Architecture of the protein-conducting channel associated with the translating 80S ribosome

Cryo-EM structure of the inactive ribosome complex accumulated in chick embryo cells in cold-stress conditions

Here, we present the high-resolution structure of the Gallus gallus 80S ribosome obtained from cold-treated chicken embryos. The translationally inactive ribosome complex contains elongation factor eEF2 with GDP, SERPINE1 mRNA binding protein 1 (SERBP1) and deacylated tRNA in the P/E position, showing common features with complexes already described in mammals. Modeling of most expansion segments of G. gallus 28S ribosomal RNA allowed us to identify specific features in their structural organization and to describe areas where a marked difference between mammalian and avian ribosomes could shed light on the evolution of the expansion segments. This study provides the first structure of an avian ribosome, establishing a model for future structural and functional studies on the translational machinery in Aves.

Quantitative Mass Spectrometry Characterizes Client Spectra of Components for Targeting of Membrane Proteins to and Their Insertion into the Membrane of the Human ER

To elucidate the redundancy in the components for the targeting of membrane proteins to the endoplasmic reticulum (ER) and/or their insertion into the ER membrane under physiological conditions, we previously analyzed different human cells by label-free quantitative mass spectrometry. The HeLa and HEK293 cells had been depleted of a certain component by siRNA or CRISPR/Cas9 treatment or were deficient patient fibroblasts and compared to the respective control cells by differential protein abundance analysis. In addition to clients of the SRP and Sec61 complex, we identified membrane protein clients of components of the TRC/GET, SND, and PEX3 pathways for ER targeting, and Sec62, Sec63, TRAM1, and TRAP as putative auxiliary components of the Sec61 complex. Here, a comprehensive evaluation of these previously described differential protein abundance analyses, as well as similar analyses on the Sec61-co-operating EMC and the characteristics of the topogenic sequences of the various membrane protein clients, i.e., the client spectra of the components, are reported. As expected, the analysis characterized membrane protein precursors with cleavable amino-terminal signal peptides or amino-terminal transmembrane helices as predominant clients of SRP, as well as the Sec61 complex, while precursors with more central or even carboxy-terminal ones were found to dominate the client spectra of the SND and TRC/GET pathways for membrane targeting. For membrane protein insertion, the auxiliary Sec61 channel components indeed share the client spectra of the Sec61 complex to a large extent. However, we also detected some unexpected differences, particularly related to EMC, TRAP, and TRAM1. The possible mechanistic implications for membrane protein biogenesis at the human ER are discussed and can be expected to eventually advance our understanding of the mechanisms that are involved in the so-called Sec61-channelopathies, resulting from deficient ER protein import.

Uncovering the membrane-integrated SecAN protein that plays a key role in translocating nascent outer membrane proteins

A large number of nascent polypeptides have to get across a membrane in targeting to the proper subcellular locations. The SecYEG protein complex, a homolog of the Sec61 complex in eukaryotic cells, has been viewed as the common translocon at the inner membrane for targeting proteins to three extracytoplasmic locations in Gram-negative bacteria, despite the lack of direct verification in living cells. Here, via unnatural amino acid-mediated protein-protein interaction analyses in living cells, in combination with genetic studies, we unveiled a hitherto unreported SecA^N protein that seems to be directly involved in translocationg nascent outer membrane proteins across the plasma membrane; it consists of the N-terminal 375 residues of the SecA protein and exists as a membrane-integrated homooligomer. Our new findings place multiple previous observations related to bacterial protein targeting in proper biochemical and evolutionary contexts.

The significance of bioelectricity on all levels of organization of an organism. Part 1: From the subcellular level to cells

Bioelectricity plays an essential role in the structural and functional organization of biological organisms. In this first article of our three-part series, we summarize the importance of bioelectricity for the basic structural level of biological organization, i.e. from the subcellular level (charges, ion channels, molecules and cell organelles) to cells.

Identifying Strategies to Experimentally Probe Multidimensional Dynamics in the Ribosome

The ribosome is a complex biomolecular machine that utilizes large-scale conformational rearrangements to synthesize proteins. For example, during the elongation cycle, the "head" domain of the ribosomal small subunit (SSU) is known to undergo transient rotation events that allow for movement of tRNA molecules (i.e., translocation). While the head may exhibit rigid-body-like properties, the precise relationship between experimentally accessible probes and multidimensional rotations has yet to be established. To address this gap, we perform molecular dynamics simulations of the translocation step of the elongation cycle in the ribosome, where the SSU head spontaneously undergoes rotation and tilt-like motions. With this data set (1250 simulated events), we used statistical and information-theory-based measures to identify possible single-molecule probes that can isolate SSU head rotation and head tilting. This analysis provides a molecular interpretation for previous single-molecule measurements, while establishing a framework for the design of next-generation experiments that may precisely probe the mechanistic and kinetic aspects of the ribosome.

Ca2+ signalling system initiated by endoplasmic reticulum stress stimulates PERK activation

The accumulation of unfolded proteins within the Endoplasmic Reticulum (ER) activates a signal transduction pathway termed the unfolded protein response (UPR), which attempts to restore ER homoeostasis. If this cannot be done, UPR signalling ultimately induces apoptosis. Ca²⁺ depletion in the ER is a potent inducer of ER stress. Despite the ubiquity of Ca²⁺ as an intracellular messenger, the precise mechanism(s) by which Ca²⁺ release affects the UPR remains unknown. Tethering a genetically encoded Ca²⁺ indicator (GCamP6) to the ER membrane revealed novel Ca²⁺ signalling events initiated by Ca²⁺ microdomains in human astrocytes under ER stress, induced by tunicamycin (Tm), an N-glycosylation inhibitor, as well as in a cell model deficient in all three inositol triphosphate receptor isoforms. Pharmacological and molecular studies indicate that these local events are mediated by translocons and that the Ca²⁺ microdomains impact (PKR)-like-ER kinase (PERK), an UPR sensor, activation. These findings reveal the existence of a Ca²⁺ signal mechanism by which stressor-mediated Ca²⁺ release regulates ER stress.

Signal Peptide Features Determining the Substrate Specificities of Targeting and Translocation Components in Human ER Protein Import

In human cells, approximately 30% of all polypeptides enter the secretory pathway at the level of the endoplasmic reticulum (ER). This process involves cleavable amino-terminal signal peptides (SPs) or more or less amino-terminal transmembrane helices (TMHs), which serve as targeting determinants, at the level of the precursor polypeptides and a multitude of cytosolic and ER proteins, which facilitate their ER import. Alone or in combination SPs and TMHs guarantee the initial ER targeting as well as the subsequent membrane integration or translocation. Cytosolic SRP and SR, its receptor in the ER membrane, mediate cotranslational targeting of most nascent precursor polypeptide chains to the polypeptide-conducting Sec61 complex in the ER membrane. Alternatively, fully-synthesized precursor polypeptides and certain nascent precursor polypeptides are targeted to the ER membrane by either the PEX-, SND-, or TRC-pathway. Although these targeting pathways may have overlapping functions, the question arises how relevant this is under cellular conditions and which features of SPs and precursor polypeptides determine preference for a certain pathway. Irrespective of their targeting pathway(s), most precursor polypeptides are integrated into or translocated across the ER membrane via the Sec61 channel. For some precursor polypeptides specific Sec61 interaction partners have to support the gating of the channel to the open state, again raising the question why and when this is the case. Recent progress shed light on the client spectrum and specificities of some auxiliary components, including Sec62/Sec63, TRAM1 protein, and TRAP. To address the question which precursors use a certain pathway or component in intact human cells, i.e., under conditions of fast translation rates and molecular crowding, in the presence of competing precursors, different targeting organelles, and relevant stoichiometries of the involved components, siRNA-mediated depletion of single targeting or transport components in HeLa cells was combined with label-free quantitative proteomics and differential protein abundance analysis. Here, we present a summary of the experimental approach as well as the resulting differential protein abundance analyses and discuss their mechanistic implications in light of the available structural data.

