eIF5B and eIF1A reorient initiator tRNA to allow ribosomal subunit joining

Translation initiation defines the identity and quantity of a synthesized protein. The process is dysregulated in many human diseases^1,2. A key commitment step is when the ribosomal subunits join at a translation start site on a messenger RNA to form a functional ribosome. Here, we combined single-molecule spectroscopy and structural methods using an in vitro reconstituted system to examine how the human ribosomal subunits join. Single-molecule fluorescence revealed when the universally conserved eukaryotic initiation factors eIF1A and eIF5B associate with and depart from initiation complexes. Guided by single-molecule dynamics, we visualized initiation complexes that contained both eIF1A and eIF5B using single-particle cryo-electron microscopy. The resulting structure revealed how eukaryote-specific contacts between the two proteins remodel the initiation complex to orient the initiator aminoacyl-tRNA in a conformation compatible with ribosomal subunit joining. Collectively, our findings provide a quantitative and architectural framework for the molecular choreography orchestrated by eIF1A and eIF5B during translation initiation in humans.

Dynamic competition between SARS-CoV-2 NSP1 and mRNA on the human ribosome inhibits translation initiation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a beta-CoV that recently emerged as a human pathogen and is the causative agent of the COVID-19 pandemic. A molecular framework of how the virus manipulates host cellular machinery to facilitate infection remains unclear. Here, we focus on SARS-CoV-2 NSP1, which is proposed to be a virulence factor that inhibits protein synthesis by directly binding the human ribosome. We demonstrate biochemically that NSP1 inhibits translation of model human and SARS-CoV-2 messenger RNAs (mRNAs). NSP1 specifically binds to the small (40S) ribosomal subunit, which is required for translation inhibition. Using single-molecule fluorescence assays to monitor NSP1-40S subunit binding in real time, we determine that eukaryotic translation initiation factors (eIFs) allosterically modulate the interaction of NSP1 with ribosomal preinitiation complexes in the absence of mRNA. We further elucidate that NSP1 competes with RNA segments downstream of the start codon to bind the 40S subunit and that the protein is unable to associate rapidly with 80S ribosomes assembled on an mRNA. Collectively, our findings support a model where NSP1 proteins from viruses in at least two subgenera of beta-CoVs associate with the open head conformation of the 40S subunit to inhibit an early step of translation, by preventing accommodation of mRNA within the entry channel.

Dynamic competition between SARS-CoV-2 NSP1 and mRNA on the human ribosome inhibits translation initiation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a beta-CoV that recently emerged as a human pathogen and is the causative agent of the COVID-19 pandemic. A molecular framework of how the virus manipulates host cellular machinery to facilitate infection remains unclear. Here, we focus on SARS-CoV-2 NSP1, which is proposed to be a virulence factor that inhibits protein synthesis by directly binding the human ribosome. We demonstrate biochemically that NSP1 inhibits translation of model human and SARS-CoV-2 messenger RNAs (mRNAs). NSP1 specifically binds to the small (40S) ribosomal subunit, which is required for translation inhibition. Using single-molecule fluorescence assays to monitor NSP1-40S subunit binding in real time, we determine that eukaryotic translation initiation factors (eIFs) allosterically modulate the interaction of NSP1 with ribosomal preinitiation complexes in the absence of mRNA. We further elucidate that NSP1 competes with RNA segments downstream of the start codon to bind the 40S subunit and that the protein is unable to associate rapidly with 80S ribosomes assembled on an mRNA. Collectively, our findings support a model where NSP1 proteins from viruses in at least two subgenera of beta-CoVs associate with the open head conformation of the 40S subunit to inhibit an early step of translation, by preventing accommodation of mRNA within the entry channel.

RPS25 is required for efficient RAN translation of C9orf72 and other neurodegenerative disease-associated nucleotide repeats

Nucleotide repeat expansions in the C9orf72 gene are the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia. Unconventional translation (RAN translation) of C9orf72 repeats generates dipeptide repeat proteins that can cause neurodegeneration. We performed a genetic screen for regulators of RAN translation and identified small ribosomal protein subunit 25 (RPS25), presenting a potential therapeutic target for C9orf72-related amyotrophic lateral sclerosis and frontotemporal dementia and other neurodegenerative diseases caused by nucleotide repeat expansions.

