April 21, 2020  |  

Short translational ramp determines efficiency of protein synthesis

It is generally assumed that translation efficiency is governed by translation initiation. However, the efficiency of protein synthesis is regulated by multiple factors including tRNA abundance, codon composition, mRNA motifs and amino-acid sequence1textendash4. These factors influence the rate of protein synthesis beyond the initiation phase of translation, typically by modulating the rate of peptide-bond formation and to a lesser extent that of translocation. The slowdown in translation during the early elongation phase, known as the 5textquoteright translational ramp, likely contributes to the efficiency of protein synthesis 5textendash9. Multiple mechanisms, which could explain the molecular basis for this translational ramp, have been proposed that include tRNA abundance bias6,9, the rate of translation initiation10textendash15, mRNA and ribosome structure 11,12,14,16textendash18, or retention of initiation factors during early elongation events 19. Here, we show that the amount of synthesized protein (translation efficiency) depends on a short translational ramp that comprises the first 5 codons in mRNA. Using a library of more than 250,000 reporter sequences combined with in vitro and in vivo protein expression assays, we show that differences in the short ramp can lead to 3 to 4 orders of magnitude changes in protein abundance. The observed difference is not dependent on tRNA abundance, efficiency of translation initiation, or overall mRNA structure. Instead, we show that translation is regulated by amino-acid-sequence composition and local mRNA sequence. Single-molecule measurements of translation kinetics indicate substantial pausing of ribosome and abortion of protein synthesis on the 4th or 5th codon for distinct amino acid or nucleotide compositions. Introduction of preferred sequence motifs, only at the exact positions within the mRNA, improves protein synthesis for recombinant proteins, indicating an evolutionarily conserved mechanism for controlling translational efficiency.


October 23, 2019  |  

Real-time observation of flexible domain movements in CRISPR-Cas9.

The CRISPR-associated protein Cas9 is widely used for genome editing because it cleaves target DNA through the assistance of a single-guide RNA (sgRNA). Structural studies have revealed the multi-domain architecture of Cas9 and suggested sequential domain movements of Cas9 upon binding to the sgRNA and the target DNA These studies also hinted at the flexibility between domains; however, it remains unclear whether these flexible movements occur in solution. Here, we directly observed dynamic fluctuations of multiple Cas9 domains, using single-molecule FRET We found that the flexible domain movements allow Cas9 to adopt transient conformations beyond those captured in the crystal structures. Importantly, the HNH nuclease domain only accessed the DNA cleavage position during such flexible movements, suggesting the importance of this flexibility in the DNA cleavage process. Our FRET data also revealed the conformational flexibility of apo-Cas9, which may play a role in the assembly with the sgRNA Collectively, our results highlight the potential role of domain fluctuations in driving Cas9-catalyzed DNA cleavage.© 2018 The Authors. Published under the terms of the CC BY NC ND 4.0 license.


September 22, 2019  |  

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.© The Author(s) 2017. Published by Oxford University Press on behalf of Nucleic Acids Research.


September 22, 2019  |  

2′-O-methylation in mRNA disrupts tRNA decoding during translation elongation.

Chemical modifications of mRNA may regulate many aspects of mRNA processing and protein synthesis. Recently, 2′-O-methylation of nucleotides was identified as a frequent modification in translated regions of human mRNA, showing enrichment in codons for certain amino acids. Here, using single-molecule, bulk kinetics and structural methods, we show that 2′-O-methylation within coding regions of mRNA disrupts key steps in codon reading during cognate tRNA selection. Our results suggest that 2′-O-methylation sterically perturbs interactions of ribosomal-monitoring bases (G530, A1492 and A1493) with cognate codon-anticodon helices, thereby inhibiting downstream GTP hydrolysis by elongation factor Tu (EF-Tu) and A-site tRNA accommodation, leading to excessive rejection of cognate aminoacylated tRNAs in initial selection and proofreading. Our current and prior findings highlight how chemical modifications of mRNA tune the dynamics of protein synthesis at different steps of translation elongation.


September 22, 2019  |  

Dimer arrangement and monomer flattening determine actin filament formation

Actin filament dynamics underlie key cellular processes, such as cell motility. Although actin filament elongation has been extensively studied under the past decades, the mechanism of filament nucleation remains unclear. Here, we immobilized gelsolin, a pointed-end nucleator, at the bottom of zero-mode waveguides to directly monitor the early steps of filament assembly. Our data revealed extensive dynamics and that only one, of two populations, elongates. Annalysis of the kinetics revealed a more stable trimer but a less stable tetramer in the elongating population compared to the non-elongating one. Furthermore, blocking flattening, the conformational change associated with filament formation, prevented the formation of both types of assemblies. Thus, flattening and the initial monomer arrangement determine gelsolin-mediated filament initiation.


July 7, 2019  |  

Cotranslational protein folding inside the ribosome exit tunnel.

