Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a key role in regulating the levels of plasma low-density lipoprotein cholesterol (LDL-C). Here, we demonstrate that the compound PF-06446846 inhibits translation of PCSK9 by inducing the ribosome to stall around codon 34, mediated by the sequence of the nascent chain within the exit tunnel. We further show that PF-06446846 reduces plasma PCSK9 and total cholesterol levels in rats following oral dosing. Using ribosome profiling, we demonstrate that PF-06446846 is highly selective for the inhibition of PCSK9 translation. The mechanism of action employed by PF-06446846 reveals a previously unexpected tunability of the human ribosome that allows small molecules to specifically block translation of individual transcripts.

Deletion of the gene for protein L27 from the E. coli chromosome results in severe defects in cell growth. This deficiency is corrected by the expression of wild-type (wt) protein L27 from a plasmid. Examination of strains expressing L27 variants truncated at the N terminus reveals that the absence of as few as three amino acids leads to a decrease in growth rate, an impairment in peptidyl transferase activity, and a sharp decline in the labeling of L27 from the 3' end of a photoreactive tRNA at the ribosomal P site. These findings suggest that the flexible N-terminal sequence of L27, which protrudes onto the interface of the bacterial 50S subunit, can reach the peptidyl transferase active site and contribute to its function, possibly by helping to correctly position tRNA substrates at the catalytic site.

SummaryIn contrast to enteric bacteria, chemotaxis in Rhodobacter sphaeroides requires transport and partial metabolism of chemoattractants. Although a chemotaxis operon has been identified containing homologues of the enteric cheA, cheW, cheR genes and two homologues of the cheY gene, deletion of the entire chemotaxis operon had only minor effects on chemotactic behaviour under the conditions tested. Responses to sugars were enhanced in tethered cells but in all other chemotaxis assays behaviour of the operon deletion mutant was wild type. The mutant also showed wild-type responses to weak organic acids such as acetate and propionate, the dominant chemoattractants for this organism, under all conditions. This is in direct contrast to the enterics in which CheA, CheW and CheY are absolutely essential for taxis to PTS sugars, oxygen and MCP-dependent chemoeffectors. The operon deletion mutant was subjected to Tn5 transposon mutagenesis and new mutants selected using a chemotaxis and phototaxis screen. One mutant, JPA203, was non-chemotactic on swarm plates and showed inverted responses when tethered or subjected to changes in light intensity. Characterization of the Tn5 insertion in JPA203 identified a second chemotaxis operon in R. sphaeroides that contains homologues of cheY, cheA and cheR, the first homologue of cheB and two homologues of cheW. The new genes were labelled orf10, cheY III , cheA II , cheW II , cheW III , cheR II , cheB and tlpC. When introduced into a wild-type background, deletion of cheA II produced a chemotaxis minus phenotype in R. sphaeroides, suggesting that cheA II forms part of a dominant chemotactic pathway, whereas the earlier identified operon plays only a minor role under laboratory conditions. The data presented here support the existence of two chemosensory pathways in R. sphaeroides, a feature that so far is unique in bacterial chemotaxis.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that downregulates low-density lipoprotein (LDL) receptor (LDL-R) levels on the surface of hepatocytes, resulting in decreased clearance of LDL-cholesterol (LDL-C). Phenotypic screening of a small-molecule compound collection was used to identify an inhibitor of PCSK9 secretion, (R)-N-(isoquinolin-1-yl)-3-(4-methoxyphenyl)-N-(piperidin-3-yl)propanamide (R-IMPP), which was shown to stimulate uptake of LDL-C in hepatoma cells by increasing LDL-R levels, without altering levels of secreted transferrin. Systematic investigation of the mode of action revealed that R-IMPP did not decrease PCSK9 transcription or increase PCSK9 degradation, but instead caused transcript-dependent inhibition of PCSK9 translation. In support of this surprising mechanism of action, we found that R-IMPP was able to selectively bind to human, but not E. coli, ribosomes. This study opens a new avenue for the development of drugs that modulate the activity of target proteins by mechanisms involving inhibition of eukaryotic translation.

SUMMARY The assembly of bacterial ribosomes is viewed with increasing interest as a potential target for new antibiotics. The in vivo synthesis and assembly of ribosomes are briefly reviewed here, highlighting the many ways in which assembly can be perturbed. The process is compared with the model in vitro process from which much of our knowledge is derived. The coordinate synthesis of the ribosomal components is essential for their ordered and efficient assembly; antibiotics interfere with this coordination and therefore affect assembly. It has also been claimed that the binding of antibiotics to nascent ribosomes prevents their assembly. These two contrasting models of antibiotic action are compared and evaluated. Finally, the suitability and tractability of assembly as a drug target are assessed.