Pore formation by pore forming membrane proteins towards infections

Over the last 25 years, the biology of membrane proteins, including the PFPs-membranes interactions is seeking attention for the development of successful drug molecules against a number of infectious diseases. Pore forming toxins (PFTs), the largest family of PFPs are considered as a group of virulence factors produced in a large number of pathogenic systems which include streptococcus, pneumonia, Staphylococcus aureus, E. coli, Mycobacterium tuberculosis, group A and B streptococci, Corynebacterium diphtheria and many more. PFTs are generally utilized by the disease causing pathogens to disrupt the host first line of defense i.e. host cell membranes through pore formation strategy. Although, pore formation is the principal mode of action of the PFTs but they can have additional adverse effects on the hosts including immune evasion. Recently, structural investigation of different PFTs have imparted the molecular mechanistic insights into how PFTs get transformed from its inactive state to active toxic state. On the basis of their structural entity, PFTs have been classified in different types and their mode of actions alters in terms of pore formation and corresponding cellular toxicity. Although pathogen genome analysis can identify the probable PFTs depending upon their structural diversity, there are so many PFTs which utilize the local environmental conditions to generate their pore forming ability using a novel strategy which is known as "conformational switch" of a protein. This conformational switch is considered as characteristics of the phase shifting proteins which were often utilized by many pathogenic systems to protect them from the invaders through allosteric communication between distant regions of the protein. In this chapter, we discuss the structure function relationships of PFTs and how activity of PFTs varies with the change in the environmental conditions has been explored. Finally, we demonstrate these structural insights to develop therapeutic potential to treat the infections caused by multidrug resistant pathogens.

The Arabidopsis 2'-O-Ribose-Methylation and Pseudouridylation Landscape of rRNA in Comparison to Human and Yeast

Eukaryotic ribosome assembly starts in the nucleolus, where the ribosomal DNA (rDNA) is transcribed into the 35S pre-ribosomal RNA (pre-rRNA). More than two-hundred ribosome biogenesis factors (RBFs) and more than two-hundred small nucleolar RNAs (snoRNA) catalyze the processing, folding and modification of the rRNA in Arabidopsis thaliana. The initial pre-ribosomal 90S complex is formed already during transcription by association of ribosomal proteins (RPs) and RBFs. In addition, small nucleolar ribonucleoprotein particles (snoRNPs) composed of snoRNAs and RBFs catalyze the two major rRNA modification types, 2'-O-ribose-methylation and pseudouridylation. Besides these two modifications, rRNAs can also undergo base methylations and acetylation. However, the latter two modifications have not yet been systematically explored in plants. The snoRNAs of these snoRNPs serve as targeting factors to direct modifications to specific rRNA regions by antisense elements. Today, hundreds of different sites of modifications in the rRNA have been described for eukaryotic ribosomes in general. While our understanding of the general process of ribosome biogenesis has advanced rapidly, the diversities appearing during plant ribosome biogenesis is beginning to emerge. Today, more than two-hundred RBFs were identified by bioinformatics or biochemical approaches, including several plant specific factors. Similarly, more than two hundred snoRNA were predicted based on RNA sequencing experiments. Here, we discuss the predicted and verified rRNA modification sites and the corresponding identified snoRNAs on the example of the model plant Arabidopsis thaliana. Our summary uncovers the plant modification sites in comparison to the human and yeast modification sites.

Quantitative Proteomics and Differential Protein Abundance Analysis after Depletion of Putative mRNA Receptors in the ER Membrane of Human Cells Identifies Novel Aspects of mRNA Targeting to the ER

In human cells, one-third of all polypeptides enter the secretory pathway at the endoplasmic reticulum (ER). The specificity and efficiency of this process are guaranteed by targeting of mRNAs and/or polypeptides to the ER membrane. Cytosolic SRP and its receptor in the ER membrane facilitate the cotranslational targeting of most ribosome-nascent precursor polypeptide chain (RNC) complexes together with the respective mRNAs to the Sec61 complex in the ER membrane. Alternatively, fully synthesized precursor polypeptides are targeted to the ER membrane post-translationally by either the TRC, SND, or PEX19/3 pathway. Furthermore, there is targeting of mRNAs to the ER membrane, which does not involve SRP but involves mRNA- or RNC-binding proteins on the ER surface, such as RRBP1 or KTN1. Traditionally, the targeting reactions were studied in cell-free or cellular assays, which focus on a single precursor polypeptide and allow the conclusion of whether a certain precursor can use a certain pathway. Recently, cellular approaches such as proximity-based ribosome profiling or quantitative proteomics were employed to address the question of which precursors use certain pathways under physiological conditions. Here, we combined siRNA-mediated depletion of putative mRNA receptors in HeLa cells with label-free quantitative proteomics and differential protein abundance analysis to characterize RRBP1- or KTN1-involving precursors and to identify possible genetic interactions between the various targeting pathways. Furthermore, we discuss the possible implications on the so-called TIGER domains and critically discuss the pros and cons of this experimental approach.

Complexity and Specificity of Sec61-Channelopathies: Human Diseases Affecting Gating of the Sec61 Complex

The rough endoplasmic reticulum (ER) of nucleated human cells has crucial functions in protein biogenesis, calcium (Ca2+) homeostasis, and signal transduction. Among the roughly one hundred components, which are involved in protein import and protein folding or assembly, two components stand out: The Sec61 complex and BiP. The Sec61 complex in the ER membrane represents the major entry point for precursor polypeptides into the membrane or lumen of the ER and provides a conduit for Ca2+ ions from the ER lumen to the cytosol. The second component, the Hsp70-type molecular chaperone immunoglobulin heavy chain binding protein, short BiP, plays central roles in protein folding and assembly (hence its name), protein import, cellular Ca2+ homeostasis, and various intracellular signal transduction pathways. For the purpose of this review, we focus on these two components, their relevant allosteric effectors and on the question of how their respective functional cycles are linked in order to reconcile the apparently contradictory features of the ER membrane, selective permeability for precursor polypeptides, and impermeability for Ca2+. The key issues are that the Sec61 complex exists in two conformations: An open and a closed state that are in a dynamic equilibrium with each other, and that BiP contributes to its gating in both directions in cooperation with different co-chaperones. While the open Sec61 complex forms an aqueous polypeptide-conducting- and transiently Ca2+-permeable channel, the closed complex is impermeable even to Ca2+. Therefore, we discuss the human hereditary and tumor diseases that are linked to Sec61 channel gating, termed Sec61-channelopathies, as disturbances of selective polypeptide-impermeability and/or aberrant Ca2+-permeability.