Fluorescently-tagged human eIF3 for single-molecule spectroscopy

Human translation initiation relies on the combined activities of numerous ribosome-associated eukaryotic initiation factors (eIFs). The largest factor, eIF3, is an ∼800 kDa multiprotein complex that orchestrates a network of interactions with the small 40S ribosomal subunit, other eIFs, and mRNA, while participating in nearly every step of initiation. How these interactions take place during the time course of translation initiation remains unclear. Here, we describe a method for the expression and affinity purification of a fluorescently-tagged eIF3 from human cells. The tagged eIF3 dodecamer is structurally intact, functions in cell-based assays, and interacts with the HCV IRES mRNA and the 40S-IRES complex in vitro. By tracking the binding of single eIF3 molecules to the HCV IRES RNA with a zero-mode waveguides-based instrument, we show that eIF3 samples both wild-type IRES and an IRES that lacks the eIF3-binding region, and that the high-affinity eIF3-IRES interaction is largely determined by slow dissociation kinetics. The application of single-molecule methods to more complex systems involving eIF3 may unveil dynamics underlying mRNA selection and ribosome loading during human translation initiation.

Expanding single-molecule fluorescence spectroscopy to capture complexity in biology

Fundamental biological processes are driven by diverse molecular machineries. In recent years, single-molecule fluorescence spectroscopy has matured as a unique tool in biology to study how structural dynamics of molecular complexes drive various biochemical reactions. In this review, we highlight underlying developments in single-molecule fluorescence methods that enable deep biological investigations. Recent progress in these methods points toward increasing complexity of measurements to capture biological processes in a living cell, where multiple processes often occur simultaneously and are mechanistically coupled.

How Messenger RNA and Nascent Chain Sequences Regulate Translation Elongation

Translation elongation is a highly coordinated, multistep, multifactor process that ensures accurate and efficient addition of amino acids to a growing nascent-peptide chain encoded in the sequence of translated messenger RNA (mRNA). Although translation elongation is heavily regulated by external factors, there is clear evidence that mRNA and nascent-peptide sequences control elongation dynamics, determining both the sequence and structure of synthesized proteins. Advances in methods have driven experiments that revealed the basic mechanisms of elongation as well as the mechanisms of regulation by mRNA and nascent-peptide sequences. In this review, we highlight how mRNA and nascent-peptide elements manipulate the translation machinery to alter the dynamics and pathway of elongation.

A novel quantitative microarray antibody capture assay identifies an extremely high hepatitis delta virus prevalence among hepatitis B virus-infected mongolians

Hepatitis delta virus (HDV) causes the most severe form of human viral hepatitis. HDV requires a hepatitis B virus (HBV) coinfection to provide HDV with HBV surface antigen envelope proteins. The net effect of HDV is to make the underlying HBV disease worse, including higher rates of hepatocellular carcinoma. Accurate assessments of current HDV prevalence have been hampered by the lack of readily available and reliable quantitative assays, combined with the absence of a Food and Drug Administration-approved therapy. We sought to develop a convenient assay for accurately screening populations and to use this assay to determine HDV prevalence in a population with abnormally high rates of hepatocellular carcinoma. We developed a high-throughput quantitative microarray antibody capture assay for anti-HDV immunoglobulin G wherein recombinant HDV delta antigen is printed by microarray on slides coated with a noncontinuous, nanostructured plasmonic gold film, enabling quantitative fluorescent detection of anti-HDV antibody in small aliquots of patient serum. This assay was then used to screen all HBV-infected patients identified in a large randomly selected cohort designed to represent the Mongolian population. We identified two quantitative thresholds of captured antibody that were 100% predictive of the sample either being positive on standard western blot or harboring HDV RNA detectable by real-time quantitative PCR. Subsequent screening of the HBV+ cohort revealed that a remarkable 57% were RNA+ and an additional 4% were positive on western blot alone.

