Cryo-EM structure of the ancient eukaryotic ribosome from the human parasite Giardia lamblia.

Giardiasis is a disease caused by the protist Giardia lamblia. As no human vaccines have been approved so far against it, and resistance to current drugs is spreading, new strategies for combating giardiasis need to be developed. The G. lamblia ribosome may provide a promising therapeutic target due to its distinct sequence differences from ribosomes of most eukaryotes and prokaryotes. Here, we report the cryo-electron microscopy structure of the G. lamblia (WB strain) ribosome determined at 2.75 Å resolution. The ribosomal RNA is the shortest known among eukaryotes, and lacks nearly all the eukaryote-specific ribosomal RNA expansion segments. In contrast, the ribosomal proteins are typically eukaryotic with some species-specific insertions/extensions. Most typical inter-subunit bridges are maintained except for one missing contact site. Unique structural features are located mainly at the ribosome's periphery. These may be exploited as target sites for the design of new compounds that inhibit selectively the parasite's ribosomal activity.

Ribosome-binding and anti-microbial studies of the mycinamicins, 16-membered macrolide antibiotics from Micromonospora griseorubida.

Macrolides have been effective clinical antibiotics for over 70 years. They inhibit protein biosynthesis in bacterial pathogens by narrowing the nascent protein exit tunnel in the ribosome. The macrolide class of natural products consist of a macrolactone ring linked to one or more sugar molecules. Most of the macrolides used currently are semi-synthetic erythromycin derivatives, composed of a 14- or 15-membered macrolactone ring. Rapidly emerging resistance in bacterial pathogens is among the most urgent global health challenges, which render many antibiotics ineffective, including next-generation macrolides. To address this threat and advance a longer-term plan for developing new antibiotics, we demonstrate how 16-membered macrolides overcome erythromycin resistance in clinically isolated Staphylococcus aureus strains. By determining the structures of complexes of the large ribosomal subunit of Deinococcus radiodurans (D50S) with these 16-membered selected macrolides, and performing anti-microbial studies, we identified resistance mechanisms they may overcome. This new information provides important insights toward the rational design of therapeutics that are effective against drug resistant human pathogens.

Structure of Pseudomonas aeruginosa ribosomes from an aminoglycoside-resistant clinical isolate.

Resistance to antibiotics has become a major threat to modern medicine. The ribosome plays a fundamental role in cell vitality by the translation of the genetic code into proteins; hence, it is a major target for clinically useful antibiotics. We report here the cryo-electron microscopy structures of the ribosome of a pathogenic aminoglycoside (AG)-resistant Pseudomonas aeruginosa strain, as well as of a nonresistance strain isolated from a cystic fibrosis patient. The structural studies disclosed defective ribosome complex formation due to a conformational change of rRNA helix H69, an essential intersubunit bridge, and a secondary binding site of the AGs. In addition, a stable conformation of nucleotides A1486 and A1487, pointing into helix h44, is created compared to a non-AG-bound ribosome. We suggest that altering the conformations of ribosomal protein uL6 and rRNA helix H69, which interact with initiation-factor IF2, interferes with proper protein synthesis initiation.

Structural insights of lincosamides targeting the ribosome of Staphylococcus aureus.

Antimicrobial resistance within a wide range of pathogenic bacteria is an increasingly serious threat to global public health. Among these pathogenic bacteria are the highly resistant, versatile and possibly aggressive bacteria, Staphylococcus aureus. Lincosamide antibiotics were proved to be effective against this pathogen. This small, albeit important group of antibiotics is mostly active against Gram-positive bacteria, but also used against selected Gram-negative anaerobes and protozoa. S. aureus resistance to lincosamides can be acquired by modifications and/or mutations in the rRNA and rProteins. Here, we present the crystal structures of the large ribosomal subunit of S. aureus in complex with the lincosamides lincomycin and RB02, a novel semisynthetic derivative and discuss the biochemical aspects of the in vitro potency of various lincosamides. These results allow better understanding of the drugs selectivity as well as the importance of the various chemical moieties of the drug for binding and inhibition.

Avilamycin and evernimicin induce structural changes in rProteins uL16 and CTC that enhance the inhibition of A-site tRNA binding.