Interaction Networks of Ribosomal Expansion Segments in Kinetoplastids

Expansion segments (ES) are insertions of a few to hundreds of nucleotides at discrete locations on eukaryotic ribosomal RNA (rRNA) chains. Some cluster around 'hot spots' involved in translation regulation and some may participate in biogenesis. Whether ES play the same roles in different organisms is currently unclear, especially since their size may vary dramatically from one species to another and very little is known about their functions. Most likely, ES variation is linked to adaptation to a particular environment. In this chapter, we compare the interaction networks of ES from four kinetoplastid parasites, which have evolved in diverse insect vectors and mammalian hosts: Trypanosoma cruzi, Trypanosoma brucei, Leishmania donovani and Leishmania major. Here, we comparatively analyze ribosome structures from these representative kinetoplastids and ascertain meaningful structural differences from mammalian ribosomes. We base our analysis on sequence alignments and three-dimensional structures of 80S ribosomes solved by cryo-electron microscopy (cryo-EM). Striking differences in size are observed between ribosomes of different parasites, indicating that not all ES are expanded equally. Larger ES are not always matched by large surrounding ES or proteins extensions in their vicinity, a particularity that may lead to clues about their biological function. ES display different species-specific patterns of conservation, which underscore the density of their interaction network at the surface of the ribosome. Making sense of the conservation patterns of ES is part of a global effort to lay the basis for functional studies aimed at discovering unique kinetoplastid-specific sites suitable for therapeutic applications against these human and often animal pathogens.

microRNAs Mediated Regulation of the Ribosomal Proteins and its Consequences on the Global Translation of Proteins

Ribosomal proteins (RPs) are mostly derived from the energy-consuming enzyme families such as ATP-dependent RNA helicases, AAA-ATPases, GTPases and kinases, and are important structural components of the ribosome, which is a supramolecular ribonucleoprotein complex, composed of Ribosomal RNA (rRNA) and RPs, coordinates the translation and synthesis of proteins with the help of transfer RNA (tRNA) and other factors. Not all RPs are indispensable; in other words, the ribosome could be functional and could continue the translation of proteins instead of lacking in some of the RPs. However, the lack of many RPs could result in severe defects in the biogenesis of ribosomes, which could directly influence the overall translation processes and global expression of the proteins leading to the emergence of different diseases including cancer. While microRNAs (miRNAs) are small non-coding RNAs and one of the potent regulators of the post-transcriptional gene expression, miRNAs regulate gene expression by targeting the 3' untranslated region and/or coding region of the messenger RNAs (mRNAs), and by interacting with the 5' untranslated region, and eventually finetune the expression of approximately one-third of all mammalian genes. Herein, we highlighted the significance of miRNAs mediated regulation of RPs coding mRNAs in the global protein translation.

Quantitative Proteomics Links the LRRC59 Interactome to mRNA Translation on the ER Membrane

Protein synthesis on the endoplasmic reticulum (ER) requires the dynamic coordination of numerous cellular components. Together, resident ER membrane proteins, cytoplasmic translation factors, and both integral membrane and cytosolic RNA-binding proteins operate in concert with membrane-associated ribosomes to facilitate ER-localized translation. Little is known, however, regarding the spatial organization of ER-localized translation. This question is of growing significance as it is now known that ER-bound ribosomes contribute to secretory, integral membrane, and cytosolic protein synthesis alike. To explore this question, we utilized quantitative proximity proteomics to identify neighboring protein networks for the candidate ribosome interactors SEC61β (subunit of the protein translocase), RPN1 (oligosaccharyltransferase subunit), SEC62 (translocation integral membrane protein), and LRRC59 (ribosome binding integral membrane protein). Biotin labeling time course studies of the four BioID reporters revealed distinct labeling patterns that intensified but only modestly diversified as a function of labeling time, suggesting that the ER membrane is organized into discrete protein interaction domains. Whereas SEC61β and RPN1 reporters identified translocon-associated networks, SEC62 and LRRC59 reporters revealed divergent protein interactomes. Notably, the SEC62 interactome is enriched in redox-linked proteins and ER luminal chaperones, with the latter likely representing proximity to an ER luminal chaperone reflux pathway. In contrast, the LRRC59 interactome is highly enriched in SRP pathway components, translation factors, and ER-localized RNA-binding proteins, uncovering a functional link between LRRC59 and mRNA translation regulation. Importantly, analysis of the LRRC59 interactome by native immunoprecipitation identified similar protein and functional enrichments. Moreover, [³⁵S]-methionine incorporation assays revealed that siRNA silencing of LRRC59 expression reduced steady state translation levels on the ER by ca. 50%, and also impacted steady state translation levels in the cytosol compartment. Collectively, these data reveal a functional domain organization for the ER and identify a key role for LRRC59 in the organization and regulation of local translation.

Understanding the mammalian TRAP complex function(s)

In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).

Structural insights into the mechanism of a novel protein targeting pathway in Gram-negative bacteria

Many nascent polypeptides synthesized in the cytoplasm are translocated across membranes via a specific ‘translocon’ composed of protein complexes. Recently, a novel targeting pathway for the outer membrane β‐barrel proteins (OMPs) in Gram‐negative bacteria was discovered. The cell envelope of Gram‐negative bacteria is composed of the inner (plasma) membrane (IM) and the outer membrane (OM). In this new pathway, a SecA^N protein, which is mainly present in the IM as a homo‐oligomer, translocates nascent OMPs across the IM; at the same time, SecA^N directly interacts with the β‐barrel assembly machinery (BAM) complex embedded within the OM. A supercomplex (containing SecA^N, the BAM complex and many other proteins) spans the IM and OM, and is involved in the biogenesis of OMPs. Investigation of the function of SecA^N and the supercomplex, as well as the translocation mechanism, will require elucidation of their structures. However, no such structures are available. Therefore, here, I describe the use of protein modeling to build homology models for SecA^N and theoretical structures for the core‐complex composed of SecA^N and the BAM complex, which is a key part of the supercomplex. The modeling data are consistent with previous experimental observations and demonstrated a conformational change of the core‐complex. I conclude by proposing mechanisms for how SecA^N and the supercomplex function in the biogenesis of OMPs.

MetAP-like Ebp1 occupies the human ribosomal tunnel exit and recruits flexible rRNA expansion segments

Human Ebp1 is a member of the proliferation-associated 2G4 (PA2G4) family and plays an important role in cancer regulation. Ebp1 shares the methionine aminopeptidase (MetAP) fold and binds to mature 80S ribosomes for translational control. Here, we present a cryo-EM single particle analysis reconstruction of Ebp1 bound to non-translating human 80S ribosomes at a resolution range from 3.3 to ~8 Å. Ebp1 blocks the tunnel exit with major interactions to the general uL23/uL29 docking site for nascent chain-associated factors complemented by eukaryote-specific eL19 and rRNA helix H59. H59 is defined as dynamic adaptor undergoing significant remodeling upon Ebp1 binding. Ebp1 recruits rRNA expansion segment ES27L to the tunnel exit via specific interactions with rRNA consensus sequences. The Ebp1-ribosome complex serves as a template for MetAP binding and provides insights into the structural principles for spatial coordination of co-translational events and molecular triage at the ribosomal tunnel exit.