CONCLUSION

The quantitative microarray antibody capture assay's unique performance characteristics make it ideal for population screening; its application to the Mongolian HBV surface antigen-positive population reveals an apparent ∼60% prevalence of HDV coinfection among these HBV-infected Mongolian subjects, which may help explain the extraordinarily high rate of hepatocellular carcinoma in Mongolia. (Hepatology 2017;66:1739-1749).

Dynamics of IRES-mediated translation

Viral internal ribosome entry sites (IRESs) are unique RNA elements, which use stable and dynamic RNA structures to recruit ribosomes and drive protein synthesis. IRESs overcome the high complexity of the canonical eukaryotic translation initiation pathway, often functioning with a limited set of eukaryotic initiation factors. The simplest types of IRESs are typified by the cricket paralysis virus intergenic region (CrPV IGR) and hepatitis C virus (HCV) IRESs, both of which independently form high-affinity complexes with the small (40S) ribosomal subunit and bypass the molecular processes of cap-binding and scanning. Owing to their simplicity and ribosomal affinity, the CrPV and HCV IRES have been important models for structural and functional studies of the eukaryotic ribosome during initiation, serving as excellent targets for recent technological breakthroughs in cryogenic electron microscopy (cryo-EM) and single-molecule analysis. High-resolution structural models of ribosome : IRES complexes, coupled with dynamics studies, have clarified decades of biochemical research and provided an outline of the conformational and compositional trajectory of the ribosome during initiation. Here we review recent progress in the study of HCV- and CrPV-type IRESs, highlighting important structural and dynamics insights and the synergy between cryo-EM and single-molecule studies.This article is part of the themed issue 'Perspectives on the ribosome'.

Secondary structural analysis of the carboxyl-terminal domain from different connexin isoforms

The connexin carboxyl-terminal (CxCT) domain plays a role in the trafficking, localization, and turnover of gap junction channels, as well as the level of gap junction intercellular communication via numerous post-translational modifications and protein-protein interactions. As a key player in the regulation of gap junctions, the CT presents itself as a target for manipulation intended to modify function. Specific to intrinsically disordered proteins, identifying residues whose secondary structure can be manipulated will be critical toward unlocking the therapeutic potential of the CxCT domain. To accomplish this goal, we used biophysical methods to characterize CxCT domains attached to their fourth transmembrane domain (TM4). Circular dichroism and nuclear magnetic resonance were complementary in demonstrating the connexin isoforms that form the greatest amount of α-helical structure in their CT domain (Cx45 > Cx43 > Cx32 > Cx50 > Cx37 ≈ Cx40 ≈ Cx26). Studies compared the influence of 2,2,2-trifluoroethanol, pH, phosphorylation, and mutations (Cx32, X-linked Charcot-Marie Tooth disease; Cx26, hearing loss) on the TM4-CxCT structure. While pH modestly influences the CT structure, a major structural change was associated with phosphomimetic substitutions. Since most connexin CT domains are phosphorylated throughout their life cycle, studies of phospho-TM4-CxCT isoforms will be critical toward understanding the role that structure plays in regulating gap junction function.

Multiple Parallel Pathways of Translation Initiation on the CrPV IRES

The complexity of eukaryotic translation allows fine-tuned regulation of protein synthesis. Viruses use internal ribosome entry sites (IRESs) to minimize or, like the CrPV IRES, eliminate the need for initiation factors. Here, by exploiting the CrPV IRES, we observed the entire process of initiation and transition to elongation in real time. We directly tracked the CrPV IRES, 40S and 60S ribosomal subunits, and tRNA using single-molecule fluorescence spectroscopy and identified multiple parallel initiation pathways within the system. Our results distinguished two pathways of 80S:CrPV IRES complex assembly that produce elongation-competent complexes. Following 80S assembly, the requisite eEF2-mediated translocation results in an unstable intermediate that is captured by binding of the elongator tRNA. Whereas initiation can occur in the 0 and +1 frames, the arrival of the first tRNA defines the reading frame and strongly favors 0 frame initiation. Overall, even in the simplest system, an intricate reaction network regulates translation initiation.

