ribosomal Protein L11 in Class I Release Factor-mediated Translation Termination and Translational Accuracy

It has been suggested from in vivo and cryoelectron micrographic studies that the large ribosomal subunit protein L11 and its N-terminal domain play an important role in peptide release by, in particular, the class I release factor RF1. In this work, we have studied in vitro the role of L11 in translation termination with ribosomes from a wild type strain (WT-L11), an L11 knocked-out strain (ΔL11), and an L11 N terminus truncated strain (Cter-L11). Our data show 4-6-fold reductions in termination efficiency (kcat/Km) of RF1, but not of RF2, on ΔL11 and Cter-L11 ribosomes compared with wild type. There is, at the same time, no effect of these L11 alterations on the maximal rate of ester bond cleavage by either RF1 or RF2. The rates of dissociation of RF2 but not of RF1 from the ribosome after peptide release are somewhat reduced by the L11 changes irrespective of the presence of RF3, and they cause a 2-fold decrease in the missense error. Our results suggest that the L11 modifications increase nonsense suppression at UAG codons because of the reduced termination efficiency of RF1 and that they decrease nonsense suppression at UGA codons because of a decreased missense error level.

Organization of ribosomes and nucleoids in Escherichia coli cells during growth and in quiescence.

Background: We studied ribosome and nucleoid distribution in Escherichia coli under growth and quiescence. Results: Spatially segregated ribosomes and nucleoids show drastically altered distribution in stationary phase or when treated with drugs affecting translation, transcription, nucleoid-topology, or cytoskeleton. Ribosome inheritance in daughter cells is frequently unequal. Conclusion: Cellular growth processes modulate ribosome and nucleoid distribution. Significance: This provides insight into subcellular organization of molecular machines. We have examined the distribution of ribosomes and nucleoids in live Escherichia coli cells under conditions of growth, division, and in quiescence. In exponentially growing cells translating ribosomes are interspersed among and around the nucleoid lobes, appearing as alternative bands under a fluorescence microscope. In contrast, inactive ribosomes either in stationary phase or after treatment with translation inhibitors such as chloramphenicol, tetracycline, and streptomycin gather predominantly at the cell poles and boundaries with concomitant compaction of the nucleoid. However, under all conditions, spatial segregation of the ribosomes and the nucleoids is well maintained. In dividing cells, ribosomes accumulate on both sides of the FtsZ ring at the mid cell. However, the distribution of the ribosomes among the new daughter cells is often unequal. Both the shape of the nucleoid and the pattern of ribosome distribution are also modified when the cells are exposed to rifampicin (transcription inhibitor), nalidixic acid (gyrase inhibitor), or A22 (MreB-cytoskeleton disruptor). Thus we conclude that the intracellular organization of the ribosomes and the nucleoids in bacteria are dynamic and critically dependent on cellular growth processes (replication, transcription, and translation) as well as on the integrity of the MreB cytoskeleton.