The ErbB3 receptor binding protein Ebp1 binds to ribosomes and is linked to translational control. Here, the authors present the cryo-EM structure of human Ebp1 bound to a non-translating 80S ribosome and find that Ebp1 blocks the tunnel exit and recruits the rRNA expansion segment ES27L to the tunnel exit.

A clearer picture of the ER translocon complex

The endoplasmic reticulum (ER) translocon complex is the main gate into the secretory pathway, facilitating the translocation of nascent peptides into the ER lumen or their integration into the lipid membrane. Protein biogenesis in the ER involves additional processes, many of them occurring co-translationally while the nascent protein resides at the translocon complex, including recruitment of ER-targeted ribosome-nascent-chain complexes, glycosylation, signal peptide cleavage, membrane protein topogenesis and folding. To perform such varied functions on a broad range of substrates, the ER translocon complex has different accessory components that associate with it either stably or transiently. Here, we review recent structural and functional insights into this dynamically constituted central hub in the ER and its components. Recent cryo-electron microscopy (EM) studies have dissected the molecular organization of the co-translational ER translocon complex, comprising the Sec61 protein-conducting channel, the translocon-associated protein complex and the oligosaccharyl transferase complex. Complemented by structural characterization of the post-translational import machinery, key molecular principles emerge that distinguish co- and post-translational protein import and biogenesis. Further cryo-EM structures promise to expand our mechanistic understanding of the various biochemical functions involving protein biogenesis and quality control in the ER.

ER import of small human presecretory proteins: components and mechanisms

Protein transport into the mammalian endoplasmic reticulum (ER) used to be seen as strictly cotranslational, that is temporarily and mechanistically coupled to protein synthesis. In the course of the last decades, however, several classes of precursors of soluble and membrane proteins were found to be post-translationally imported into the ER, without any involvement of the ribosome. The first such class to be identified were the small presecretory proteins; tail-anchored membrane proteins followed next. In both classes, the inherent address tag is released from the translating ribosome before the initiation of ER import, as part of the fully synthesized precursor. In small presecretory proteins, the information for ER targeting and -translocation via the polypeptide-conducting Sec61-channel is encoded by a classical N-terminal signal peptide, which is released from the ribsosome before targeting due to the small size of the full-length precursor. Here, we discuss the current state of research on targeting and translocation of small presecretory proteins into the mammalian ER. In closing, we present a unifying hypothesis for ER protein translocation in terms of an energy diagram for Sec61-channel gating.

Structure of the 80S ribosome-Xrn1 nuclease complex

Messenger RNA (mRNA) homeostasis represents an essential part of gene expression, in which the generation of mRNA by RNA polymerase is counter-balanced by its degradation by nucleases. The conserved 5'-to-3' exoribonuclease Xrn1 has a crucial role in eukaryotic mRNA homeostasis by degrading decapped or cleaved mRNAs post-translationally and, more surprisingly, also co-translationally. Here we report that active Xrn1 can directly and specifically interact with the translation machinery. A cryo-electron microscopy structure of a programmed Saccharomyces cerevisiae 80S ribosome-Xrn1 nuclease complex reveals how the conserved core of Xrn1 enables binding at the mRNA exit site of the ribosome. This interface provides a conduit for channelling of the mRNA from the ribosomal decoding site directly into the active center of the nuclease, thus separating mRNA decoding from degradation by only 17 ± 1 nucleotides. These findings explain how rapid 5'-to-3' mRNA degradation is coupled efficiently to its final round of mRNA translation.

Collided ribosomes form a unique structural interface to induce Hel2-driven quality control pathways

Ribosome stalling triggers quality control pathways targeting the mRNA (NGD: no-go decay) and the nascent polypeptide (RQC: ribosome-associated quality control). RQC requires Hel2-dependent uS10 ubiquitination and the RQT complex in yeast. Here, we report that Hel2-dependent uS10 ubiquitination and Slh1/Rqt2 are crucial for RQC and NGD induction within a di-ribosome (disome) unit, which consists of the leading stalled ribosome and the following colliding ribosome. Hel2 preferentially ubiquitinated a disome over a monosome on a quality control inducing reporter mRNA in an in vitro translation reaction. Cryo-EM analysis of the disome unit revealed a distinct structural arrangement suitable for recognition and modification by Hel2. The absence of the RQT complex or uS10 ubiquitination resulted in the elimination of NGD within the disome unit. Instead, we observed Hel2-mediated cleavages upstream of the disome, governed by initial Not4-mediated monoubiquitination of eS7 and followed by Hel2-mediated K63-linked polyubiquitination. We propose that Hel2-mediated ribosome ubiquitination is required both for canonical NGD (NGD^{RQC +}) and RQC coupled to the disome and that RQC-uncoupled NGD outside the disome (NGD^{RQC -}) can occur in a Not4-dependent manner.

Ribosome-NatA architecture reveals that rRNA expansion segments coordinate N-terminal acetylation

The majority of eukaryotic proteins are N-terminally α-acetylated by N-terminal acetyltransferases (NATs). Acetylation usually occurs co-translationally and defects have severe consequences. Nevertheless, it is unclear how these enzymes act in concert with the translating ribosome. Here, we report the structure of a native ribosome-NatA complex from Saccharomyces cerevisiae. NatA (comprising Naa10, Naa15 and Naa50) displays a unique mode of ribosome interaction by contacting eukaryotic-specific ribosomal RNA expansion segments in three out of four binding patches. Thereby, NatA is dynamically positioned directly underneath the ribosomal exit tunnel to facilitate modification of the emerging nascent peptide chain. Methionine amino peptidases, but not chaperones or signal recognition particle, would be able to bind concomitantly. This work assigns a function to the hitherto enigmatic ribosomal RNA expansion segments and provides mechanistic insights into co-translational protein maturation by N-terminal acetylation.

emClarity: software for high-resolution cryo-electron tomography and subtomogram averaging

Macromolecular complexes are intrinsically flexible and often challenging to purify for structure determination by single particle cryoEM. Such complexes may be studied using cryo-electron tomography combined with sub-tomogram alignment and classification, which in exceptional cases reaches sub-nanometer resolution, yielding insight into structure-function relationships. Extending this approach to specimens that exhibit conformational or compositional heterogeneity, and that may be present at low abundance, remains challenging. To address this challenge, we developed emClarity (https://github.com/bHimes/emClarity/wiki), a GPU-accelerated image processing package, which features an iterative tomographic tilt-series refinement algorithm using sub-tomograms as fiducial markers and a 3D-samping function compensated, multi-scale Principle Component Analysis classification method. We demonstrate substantial improvements in the resolution of maps and in the separation of different functional states of macromolecular complexes, compared to those generated using current state-of-the-art software.