MinireviewThe Ribosome in Focus ation of both subunits. The main findings about the Bruce A. Maguire and Robert A. Zimmermann structure and function of the ribosome up until 1999 are Department of Biochemistry and Molecular Biology summarized in a recent symposium volume (Garrett et University of Massachusetts al., 2000). Amherst, Massachusetts 01003 Morphology of the Ribosomal Subunits Prior to the crystal structure, four decades of painstaking research had led to a general understanding of ribo-The recent solution of ribosome structure at atomic ressome morphology and function. Electron microscopy of olution by X-ray crystallography represents a quantum negatively stained 50S and 30S subunits from E. coli leap in our understanding of the mechanism of protein showed that the former possesses a chunky, almost synthesis. Notably, the major functional sites are mainly spherical, body adorned with a central protuberance composed of RNA and the activity that catalyzes peptide and two flanking arms, while the latter is characterized bond formation appears to be a ribozyme. A high-resoluby a body and head, joined by a narrow neck, and a tion crystallographic structure seemed little more than platform that extends from the body on one side. The a pipe dream to many when Ada Yonath pioneered the positions of the ribosomal proteins relative to these enwork in the late 1970s. Although high-resolution diffracvelopes were located by immune electron microscopy tion patterns were obtained within a few years, solving and neutron scattering, and the placement of functional the phasing of the patterns remained the major sticking sites, including those for peptidyl transfer, decoding, point. Improvements in computing, the use of extremo-A-, P-, and E-site tRNAs, and the protein cofactors that phile ribosomes and cryo-temperatures to stabilize stimulate initiation, elongation, and termination were incrystals in the X-ray beam, and the advent of tunable ferred from a wide array of experimental approaches. high-energy synchrotron radiation all contributed to The informative, if somewhat blurred, picture of the riboprogress, while low-resolution structures obtained by some that emerged from these studies was subsecryo electron microscopy (cryo-EM) helped solve the quently improved by images derived from cryo-EM and phasing (Ban et al., 1998; Harms et al., 1999). Three has now been brought into sharp focus by the highother groups entered the fray in the mid 1990s, and the resolution crystal structures of the 50S and 30S subpace of progress in the last couple of years has been units. Solution of these structures is a salutary achievebreathtaking. To date, the 50S subunit from the haloment in structural biology, at once providing a basis philic archaeon Haloarcula marismortui has been solved for explaining in detail the vast amount of biophysical, at 2.4 A ˚(Ban et al., 2000) and the 30S subunit from biochemical, and genetic data on the E. coli ribosome the thermophilic (eu)bacterium Thermus thermophilus that have been accumulated over t...

We have developed a novel chromatography for the rapid isolation of active ribosomes from bacteria without the use of harsh conditions or lengthy procedures that damage ribosomes. Ribosomes interact with an alkyl linker attached to the resin, apparently through their RNA component. Examples are given with ribosomes from Escherichia coli, Deinococcus radiodurans, and with clinical isolates of Streptococcus pneumoniae and methicillin-resistant Staphylococcus aureus (MRSA). The ribosomes obtained by this method are unusually intact, so that highly active ribosomes can now be isolated from the clinical isolates, enabling significantly improved in vitro functional assays that will greatly assist the discovery and development of new ribosomally targeted antibiotics.

Here we describe a preparative differential centrifugation protocol for the isolation of ribosomes from a crude cell homogenate. The subcellular fraction obtained is enriched in ribosome monomers and polysomes. The protocol has been optimized for the homogenization and collection of the ribosomal fraction from prokaryotic cells, mammalian and plant tissues, reticulocytes, and chloroplasts. The quality of the ribosomal preparation is enhanced by the removal of the remaining cellular components and adsorbed proteins by pelleting through a sucrose cushion with a high concentration of monovalent salts, NH 4 Cl or KCl. The different components of the ribosomal fraction isolated using this protocol can be further purified by sucrose gradient centrifugation. MATERIALSIt is essential that you consult the appropriate Material Safety Data Sheets and your institution's Environmental Health and Safety Office for proper handling of equipment and hazardous materials used in this protocol.RECIPES: Please see the end of this protocol for recipes indicated by . Additional recipes can be found online at http://cshprotocols.cshlp.org/site/recipes. ReagentsCell culture (with appropriate growth medium) or tissue (mammalian or plant) of interestOne of the most critical factors for optimal ribosome purification is the quality of the cells from which the ribosomes are to be isolated. The best material is usually obtained from cells growing rapidly in a rich medium at their optimal temperature and harvested in early to mid-log phase long before growth and translation start to slow. For E. coli, strains lacking RNase I, such as MRE600, A19, and Q13, are preferred.Chloramphenicol or cycloheximide (optional; see Step 1) Chloroplast lysis buffer DNase (RNase-free) Dry ice/ethanol bath (optional; see Step 1) Homogenization buffer appropriate for preparation of interest (see Step 3) Chloroplast homogenization buffer French press buffer for prokaryotic 70S ribosomes French press buffer for prokaryotic polysomes Mammalian ribosome homogenization buffer