At what point during translation do proteins fold? It is well established that proteins can fold cotranslationally outside the ribosome exit tunnel, whereas studies of folding inside the exit tunnel have so far detected only the formation of helical secondary structure and collapsed or partially structured folding intermediates. Here, using a combination of cotranslational nascent chain force measurements, inter-subunit fluorescence resonance energy transfer studies on single translating ribosomes, molecular dynamics simulations, and cryoelectron microscopy, we show that a small zinc-finger domain protein can fold deep inside the vestibule of the ribosome exit tunnel. Thus, for small protein domains, the ribosome itself can provide the kind of sheltered folding environment that chaperones provide for larger proteins. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.


July 7, 2019  |  

Coupling of mRNA structure rearrangement to ribosome movement during bypassing of non-coding regions.

Nearly half of the ribosomes translating a particular bacteriophage T4 mRNA bypass a region of 50 nt, resuming translation 3′ of this gap. How this large-scale, specific hop occurs and what determines whether a ribosome bypasses remain unclear. We apply single-molecule fluorescence with zero-mode waveguides to track individual Escherichia coli ribosomes during translation of T4’s gene 60 mRNA. Ribosomes that bypass are characterized by a 10- to 20-fold longer pause in a non-canonical rotated state at the take-off codon. During the pause, mRNA secondary structure rearrangements are coupled to ribosome forward movement, facilitated by nascent peptide interactions that disengage the ribosome anticodon-codon interactions for slippage. Close to the landing site, the ribosome then scans mRNA in search of optimal base-pairing interactions. Our results provide a mechanistic and conformational framework for bypassing, highlighting a non-canonical ribosomal state to allow for mRNA structure refolding to drive large-scale ribosome movements. Copyright © 2015 Elsevier Inc. All rights reserved.


July 7, 2019  |  

Synergistic effect of ATP for RuvA-RuvB-Holliday junction DNA complex formation.

The Escherichia coli RuvB hexameric ring motor proteins, together with RuvAs, promote branch migration of Holliday junction DNA. Zero mode waveguides (ZMWs) constitute of nanosized holes and enable the visualization of a single fluorescent molecule under micromolar order of the molecules, which is applicable to characterize the formation of RuvA-RuvB-Holliday junction DNA complex. In this study, we used ZMWs and counted the number of RuvBs binding to RuvA-Holliday junction DNA complex. Our data demonstrated that different nucleotide analogs increased the amount of Cy5-RuvBs binding to RuvA-Holliday junction DNA complex in the following order: no nucleotide, ADP, ATP?S, and mixture of ADP and ATP?S. These results suggest that not only ATP binding to RuvB but also ATP hydrolysis by RuvB facilitates a stable RuvA-RuvB-Holliday junction DNA complex formation.


July 7, 2019  |  

Dynamic pathways of -1 translational frameshifting.

Spontaneous changes in the reading frame of translation are rare (frequency of 10(-3) to 10(-4) per codon), but can be induced by specific features in the messenger RNA (mRNA). In the presence of mRNA secondary structures, a heptanucleotide ‘slippery sequence’ usually defined by the motif X XXY YYZ, and (in some prokaryotic cases) mRNA sequences that base pair with the 3′ end of the 16S ribosomal rRNA (internal Shine-Dalgarno sequences), there is an increased probability that a specific programmed change of frame occurs, wherein the ribosome shifts one nucleotide backwards into an overlapping reading frame (-1 frame) and continues by translating a new sequence of amino acids. Despite extensive biochemical and genetic studies, there is no clear mechanistic description for frameshifting. Here we apply single-molecule fluorescence to track the compositional and conformational dynamics of individual ribosomes at each codon during translation of a frameshift-inducing mRNA from the dnaX gene in Escherichia coli. Ribosomes that frameshift into the -1 frame are characterized by a tenfold longer pause in elongation compared to non-frameshifted ribosomes, which translate through unperturbed. During the pause, interactions of the ribosome with the mRNA stimulatory elements uncouple EF-G catalysed translocation from normal ribosomal subunit reverse-rotation, leaving the ribosome in a non-canonical intersubunit rotated state with an exposed codon in the aminoacyl-tRNA site (A site). tRNA(Lys) sampling and accommodation to the empty A site and EF-G action either leads to the slippage of the tRNAs into the -1 frame or maintains the ribosome into the 0 frame. Our results provide a general mechanistic and conformational framework for -1 frameshifting, highlighting multiple kinetic branchpoints during elongation.


July 7, 2019  |  

The dynamics of SecM-induced translational stalling.

SecM is an E. coli secretion monitor capable of stalling translation on the prokaryotic ribosome without cofactors. Biochemical and structural studies have demonstrated that the SecM nascent chain interacts with the 50S subunit exit tunnel to inhibit peptide bond formation. However, the timescales and pathways of stalling on an mRNA remain undefined. To provide a dynamic mechanism for stalling, we directly tracked the dynamics of elongation on ribosomes translating the SecM stall sequence (FSTPVWISQAQGIRAGP) using single-molecule fluorescence techniques. Within 1 min, three peptide-ribosome interactions work cooperatively over the last five codons of the SecM sequence, leading to severely impaired elongation rates beginning from the terminal proline and lasting four codons. Our results suggest that stalling is tightly linked to the dynamics of elongation and underscore the roles that the exit tunnel and nascent chain play in controlling fundamental steps in translation. opyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.