The endoplasmic reticulum-residing chaperone BiP is short-lived and metabolized through N-terminal arginylation

BiP and other endoplasmic reticulum (ER)-resident proteins are thought to be metabolically stable and to function primarily in the ER lumen. We sought to assess how the abundance of these proteins dynamically fluctuates in response to various stresses and how their subpopulations are relocated to non-ER compartments such as the cytosol. We showed that the molecular chaperone BiP (also known as GRP78) was short-lived under basal conditions and ER stress. The turnover of BiP was in part driven by its amino-terminal arginylation (Nt-arginylation) by the arginyltransferase ATE1, which generated an autophagic N-degron of the N-end rule pathway. ER stress elicited the formation of R-BiP, an effect that was increased when the proteasome was also inhibited. Nt-arginylation correlated with the cytosolic relocalization of BiP under the types of stress tested. The cytosolic relocalization of BiP did not require the functionality of the unfolded protein response or the Sec61- or Derlin1-containing translocon. A key inhibitor of the turnover and Nt-arginylation of BiP was HERP (homocysteine-responsive ER protein), a 43-kDa ER membrane-integrated protein that is an essential component of ER-associated protein degradation. Pharmacological inhibition of the ER-Golgi secretory pathway also suppressed R-BiP formation. Finally, we showed that cytosolic R-BiP induced by ER stress and proteasomal inhibition was routed to autophagic vacuoles and possibly additional metabolic fates. These results suggest that Nt-arginylation is a posttranslational modification that modulates the function, localization, and metabolic fate of ER-resident proteins.

Structural insights into species-specific features of the ribosome from the human pathogen Mycobacterium tuberculosis

Ribosomes from Mycobacterium tuberculosis (Mtb) possess species-specific ribosomal RNA (rRNA) expansion segments and ribosomal proteins (rProtein). Here, we present the near-atomic structures of the Mtb 50S ribosomal subunit and the complete Mtb 70S ribosome, solved by cryo-electron microscopy. Upon joining of the large and small ribosomal subunits, a 100-nt long expansion segment of the Mtb 23S rRNA, named H54a or the ‘handle’, switches interactions from with rRNA helix H68 and rProtein uL2 to with rProtein bS6, forming a new intersubunit bridge ‘B9’. In Mtb 70S, bridge B9 is mostly maintained, leading to correlated motions among the handle, the L1 stalk and the small subunit in the rotated and non-rotated states. Two new protein densities were discovered near the decoding center and the peptidyl transferase center, respectively. These results provide a structural basis for studying translation in Mtb as well as developing new tuberculosis drugs.

Preribosomes escaping from the nucleus are caught during translation by cytoplasmic quality control

Assembly of fully functional ribosomes is a prerequisite for failsafe translation. This explains why maturing preribosomal subunits have to pass through an array of quality-control checkpoints, including nuclear export, to ensure that only properly assembled ribosomes engage in translation. Despite these safeguards, we found that nuclear pre-60S particles unable to remove a transient structure composed of ITS2 pre-rRNA and associated assembly factors, termed the 'foot', escape to the cytoplasm, where they can join with mature 40S subunits to catalyze protein synthesis. However, cells harboring these abnormal ribosomes show translation defects indicated by the formation of 80S ribosomes poised with pre-60S subunits carrying tRNAs in trapped hybrid states. To overcome this translational stress, the cytoplasmic surveillance machineries RQC and Ski-exosome target these malfunctioning ribosomes. Thus, pre-60S subunits that escape nuclear quality control can enter translation, but are caught by cytoplasmic surveillance mechanisms.

Eukaryotic Ribosomal Expansion Segments as Antimicrobial Targets

Diversity in eukaryotic rRNA structure and function offers possibilities of therapeutic targets. Unlike ribosomes of prokaryotes, eukaryotic ribosomes contain species-specific rRNA expansion segments (ESs) with idiosyncratic structures and functions that are essential and specific to some organisms. Here we investigate expansion segment 7 (ES7), one of the largest and most variable expansions of the eukaryotic ribosome. We hypothesize that ES7 of the pathogenic fungi Candida albicans (ES7_CA) could be a prototypic drug target. We show that isolated ES7_CA folds reversibly to a native-like state. We developed a fluorescence displacement assay using an RNA binding fluorescent probe, F-neo. F-neo binds tightly to ES7_CA with a K_d of 2.5 × 10^-9 M but binds weakly to ES7 of humans (ES7_HS) with a K_d estimated to be greater than 7 μM. The fluorescence displacement assay was used to investigate the affinities of a library of peptidic aminosugar conjugates (PAs) for ES7_CA. For conjugates with highest affinities for ES7_CA (NeoRH, NeoFH, and NeoYH), the lowest dose needed to induce mortality in C. albicans (minimum inhibitory concentration, MIC) was determined. PAs with the lowest MIC values were tested for cytotoxicity in HEK293T cells. Molecules with high affinity for ES7_CA in vitro induce mortality in C. albicans but not in HEK293T cells. The results are consistent with the hypothesis that ESs represent useful targets for chemotherapeutics directed against eukaryotic pathogens.

The cryo-EM structure of a ribosome-Ski2-Ski3-Ski8 helicase complex

Ski2-Ski3-Ski8 (Ski) is a helicase complex functioning with the RNA-degrading exosome to mediate the 3'-5' messenger RNA (mRNA) decay in turnover and quality-control pathways. We report that the Ski complex directly associates with 80S ribosomes presenting a short mRNA 3' overhang. We determined the structure of an endogenous ribosome-Ski complex using cryo-electron microscopy (EM) with a local resolution of the Ski complex ranging from 4 angstroms (Å) in the core to about 10 Å for intrinsically flexible regions. Ribosome binding displaces the autoinhibitory domain of the Ski2 helicase, positioning it in an open conformation near the ribosomal mRNA entry tunnel. We observe that the mRNA 3' overhang is threaded directly from the small ribosomal subunit to the helicase channel of Ski2, primed for ongoing exosome-mediated 3'-5' degradation.

Mechanistic insight into eukaryotic 60S ribosomal subunit biogenesis by cryo-electron microscopy

Eukaryotic ribosomes, the protein-producing factories of the cell, are composed of four ribosomal RNA molecules and roughly 80 proteins. Their biogenesis is a complex process that involves more than 200 biogenesis factors that facilitate the production, modification, and assembly of ribosomal components and the structural transitions along the maturation pathways of the pre-ribosomal particles. Here, I review recent structural and mechanistic insights into the biogenesis of the large ribosomal subunit that were furthered by cryo-electron microscopy of natively purified pre-60S particles and in vitro reconstituted ribosome assembly factor complexes. Combined with biochemical, genetic, and previous structural data, these structures have provided detailed insights into the assembly and maturation of the central protuberance of the 60S subunit, the network of biogenesis factors near the ribosomal tunnel exit, and the functional activation of the large ribosomal subunit during cytoplasmic maturation.