Two structurally unique ribosomal antibiotics belonging to the orthosomycin family, avilamycin and evernimicin, possess activity against Enterococci, Staphylococci, and Streptococci, and other Gram-positive bacteria. Here, we describe the high-resolution crystal structures of the eubacterial large ribosomal subunit in complex with them. Their extended binding sites span the A-tRNA entrance corridor, thus inhibiting protein biosynthesis by blocking the binding site of the A-tRNA elbow, a mechanism not shared with other known antibiotics. Along with using the ribosomal components that bind and discriminate the A-tRNA-namely, ribosomal RNA (rRNA) helices H89, H91, and ribosomal proteins (rProtein) uL16-these structures revealed novel interactions with domain 2 of the CTC protein, a feature typical to various Gram-positive bacteria. Furthermore, analysis of these structures explained how single nucleotide mutations and methylations in helices H89 and H91 confer resistance to orthosomycins and revealed the sequence variations in 23S rRNA nucleotides alongside the difference in the lengths of the eukaryotic and prokaryotic α1 helix of protein uL16 that play a key role in the selectivity of those drugs. The accurate interpretation of the crystal structures that could be performed beyond that recently reported in cryo-EM models provide structural insights that may be useful for the design of novel pathogen-specific antibiotics, and for improving the potency of orthosomycins. Because both drugs are extensively metabolized in vivo, their environmental toxicity is very low, thus placing them at the frontline of drugs with reduced ecological hazards.

Structural insights into species-specific features of the ribosome from the pathogen Staphylococcus aureus.

The emergence of bacterial multidrug resistance to antibiotics threatens to cause regression to the preantibiotic era. Here we present the crystal structure of the large ribosomal subunit from Staphylococcus aureus, a versatile Gram-positive aggressive pathogen, and its complexes with the known antibiotics linezolid and telithromycin, as well as with a new, highly potent pleuromutilin derivative, BC-3205. These crystal structures shed light on specific structural motifs of the S. aureus ribosome and the binding modes of the aforementioned antibiotics. Moreover, by analyzing the ribosome structure and comparing it with those of nonpathogenic bacterial models, we identified some unique internal and peripheral structural motifs that may be potential candidates for improving known antibiotics and for use in the design of selective antibiotic drugs against S. aureus.

A Recombinant Collagen-mRNA Platform for Controllable Protein Synthesis.

We have developed a collagen-mRNA platform for controllable protein production that is intended to be less prone to the problems associated with commonly used mRNA therapy as well as with collagen skin-healing procedures. A collagen mimic was constructed according to a recombinant method and was used as scaffold for translating mRNA chains into proteins. Cysteines were genetically inserted into the collagen chain at positions allowing efficient ribosome translation activity while minimizing mRNA misfolding and degradation. Enhanced green fluorescence protein (eGFP) mRNA bound to collagen was successfully translated by cell-free Escherichia coli ribosomes. This system enabled an accurate control of specific protein synthesis by monitoring expression time and level. Luciferase-mRNA was also translated on collagen scaffold by eukaryotic cell extracts. Thus we have demonstrated the feasibility of controllable protein synthesis on collagen scaffolds by ribosomal machinery.

Crystal structure of the synergistic antibiotic pair, lankamycin and lankacidin, in complex with the large ribosomal subunit.

The structures of the large ribosomal subunit of Deinococcus radiodurans (D50S) in complex with the antibiotic lankamycin (3.2 Å) and a double antibiotic complex of lankamycin and lankacidin C (3.45 Å) have been determined, in continuation of previous crystallographic studies on lankacidin-D50S complex. These two drugs have been previously reported to inhibit ribosomal function with mild synergistic effect. Lankamycin, a member of the macrolide family, binds in a similar manner to erythromycin. However, when in complex with lankacidin, lankamycin is located so that it can form interactions with lankacidin in the adjacent ribosomal binding site. When compared to the well-documented synergistic antibiotics, Streptogramins A and B, the pair of lankacidin and lankamycin bind in similar sites, the peptidyl transferase center and nascent peptide exit tunnel, respectively. Herein, we discuss the structural basis for antibiotic synergism and highlight the key factors involved in ribosomal inhibition.

The structure of ribosome-lankacidin complex reveals ribosomal sites for synergistic antibiotics.