Structural Studies of the Nedd4 WW Domains and Their Selectivity for the Connexin43 (Cx43) Carboxyl Terminus

Neuronal precursor cell-expressed developmentally down-regulated 4 (Nedd4) was the first ubiquitin protein ligase identified to interact with connexin43 (Cx43), and its suppressed expression results in accumulation of gap junction plaques at the plasma membrane. Nedd4-mediated ubiquitination of Cx43 is required to recruit Eps15 and target Cx43 to the endocytic pathway. Although the Cx43 residues that undergo ubiquitination are still unknown, in this study we address other unresolved questions pertaining to the molecular mechanisms mediating the direct interaction between Nedd4 (WW1-3 domains) and Cx43 (carboxyl terminus (CT)). All three WW domains display a similar three antiparallel β-strand structure and interact with the same Cx43CT(283)PPXY(286)sequence. Although Tyr(286)is essential for the interaction, MAPK phosphorylation of the preceding serine residues (Ser(P)(279)and Ser(P)(282)) increases the binding affinity by 2-fold for the WW domains (WW2 > WW3 ≫ WW1). The structure of the WW2·Cx43CT(276-289)(Ser(P)(279), Ser(P)(282)) complex reveals that coordination of Ser(P)(282)with the end of β-strand 3 enables Ser(P)(279)to interact with the back face of β-strand 3 (Tyr(286)is on the front face) and loop 2, forming a horseshoe-shaped arrangement. The close sequence identity of WW2 with WW1 and WW3 residues that interact with the Cx43CT PPXY motif and Ser(P)(279)/Ser(P)(282)strongly suggests that the significantly lower binding affinity of WW1 is the result of a more rigid structure. This study presents the first structure illustrating how phosphorylation of the Cx43CT domain helps mediate the interaction with a molecular partner involved in gap junction regulation.

¹H, ¹³C, and ¹⁵N backbone resonance assignments of the connexin43 carboxyl terminal domain attached to the 4th transmembrane domain in detergent micelles

Gap junctions are specialized membrane channels that enable coordination of cellular functions and whole-organ responses by facilitating both molecular and electrical communication between neighboring cells. Connexin43 (Cx43) is the most widely expressed and well-studied gap junction protein. In the heart, Cx43 is essential for normal cardiac development and function. Studies using a soluble version of the Cx43 carboxyl-terminal domain (Cx43CT; S255-I382) have established the central role it plays in channel regulation. However, in purifying and characterizing a more 'native-like' construct (Cx43CT attached to the fourth transmembrane domain (TM4-Cx43CT; D196-I382)), we have identified that the TM4-Cx43CT is a better model than the soluble Cx43CT to further investigate the mechanisms governing Cx43 channel regulation. Here, we report the backbone (1)H, (15)N, and (13)C assignments and predicted secondary structure of the TM4-Cx43CT. Assignment of the TM4-Cx43CT is a key step towards a better understanding of the structural basis of Cx43 regulation, which will lead to improved strategies for modulation of junctional communication that has been altered due to disease or ischemic injury.

An Escherichia coli strain for expression of the connexin45 carboxyl terminus attached to the 4th transmembrane domain

A major problem for structural characterization of membrane proteins, such as connexins, by nuclear magnetic resonance (NMR) occurs at the initial step of the process, the production of sufficient amounts of protein. This occurs because proteins must be expressed in minimal based media. Here, we describe an expression system for membrane proteins that significantly improves yield by addressing two common problems, cell toxicity caused by protein translation and codon bias between genomes. This work provides researchers with a cost-effective tool for NMR and other biophysical studies, to use when faced with little-to-no expression of eukaryotic membrane proteins in Escherichia coli expression systems.