Eukaryote-specific rRNA expansion segments function in ribosome biogenesis

The secondary structure of ribosomal RNA (rRNA) is largely conserved across all kingdoms of life. However, eukaryotes have evolved extra blocks of rRNA sequences, relative to those of prokaryotes, called expansion segments (ES). A thorough characterization of the potential roles of ES remains to be done, possibly because of limitations in the availability of robust systems to study rRNA mutants. We sought to systematically investigate the potential functions, if any, of the ES in 25S rRNA of Saccharomyces cerevisiae by deletion mutagenesis. We deleted 14 of the 16 different eukaryote-specific ES in yeast 25S rRNA individually and assayed their phenotypes. Our results show that all but two of the ES tested are necessary for optimal growth and are required for production of 25S rRNA, suggesting that ES play roles in ribosome biogenesis. Further, we classified expansion segments into groups that participate in early nucleolar, middle, and late nucleoplasmic steps of ribosome biogenesis, by assaying their pre-rRNA processing phenotypes. This study is the first of its kind to systematically identify the functions of eukaryote-specific expansion segments by showing that they play roles in specific steps of ribosome biogenesis. The catalog of phenotypes we identified, combined with previous investigations of the roles ribosomal proteins in large subunit biogenesis, leads us to infer that assembling ribosomes are composed of distinct RNA and protein structural neighborhood clusters that participate in specific steps of ribosome biogenesis.

Toward a structural understanding of co-translational protein translocation

The translocation of most eukaryotic secreted and integral membrane proteins occurs co-translationally at the endoplasmic reticulum (ER). These nascent polypeptides are recognized on the ribosome by the signal recognition particle (SRP), targeted to the ER, and translocated across or inserted into the membrane by the Sec61 translocation channel. Structural analysis of these co-translational processes has been challenging due to the size, complexity, and flexibility of the targeting and translocation machinery. Recent technological advances in cryo-electron microscopy (cryo-EM) have resulted in increasingly powerful tools to study large, heterogeneous, and low-abundance samples. These advances are being utilized to obtain near-atomic resolution reconstructions of functional translation, targeting, and translocation intermediates, paving the way to a mechanistic understanding of protein biogenesis.

Homoiterons and expansion in ribosomal RNAs

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Homoiterons like GGGGGGG stabilize ribosomal RNAs of thermophile prokaryotes.

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In eukaryotes, homoiterons are much more abundant in RNA of the larger subunit (LSU).

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The LSU repeats increase with phylogenetic rank to 28% entire RNA sequence in hominids.

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In mammal LSU RNAs, these repeats constitute 45% of the massive expansion segments.

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These repeats may help in anchoring of ribosomes and export of secretory proteins.

Ribosomal RNAs in both prokaryotes and eukaryotes feature numerous repeats of three or more nucleotides with the same nucleobase (homoiterons). In prokaryotes these repeats are much more frequent in thermophile compared to mesophile or psychrophile species, and have similar frequency in both large RNAs. These features point to use of prokaryotic homoiterons in stabilization of both ribosomal subunits. The two large RNAs of eukaryotic cytoplasmic ribosomes have expanded to a different degree across the evolutionary ladder. The big RNA of the larger subunit (60S LSU) evolved expansion segments of up to 2400 nucleotides, and the smaller subunit (40S SSU) RNA acquired expansion segments of not more than 700 nucleotides. In the examined eukaryotes abundance of rRNA homoiterons generally follows size and nucleotide bias of the expansion segments, and increases with GC content and especially with phylogenetic rank. Both the nucleotide bias and frequency of homoiterons are much larger in metazoan and angiosperm LSU compared to the respective SSU RNAs. This is especially pronounced in the tetrapod vertebrates and seems to culminate in the hominid mammals. The stability of secondary structure in polyribonucleotides would significantly connect to GC content, and should also relate to G and C homoiteron content. RNA modeling points to considerable presence of homoiteron-rich double-stranded segments especially in vertebrate LSU RNAs, and homoiterons with four or more nucleotides in the vertebrate and angiosperm LSU RNAs are largely confined to the expansion segments. These features could mainly relate to protein export function and attachment of LSU to endoplasmic reticulum and other subcellular networks.

A comparative genomics study on the effect of individual amino acids on ribosome stalling

BACKGROUND

During protein synthesis, the nascent peptide chain emerges from the ribosome through the ribosomal exit tunnel. Biochemical interactions between the nascent peptide and the tunnel may stall the ribosome movement and thus affect the expression level of the protein being synthesized. Earlier studies focused on one model organism (S. cerevisiae), have suggested that certain amino acid sequences may be responsible for ribosome stalling; however, the stalling effect at the individual amino acid level across many organisms has not yet been quantified.

RESULTS

By analyzing multiple ribosome profiling datasets from different organisms (including prokaryotes and eukaryotes), we report for the first time the organism-specific amino acids that significantly lead to ribosome stalling. We show that the identity of the stalling amino acids vary across the tree of life. In agreement with previous studies, we observed a remarkable stalling signal of proline and arginine in S. cerevisiae. In addition, our analysis supports the conjecture that the stalling effect of positively charged amino acids is not universal and that in certain conditions, negative charge may also induce ribosome stalling. Finally, we show that the beginning part of the tunnel tends to undergo more interactions with the translated amino acids than other positions along the tunnel.

CONCLUSIONS

The reported results support the conjecture that the ribosomal exit tunnel interacts with various amino acids and that the nature of these interactions varies among different organisms. Our findings should contribute towards better understanding of transcript and proteomic evolution and translation elongation regulation.

Structure of a human translation termination complex

In contrast to bacteria that have two release factors, RF1 and RF2, eukaryotes only possess one unrelated release factor eRF1, which recognizes all three stop codons of the mRNA and hydrolyses the peptidyl-tRNA bond. While the molecular basis for bacterial termination has been elucidated, high-resolution structures of eukaryotic termination complexes have been lacking. Here we present a 3.8 Å structure of a human translation termination complex with eRF1 decoding a UAA(A) stop codon. The complex was formed using the human cytomegalovirus (hCMV) stalling peptide, which perturbs the peptidyltransferase center (PTC) to silence the hydrolysis activity of eRF1. Moreover, unlike sense codons or bacterial stop codons, the UAA stop codon adopts a U-turn-like conformation within a pocket formed by eRF1 and the ribosome. Inducing the U-turn conformation for stop codon recognition rationalizes how decoding by eRF1 includes monitoring geometry in order to discriminate against sense codons.

The Ribosome-Sec61 Translocon Complex Forms a Cytosolically Restricted Environment for Early Polytopic Membrane Protein Folding

Transmembrane topology of polytopic membrane proteins (PMPs) is established in the endoplasmic reticulum (ER) by the ribosome Sec61-translocon complex (RTC) through iterative cycles of translocation initiation and termination. It remains unknown, however, whether tertiary folding of transmembrane domains begins after the nascent polypeptide integrates into the lipid bilayer or within a proteinaceous environment proximal to translocon components. To address this question, we used cysteine scanning mutagenesis to monitor aqueous accessibility of stalled translation intermediates to determine when, during biogenesis, hydrophilic peptide loops of the aquaporin-4 (AQP4) water channel are delivered to cytosolic and lumenal compartments. Results showed that following ribosome docking on the ER membrane, the nascent polypeptide was shielded from the cytosol as it emerged from the ribosome exit tunnel. Extracellular loops followed a well defined path through the ribosome, the ribosome translocon junction, the Sec61-translocon pore, and into the ER lumen coincident with chain elongation. In contrast, intracellular loops (ICLs) and C-terminalresidues exited the ribosome into a cytosolically shielded environment and remained inaccessible to both cytosolic and lumenal compartments until translation was terminated. Shielding of ICL1 and ICL2, but not the C terminus, became resistant to maneuvers that disrupt electrostatic ribosome interactions. Thus, the early folding landscape of polytopic proteins is shaped by a spatially restricted environment localized within the assembled ribosome translocon complex.