Crystallographic analysis revealed that the 17-member polyketide antibiotic lankacidin produced by Streptomyces rochei binds at the peptidyl transferase center of the eubacterial large ribosomal subunit. Biochemical and functional studies verified this finding and showed interference with peptide bond formation. Chemical probing indicated that the macrolide lankamycin, a second antibiotic produced by the same species, binds at a neighboring site, at the ribosome exit tunnel. These two antibiotics can bind to the ribosome simultaneously and display synergy in inhibiting bacterial growth. The binding site of lankacidin and lankamycin partially overlap with the binding site of another pair of synergistic antibiotics, the streptogramins. Thus, at least two pairs of structurally dissimilar compounds have been selected in the course of evolution to act synergistically by targeting neighboring sites in the ribosome. These results underscore the importance of the corresponding ribosomal sites for development of clinically relevant synergistic antibiotics and demonstrate the utility of structural analysis for providing new directions for drug discovery.

A single pseudouridine on rRNA regulates ribosome structure and function in the mammalian parasite Trypanosoma brucei.

Trypanosomes are protozoan parasites that cycle between insect and mammalian hosts and are the causative agent of sleeping sickness. Here, we describe the changes of pseudouridine (Ψ) modification on rRNA in the two life stages of the parasite using four different genome-wide approaches. CRISPR-Cas9 knock-outs of all four snoRNAs guiding Ψ on helix 69 (H69) of the large rRNA subunit were lethal. A single knock-out of a snoRNA guiding Ψ530 on H69 altered the composition of the 80S monosome. These changes specifically affected the translation of only a subset of proteins. This study correlates a single site Ψ modification with changes in ribosomal protein stoichiometry, supported by a high-resolution cryo-EM structure. We propose that alteration in rRNA modifications could generate ribosomes preferentially translating state-beneficial proteins.

Mutations in RPS19 may affect ribosome function and biogenesis in Diamond Blackfan anemia.

Ribosomes, the cellular organelles translating the genetic code to proteins, are assemblies of RNA chains and many proteins (RPs) arranged in precise fine-tuned interwoven structures. Mutated ribosomal genes cause ribosomopathies, including Diamond Blackfan anemia (DBA, a rare heterogeneous red-cell aplasia connected to ribosome malfunction) or failed biogenesis. Combined bioinformatical, structural, and predictive analyses of potential consequences of possibly expressed mutations in eS19, the protein product of the highly mutated RPS19, suggest that mutations in its exposed surface could alter its positioning during assembly and consequently prevent biogenesis, implying a natural selective strategy to avoid malfunctions in ribosome assembly. A search for RPS19 pseudogenes indicated > 90% sequence identity with the wild-type, hinting at its expression in cases of absent or truncated gene products.

Structural Studies Reveal the Role of Helix 68 in the Elongation Step of Protein Biosynthesis.

The ribosome, a multicomponent assembly consisting of RNA and proteins, is a pivotal macromolecular machine that translates the genetic code into proteins. The large ribosomal subunit rRNA helix 68 (H68) is a key element in the protein synthesis process, as it coordinates the coupled movements of the actors involved in translocation, including the tRNAs and L1 stalk. Examination of cryo-electron microscopy (cryo-EM) structures of ribosomes incubated for various time durations at physiological temperatures led to the identification of functionally relevant H68 movements. These movements assist the transition of the L1 stalk between its open and closed states. H68 spatial flexibility and its significance to the protein synthesis process were confirmed through its effective targeting with antisense PNA oligomers. Our results suggest that H68 is actively involved in ribosome movements that are central to the elongation process. IMPORTANCE The mechanism that regulates the translocation step in ribosomes during protein synthesis is not fully understood. In this work, cryo-EM techniques used to image ribosomes from Staphylococcus aureus after incubation at physiological temperature allowed the identification of a conformation of the helix 68 that has never been observed so far. We then propose a mechanism in which such helix, switching between two different conformations, actively coordinates the translocation step, shedding light on the dynamics of ribosomal components. In addition, the relevance of helix 68 to ribosome function and its potential as an antibiotic target was proved by inhibiting Staphylococcus aureus ribosomes activity in vitro using oligomers with sequence complementarity.

Transportin mediates nuclear entry of DNA in vertebrate systems.