July 7, 2019  |  

High-throughput platform for real-time monitoring of biological processes by multicolor single-molecule fluorescence.

Zero-mode waveguides provide a powerful technology for studying single-molecule real-time dynamics of biological systems at physiological ligand concentrations. We customized a commercial zero-mode waveguide-based DNA sequencer for use as a versatile instrument for single-molecule fluorescence detection and showed that the system provides long fluorophore lifetimes with good signal to noise and low spectral cross-talk. We then used a ribosomal translation assay to show real-time fluidic delivery during data acquisition, showing it is possible to follow the conformation and composition of thousands of single biomolecules simultaneously through four spectral channels. This instrument allows high-throughput multiplexed dynamics of single-molecule biological processes over long timescales. The instrumentation presented here has broad applications to single-molecule studies of biological systems and is easily accessible to the biophysical community.


July 7, 2019  |  

Sequence-dependent elongation dynamics on macrolide-bound ribosomes.

The traditional view of macrolide antibiotics as plugs inside the ribosomal nascent peptide exit tunnel (NPET) has lately been challenged in favor of a more complex, heterogeneous mechanism, where drug-peptide interactions determine the fate of a translating ribosome. To investigate these highly dynamic processes, we applied single-molecule tracking of elongating ribosomes during inhibition of elongation by erythromycin of several nascent chains, including ErmCL and H-NS, which were shown to be, respectively, sensitive and resistant to erythromycin. Peptide sequence-specific changes were observed in translation elongation dynamics in the presence of a macrolide-obstructed NPET. Elongation rates were not severely inhibited in general by the presence of the drug; instead, stalls or pauses were observed as abrupt events. The dynamic pathways of nascent-chain-dependent elongation pausing in the presence of macrolides determine the fate of the translating ribosome stalling or readthrough. Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.


July 7, 2019  |  

Enhancing single-molecule fluorescence with nanophotonics.

Single-molecule fluorescence spectroscopy has become an important research tool in the life sciences but a number of limitations hinder the widespread use as a standard technique. The limited dynamic concentration range is one of the major hurdles. Recent developments in the nanophotonic field promise to alleviate these restrictions to an extent that even low affinity biomolecular interactions can be studied. After motivating the need for nanophotonics we introduce the basic concepts of nanophotonic devices such as zero mode waveguides and nanoantennas. We highlight current applications and the future potential of nanophotonic approaches when combined with biological systems and single-molecule spectroscopy. Copyright © 2014 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.


July 7, 2019  |  

The impact of aminoglycosides on the dynamics of translation elongation.

Inferring antibiotic mechanisms on translation through static structures has been challenging, as biological systems are highly dynamic. Dynamic single-molecule methods are also limited to few simultaneously measurable parameters. We have circumvented these limitations with a multifaceted approach to investigate three structurally distinct aminoglycosides that bind to the aminoacyl-transfer RNA site (A site) in the prokaryotic 30S ribosomal subunit: apramycin, paromomycin, and gentamicin. Using several single-molecule fluorescence measurements combined with structural and biochemical techniques, we observed distinct changes to translational dynamics for each aminoglycoside. While all three drugs effectively inhibit translation elongation, their actions are structurally and mechanistically distinct. Apramycin does not displace A1492 and A1493 at the decoding center, as demonstrated by a solution nuclear magnetic resonance structure, causing only limited miscoding; instead, it primarily blocks translocation. Paromomycin and gentamicin, which displace A1492 and A1493, cause significant miscoding, block intersubunit rotation, and inhibit translocation. Our results show the power of combined dynamics, structural, and biochemical approaches to elucidate the complex mechanisms underlying translation and its inhibition. Copyright © 2013 The Authors. Published by Elsevier Inc. All rights reserved.


July 7, 2019  |  

Coordinated conformational and compositional dynamics drive ribosome translocation.

During translation elongation, the ribosome compositional factors elongation factor G (EF-G; encoded by fusA) and tRNA alternately bind to the ribosome to direct protein synthesis and regulate the conformation of the ribosome. Here, we use single-molecule fluorescence with zero-mode waveguides to directly correlate ribosome conformation and composition during multiple rounds of elongation at high factor concentrations in Escherichia coli. Our results show that EF-G bound to GTP (EF-G-GTP) continuously samples both rotational states of the ribosome, binding with higher affinity to the rotated state. Upon successful accommodation into the rotated ribosome, the EF-G-ribosome complex evolves through several rate-limiting conformational changes and the hydrolysis of GTP, which results in a transition back to the nonrotated state and in turn drives translocation and facilitates release of both EF-G-GDP and E-site tRNA. These experiments highlight the power of tracking single-molecule conformation and composition simultaneously in real time.


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