Regulation by a chaperone improves substrate selectivity during cotranslational protein targeting

The ribosome exit site is a crowded environment where numerous factors contact nascent polypeptides to influence their folding, localization, and quality control. Timely and accurate selection of nascent polypeptides into the correct pathway is essential for proper protein biogenesis. To understand how this is accomplished, we probe the mechanism by which nascent polypeptides are accurately sorted between the major cotranslational chaperone trigger factor (TF) and the essential cotranslational targeting machinery, signal recognition particle (SRP). We show that TF regulates SRP function at three distinct stages, including binding of the translating ribosome, membrane targeting via recruitment of the SRP receptor, and rejection of ribosome-bound nascent polypeptides beyond a critical length. Together, these mechanisms enhance the specificity of substrate selection into both pathways. Our results reveal a multilayered mechanism of molecular interplay at the ribosome exit site, and provide a conceptual framework to understand how proteins are selected among distinct biogenesis machineries in this crowded environment.

Using Molecular Simulation to Model High-Resolution Cryo-EM Reconstructions

An explosion of new data from high-resolution cryo-electron microscopy (cryo-EM) studies has produced a large number of data sets for many species of ribosomes in various functional states over the past few years. While many methods exist to produce structural models for lower resolution cryo-EM reconstructions, high-resolution reconstructions are often modeled using crystallographic techniques and extensive manual intervention. Here, we present an automated fitting technique for high-resolution cryo-EM data sets that produces all-atom models highly consistent with the EM density. Using a molecular dynamics approach, atomic positions are optimized with a potential that includes the cross-correlation coefficient between the structural model and the cryo-EM electron density, as well as a biasing potential preserving the stereochemistry and secondary structure of the biomolecule. Specifically, we use a hybrid structure-based/ab initio molecular dynamics potential to extend molecular dynamics fitting. In addition, we find that simulated annealing integration, as opposed to straightforward molecular dynamics integration, significantly improves performance. We obtain atomistic models of the human ribosome consistent with high-resolution cryo-EM reconstructions of the human ribosome. Automated methods such as these have the potential to produce atomistic models for a large number of ribosome complexes simultaneously that can be subsequently refined manually.

Diversity and selectivity in mRNA translation on the endoplasmic reticulum

Pioneering electron microscopy studies defined two primary populations of ribosomes in eukaryotic cells: one freely dispersed through the cytoplasm and the other bound to the surface of the endoplasmic reticulum (ER). Subsequent investigations revealed a specialized function for each population, with secretory and integral membrane protein-encoding mRNAs translated on ER-bound ribosomes, and cytosolic protein synthesis was widely attributed to free ribosomes. Recent findings have challenged this view, and transcriptome-scale studies of mRNA distribution and translation have now demonstrated that ER-bound ribosomes also function in the translation of a large fraction of mRNAs that encode cytosolic proteins. These studies suggest a far more expansive role for the ER in transcriptome expression, where membrane and secretory protein synthesis represents one element of a multifaceted and dynamic contribution to post-transcriptional gene expression.

Positive and negative regulation of vertebrate separase by Cdk1-cyclin B1 may explain why securin is dispensable

Sister chromatid cohesion is established during replication by entrapment of both dsDNAs within the cohesin ring complex. It is dissolved in anaphase when separase, a giant cysteine endopeptidase, cleaves the Scc1/Rad21 subunit of cohesin, thereby triggering chromosome segregation. Separase is held inactive by association with securin until this anaphase inhibitor is destroyed at the metaphase-to-anaphase transition by ubiquitin-dependent degradation. The relevant ubiquitin ligase, the anaphase-promoting complex/cyclosome, also targets cyclin B1, thereby causing inactivation of Cdk1 and mitotic exit. Although separase is essential, securin knock-out mice are surprisingly viable and fertile. Capitalizing on our previous finding that Cdk1-cyclin B1 can also bind and inhibit separase, we investigated whether this kinase might be suitable to maintain faithful timing and execution of anaphase in the absence of securin. We found that, similar to securin, Cdk1-cyclin B1 regulates separase in both a positive and negative manner. Although securin associates with nascent separase to co-translationally assist proper folding, Cdk1-cyclin B1 acts on native state separase. Upon entry into mitosis, Cdk1-cyclin B1-dependent phosphorylation of Ser-1126 renders separase prone to inactivation by aggregation/precipitation. Stable association of Cdk1-cyclin B1 with phosphorylated separase counteracts this tendency and stabilizes separase in an inhibited yet activatable state. These opposing effects are suited to prevent premature cleavage of cohesin in early mitosis while ensuring timely activation of separase by anaphase-promoting complex/cyclosome-dependent degradation of cyclin B1. Coupling sister chromatid separation with subsequent exit from mitosis by this simplified mode might have been the common scheme of mitotic control prior to the evolution of securin.

Structure and assembly pathway of the ribosome quality control complex

During ribosome-associated quality control, stalled ribosomes are split into subunits and the 60S-housed nascent polypeptides are poly-ubiquitinated by Listerin. How this low-abundance ubiquitin ligase targets rare stall-generated 60S among numerous empty 60S is unknown. Here, we show that Listerin specificity for nascent chain-60S complexes depends on nuclear export mediator factor (NEMF). The 3.6 Å cryo-EM structure of a nascent chain-containing 60S-Listerin-NEMF complex revealed that NEMF makes multiple simultaneous contacts with 60S and peptidyl-tRNA to sense nascent chain occupancy. Structural and mutational analyses showed that ribosome-bound NEMF recruits and stabilizes Listerin's N-terminal domain, while Listerin's C-terminal RWD domain directly contacts the ribosome to position the adjacent ligase domain near the nascent polypeptide exit tunnel. Thus, highly specific nascent chain targeting by Listerin is imparted by the avidity gained from a multivalent network of context-specific individually weak interactions, highlighting a new principle of client recognition during protein quality control.

The sorting of a small potassium channel in mammalian cells can be shifted between mitochondria and plasma membrane

The two small and similar viral K(+) channels Kcv and Kesv are sorted in mammalian cells and yeast to different destinations. Analysis of the sorting pathways shows that Kcv is trafficking via the secretory pathway to the plasma membrane, while Kesv is inserted via the TIM/TOM complex to the inner membrane of mitochondria. Studies with Kesv mutants show that an N-terminal mitochondrial targeting sequence in this channel is neither necessary nor sufficient for sorting of Kesv the mitochondria. Instead the sorting of Kesv can be redirected from the mitochondria to the plasma membrane by an insertion of ≥2 amino acids in a position sensitive manner into the C-terminal transmembrane domain (TMD2) of this channel. The available data advocate the presence of a C-terminal sorting signal in TMD2 of Kesv channel, which is presumably not determined by the length of this domain.