Delivery of DNA to the cell nucleus is an essential step in many types of viral infection, transfection, gene transfer by the plant pathogen Agrobacterium tumefaciens and in strategies for gene therapy. Thus, the mechanism by which DNA crosses the nuclear pore complex (NPC) is of great interest. Using nuclei reconstituted in vitro in Xenopus egg extracts, we previously studied DNA passage through the nuclear pores using a single-molecule approach based on optical tweezers. Fluorescently labeled DNA molecules were also seen to accumulate within nuclei. Here we find that this import of DNA relies on a soluble protein receptor of the importin family. To identify this receptor, we used different pathway-specific cargoes in competition studies as well as pathway-specific dominant negative inhibitors derived from the nucleoporin Nup153. We found that inhibition of the receptor transportin suppresses DNA import. In contrast, inhibition of importin beta has little effect on the nuclear accumulation of DNA. The dependence on transportin was fully confirmed in assays using permeabilized HeLa cells and a mammalian cell extract. We conclude that the nuclear import of DNA observed in these different vertebrate systems is largely mediated by the receptor transportin. We further report that histones, a known cargo of transportin, can act as an adaptor for the binding of transportin to DNA.

Ancient machinery embedded in the contemporary ribosome.

Structural analysis, supported by biochemical, mutagenesis and computational evidence, indicates that the peptidyltransferase centre of the contemporary ribosome is a universal symmetrical pocket composed solely of rRNA. This pocket seems to be a relic of the proto-ribosome, an ancient ribozyme, which was a dimeric RNA assembly formed from self-folded RNA chains of identical, similar or different sequences. This could have occurred spontaneously by gene duplication or gene fusion. This pocket-like entity was capable of autonomously catalysing various reactions, including peptide bond formation and non-coded or semi-coded amino acid polymerization. Efforts toward the structural definition of the early entity capable of genetic decoding involve the crystallization of the small ribosomal subunit of a bacterial organism harbouring a single functional rRNA operon.

The Proto-Ribosome: an ancient nano-machine for peptide bond formation.

The ribosome is a ribozyme whose active site, the peptidyl transferase center (PTC) is situated within a highly conserved universal symmetrical region that connects all ribosomal functional centers involved in amino-acid polymerization. The linkage between this elaborate architecture and A-site tRNA position revealed that the A to P-site passage of the tRNA 3' terminus during protein synthesis is performed by a rotary motion, synchronized with the overall tRNA/mRNA sideways movement and Guided by the PTC. This rotary motion leads to suitable stereochemistry for peptide bond formation as well as for substrate mediated catalysis. Analysis of the substrate binding modes to ribosomes led to the hypothesis that the ancient ribosome produced single peptide bonds and non-coded chains, potentially in a similar manner to the modern PTC. Later in evolution, a mechanism, enabling some type of decoding genetic control triggered the emergence of the small ribosomal subunit or part of it. This seems to be the result of the appearance of reaction products that could have evolved after polypeptides capable of enzymatic function were generated sporadically, while an ancient stable RNA fold was converted into an old version of a tRNA molecule. As in the contemporary ribosome the symmetry relates only the backbone fold and nucleotides orientations but not nucleotide sequences, it emphasizes the superiority of functional requirement over sequence conservation, and indicates that the PTC may have evolved by gene fusion or gene duplication.

Biological implications of the ribosome's stunning stereochemistry.

The ribosome's striking architecture is ingeniously designed for its efficient polymerase activity in the biosynthesis of proteins, which is a prerequisite for cell vitality. This elaborate architecture is comprised of a universal symmetrical region that connects all of the ribosomal functional centers involved in protein biosynthesis. Assisted by the mobility of selected ribosomal nucleotides, the symmetrical region provides the structural tools that are required not only for peptide bond formation, but also for fast and smooth successive elongation of nascent proteins. It confines the path along which the A-tRNA 3'-end is rotated into the P-site in concert with the overall tRNA/mRNA sideways movement, thus providing the required stereochemistry for peptide bond formation and substrate-mediated catalysis. The extreme flexibility of the nucleotides that facilitate peptide bond formation is being exploited to promote antibiotic selectivity and synergism, as well as to combat antibiotic resistance.

Exit tunnel modulation as resistance mechanism of S. aureus erythromycin resistant mutant.