Effects of phosphorylation on the structure and backbone dynamics of the intrinsically disordered connexin43 C-terminal domain

Phosphorylation of the connexin43 C-terminal (Cx43CT) domain regulates gap junction intercellular communication. However, an understanding of the mechanisms by which phosphorylation exerts its effects is lacking. Here, we test the hypothesis that phosphorylation regulates Cx43 gap junction intercellular communication by mediating structural changes in the C-terminal domain. Circular dichroism and nuclear magnetic resonance were used to characterize the effects of phosphorylation on the secondary structure and backbone dynamics of soluble and membrane-tethered Cx43CT domains. Cx43CT phospho-mimetic isoforms, which have Asp substitutions at specific Ser/Tyr sites, revealed phosphorylation alters the α-helical content of the Cx43CT domain only when attached to the membrane. The changes in secondary structure are due to variations in the conformational preference and backbone flexibility of residues adjacent and distal to the site(s) of modification. In addition to the known direct effects of phosphorylation on molecular partner interactions, the data presented here suggest phosphorylation may also indirectly regulate binding affinity by altering the conformational preference of the Cx43CT domain.

A history of gap junction structure: hexagonal arrays to atomic resolution

Gap junctions are specialized membrane structures that provide an intercellular pathway for the propagation and/or amplification of signaling cascades responsible for impulse propagation, cell growth, and development. Prior to the identification of the proteins that comprise gap junctions, elucidation of channel structure began with initial observations of a hexagonal nexus connecting apposed cellular membranes. Concomitant with technological advancements spanning over 50 years, atomic resolution structures are now available detailing channel architecture and the cytoplasmic domains that have helped to define mechanisms governing the regulation of gap junctions. Highlighted in this review are the seminal structural studies that have led to our current understanding of gap junction biology.

Optimizing the solution conditions to solve the structure of the Connexin43 carboxyl terminus attached to the 4(th) transmembrane domain in detergent micelles

pH-mediated gating of Cx43 channels following an ischemic event is believed to contribute to the development of lethal cardiac arrhythmias. Studies using a soluble version of the Cx43 carboxyl-terminal domain (Cx43CT; S255-I382) have established the central role it plays in channel regulation; however, research in the authors' laboratory suggests that this construct may not be the ideal model system. Therefore, we have developed a more 'native-like' construct (Cx43CT attached to the 4th transmembrane domain [TM4-Cx43CT; G178-I382]) than the soluble Cx43CT to further investigate the mechanism(s) governing this regulation. Here, we utilize circular dichroism and nuclear magnetic resonance (NMR) were used to validate the TM4-Cx43CT for studying channel gating and optimize solution conditions for structural studies. The data indicate that, unlike the soluble Cx43CT, the TM4-Cx43CT is structurally responsive to changes in pH, suggesting the presence of the TM4 facilitates pH-induced structural alterations. Additionally, the optimal solution conditions for solving the NMR solution structure include 10% 2,2,2 trifluoroethanol and removal of the 2nd extracellular loop (G178-V196).

Purification and reconstitution of the connexin43 carboxyl terminus attached to the 4th transmembrane domain in detergent micelles

In recent years, reports have identified that many eukaryotic proteins contain disordered regions spanning greater than 30 consecutive residues in length. In particular, a number of these intrinsically disordered regions occur in the cytoplasmic segments of plasma membrane proteins. These intrinsically disordered regions play important roles in cell signaling events, as they are sites for protein-protein interactions and phosphorylation. Unfortunately, in many crystallographic studies of membrane proteins, these domains are removed because they hinder the crystallization process. Therefore, a purification procedure was developed to enable the biophysical and structural characterization of these intrinsically disordered regions while still associated with the lipid environment. The carboxyl terminal domain from the gap junction protein connexin43 attached to the 4th transmembrane domain (TM4-Cx43CT) was used as a model system (residues G178-I382). The purification was optimized for structural analysis by nuclear magnetic resonance (NMR) because this method is well suited for small membrane proteins and proteins that lack a well-structured three-dimensional fold. The TM4-Cx43CT was purified to homogeneity with a yield of approximately 6 mg/L from C41(DE3) bacterial cells, reconstituted in the anionic detergent 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-RAC-(1-glycerol)], and analyzed by circular dichroism and NMR to demonstrate that the TM4-Cx43CT was properly folded into a functional conformation by its ability to form alpha-helical structure and associate with a known binding partner, the c-Src SH3 domain, respectively.