Fidelity of cotranslational protein targeting by the signal recognition particle

Accurate folding, assembly, localization, and maturation of newly synthesized proteins are essential to all cells and require high fidelity in the protein biogenesis machineries that mediate these processes. Here, we review our current understanding of how high fidelity is achieved in one of these processes, the cotranslational targeting of nascent membrane and secretory proteins by the signal recognition particle (SRP). Recent biochemical, biophysical, and structural studies have elucidated how the correct substrates drive a series of elaborate conformational rearrangements in the SRP and SRP receptor GTPases; these rearrangements provide effective fidelity checkpoints to reject incorrect substrates and enhance the fidelity of this essential cellular pathway. The mechanisms used by SRP to ensure fidelity share important conceptual analogies with those used by cellular machineries involved in DNA replication, transcription, and translation, and these mechanisms likely represent general principles for other complex cellular pathways.

Multifunctional roles for the protein translocation machinery in RNA anchoring to the endoplasmic reticulum

Signal sequence-encoding mRNAs undergo translation-dependent localization to the endoplasmic reticulum (ER) and at the ER are anchored via translation on Sec61-bound ribosomes. Recent investigations into the composition and membrane association characteristics of ER-associated mRNAs have, however, revealed both ribosome-dependent (indirect) and ribosome-independent (direct) modes of mRNA association with the ER. These findings raise important questions regarding our understanding of how mRNAs are selected, localized, and anchored to the ER. Using semi-intact tissue culture cells, we performed a polysome solubilization screen and identified conditions that distinguish polysomes engaged in the translation of distinct cohorts of mRNAs. To gain insight into the molecular basis of direct mRNA anchoring to the ER, we performed RNA-protein UV photocross-linking studies in rough microsomes and demonstrate that numerous ER integral membrane proteins display RNA binding activity. Quantitative proteomic analyses of HeLa cytosolic and ER-bound polysome fractions identified translocon components as selective polysome-interacting proteins. Notably, the Sec61 complex was highly enriched in polysomes engaged in the translation of endomembrane organelle proteins, whereas translocon accessory proteins, such as ribophorin I, were present in all subpopulations of ER-associated polysomes. Analyses of the protein composition of oligo(dT)-selected UV photocross-linked ER protein-RNA adducts identified Sec61α,β and ribophorin I as ER-poly(A) mRNA-binding proteins, suggesting unexpected roles for the protein translocation and modification machinery in mRNA anchoring to the ER. In summary, we propose that multiple mechanisms of mRNA and ribosome association with ER operate to enable an mRNA transcriptome-wide function for the ER in protein synthesis.

Structure of the mammalian ribosome-Sec61 complex to 3.4 Å resolution

Cotranslational protein translocation is a universally conserved process for secretory and membrane protein biosynthesis. Nascent polypeptides emerging from a translating ribosome are either transported across or inserted into the membrane via the ribosome-bound Sec61 channel. Here, we report structures of a mammalian ribosome-Sec61 complex in both idle and translating states, determined to 3.4 and 3.9 Å resolution. The data sets permit building of a near-complete atomic model of the mammalian ribosome, visualization of A/P and P/E hybrid-state tRNAs, and analysis of a nascent polypeptide in the exit tunnel. Unprecedented chemical detail is observed for both the ribosome-Sec61 interaction and the conformational state of Sec61 upon ribosome binding. Comparison of the maps from idle and translating complexes suggests how conformational changes to the Sec61 channel could facilitate translocation of a secreted polypeptide. The high-resolution structure of the mammalian ribosome-Sec61 complex provides a valuable reference for future functional and structural studies.

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A near-complete atomic resolution structure of the mammalian ribosome

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Snapshot of a translating ribosome with hybrid state tRNAs and nascent polypeptide

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Structures of the Sec61 translocon bound to idle and translating ribosomes

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Molecular details of the residues involved in the ribosome-Sec61 interaction

Embracing the void--how much do we really know about targeting and translocation to the endoplasmic reticulum?

In order for a protein to enter the secretory pathway, two crucial steps must occur: it first needs to be targeted to the cytosolic surface of the endoplasmic reticulum (ER), and then be translocated across the ER membrane. Although for many years studies of targeting focused on the signal recognition particle, recent findings reveal that several alternative targeting pathways exist, some still undescribed, and some only recently elucidated. In addition, many genes implicated in the translocation step have not been assigned a specific function. Here, we will focus on the open questions regarding ER targeting and translocation, and discuss how combining classical biochemistry with systematic approaches can promote our understanding of these essential cellular steps.

On the expansion of ribosomal proteins and RNAs in eukaryotes

While the ribosome constitution is similar in all biota, there is a considerable increase in size of both ribosomal proteins (RPs) and RNAs in eukaryotes as compared to archaea and bacteria. This is pronounced in the large (60S) ribosomal subunit (LSU). In addition to enlargement (apparently maximized already in lower eukarya), the RP changes include increases in fraction, segregation and clustering of basic residues, and decrease in hydrophobicity. The acidic fraction is lower in eukaryote as compared to prokaryote RPs. In all eukaryote groups tested, the LSU RPs have significantly higher content of basic residues and homobasic segments than the SSU RPs. The vertebrate LSU RPs have much higher sequestration of basic residues than those of bacteria, archaea and even of the lower eukarya. The basic clusters are highly aligned in the vertebrate, but less in the lower eukarya, and only within families in archaea and bacteria. Increase in the basicity of RPs, besides helping transport to the nucleus, should promote stability of the assembled ribosome as well as the association with translocons and other intracellular matrix proteins. The size and GC nucleotide bias of the expansion segments of large LSU rRNAs also culminate in the vertebrate, and should support ribosome association with the endoplasmic reticulum and other intracellular networks. However, the expansion and nucleotide bias of eukaryote LSU rRNAs do not clearly correlate with changes in ionic parameters of LSU ribosomal proteins.

Co-translational mechanisms of quality control of newly synthesized polypeptides

During protein synthesis, nascent polypeptides emerge from ribosomes to fold into functional proteins. Misfolding of newly synthesized polypeptides (NSPs) at this stage leads to their aggregation. These misfolded NSPs must be expediently cleared to circumvent the deleterious effects of protein aggregation on cell physiology. To this end, a sizable portion of NSPs are ubiquitinated and rapidly degraded by the proteasome. This suggests the existence of co-translational mechanisms that play a pivotal role in the quality control of NSPs. It is generally thought that ribosomes play a central role in this process. During mRNA translation, ribosomes sense errors that lead to the accumulation of aberrant polypeptides, and serve as a hub for protein complexes that are required for optimal folding and/or proteasome-dependent degradation of misfolded polypeptides. In this review, we discuss recent findings that shed light on the molecular underpinnings of the co-translational quality control of NSPs.