The clinical use of the antibiotic erythromycin (ery) is hampered owing to the spread of resistance genes that are mostly mutating rRNA around the ery binding site at the entrance to the protein exit tunnel. Additional effective resistance mechanisms include deletion or insertion mutations in ribosomal protein uL22, which lead to alterations of the exit tunnel shape, located 16 Å away from the drug's binding site. We determined the cryo-EM structures of the Staphylococcus aureus 70S ribosome, and its ery bound complex with a two amino acid deletion mutation in its ß hairpin loop, which grants the bacteria resistance to ery. The structures reveal that, although the binding of ery is stable, the movement of the flexible shorter uL22 loop towards the tunnel wall creates a wider path for nascent proteins, thus enabling bypass of the barrier formed by the drug. Moreover, upon drug binding, the tunnel widens further.

Importin beta negatively regulates nuclear membrane fusion and nuclear pore complex assembly.

Assembly of a eukaryotic nucleus involves three distinct events: membrane recruitment, fusion to form a double nuclear membrane, and nuclear pore complex (NPC) assembly. We report that importin beta negatively regulates two of these events, membrane fusion and NPC assembly. When excess importin beta is added to a full Xenopus nuclear reconstitution reaction, vesicles are recruited to chromatin but their fusion is blocked. The importin beta down-regulation of membrane fusion is Ran-GTP reversible. Indeed, excess RanGTP (RanQ69L) alone stimulates excessive membrane fusion, leading to intranuclear membrane tubules and cytoplasmic annulate lamellae-like structures. We propose that a precise balance of importin beta to Ran is required to create a correct double nuclear membrane and simultaneously to repress undesirable fusion events. Interestingly, truncated importin beta 45-462 allows membrane fusion but produces nuclei lacking any NPCs. This reveals distinct importin beta-regulation of NPC assembly. Excess full-length importin beta and beta 45-462 act similarly when added to prefused nuclear intermediates, i.e., both block NPC assembly. The importin beta NPC block, which maps downstream of GTPgammaS and BAPTA-sensitive steps in NPC assembly, is reversible by cytosol. Remarkably, it is not reversible by 25 microM RanGTP, a concentration that easily reverses fusion inhibition. This report, using a full reconstitution system and natural chromatin substrates, significantly expands the repertoire of importin beta. Its roles now encompass negative regulation of two of the major events of nuclear assembly: membrane fusion and NPC assembly.

Atomic resolution snapshot of Leishmania ribosome inhibition by the aminoglycoside paromomycin.

Leishmania is a single-celled eukaryotic parasite afflicting millions of humans worldwide, with current therapies limited to a poor selection of drugs that mostly target elements in the parasite's cell envelope. Here we determined the atomic resolution electron cryo-microscopy (cryo-EM) structure of the Leishmania ribosome in complex with paromomycin (PAR), a highly potent compound recently approved for treatment of the fatal visceral leishmaniasis (VL). The structure reveals the mechanism by which the drug induces its deleterious effects on the parasite. We further show that PAR interferes with several aspects of cytosolic translation, thus highlighting the cytosolic rather than the mitochondrial ribosome as the primary drug target. The results also highlight unique as well as conserved elements in the PAR-binding pocket that can serve as hotspots for the development of novel therapeutics.

The cryo-EM structure of hibernating 100S ribosome dimer from pathogenic Staphylococcus aureus.

Formation of 100S ribosome dimer is generally associated with translation suppression in bacteria. Trans-acting factors ribosome modulation factor (RMF) and hibernating promoting factor (HPF) were shown to directly mediate this process in E. coli. Gram-positive S. aureus lacks an RMF homolog and the structural basis for its 100S formation was not known. Here we report the cryo-electron microscopy structure of the native 100S ribosome from S. aureus, revealing the molecular mechanism of its formation. The structure is distinct from previously reported analogs and relies on the HPF C-terminal extension forming the binding platform for the interactions between both of the small ribosomal subunits. The 100S dimer is formed through interactions between rRNA h26, h40, and protein uS2, involving conformational changes of the head as well as surface regions that could potentially prevent RNA polymerase from docking to the ribosome.Under conditions of nutrient limitation, bacterial ribosomes undergo dimerization, forming a 100S complex that is translationally inactive. Here the authors present the structural basis for formation of the 100S complexes in Gram-positive bacteria, shedding light on the mechanism of translation suppression by the ribosome-silencing factors.