Biology subjects

Eng, E. T.

Publications and source records attributed to Eng, E. T..

At least 19 recordsLinked to original sources

bioRxiv2025

Structures reveal how the Cas1-2/3 integrase captures, delivers, and integrates foreign DNA into CRISPR loci

Cas1 and Cas2 are the hallmark proteins of prokaryotic adaptive immunity. However, these two proteins are often fused to other proteins and the functional association of these fusions often remain poorly understood. Here we purify Cas1 and the Cas2/3 fusion protein from Pseudomonas aeruginosa. We determine multiple structures of the Cas1-2/3 complex at distinct stages of CRISPR adaptation. Collectively, these structures reveal a prominent, positively charged channel on one face of the integration complex that captures short fragments of foreign DNA. Foreign DNA binding triggers conformational changes in Cas2/3 that expose new DNA binding surfaces necessary for homing the DNA-bound integrase to specific CRISPR loci. The length of the foreign DNA substrate determines if Cas1-2/3 docks completely onto the CRISPR repeat to successfully catalyze two sequential transesterification reactions required for integration. Taken together, these structures clarify how the Cas1-2/3 proteins orchestrate foreign DNA capture, site-specific delivery, and integration of new DNA into the bacterial genome. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=200 SRC="FIGDIR/small/658980v1_ufig1.gif" ALT="Figure 1"> View larger version (53K): org.highwire.dtl.DTLVardef@11ad39borg.highwire.dtl.DTLVardef@3a8474org.highwire.dtl.DTLVardef@1cdf25borg.highwire.dtl.DTLVardef@1e24ff5_HPS_FORMAT_FIGEXP M_FIG C_FIG HIGHLIGHTS- A positively charged channel on the Cas1-2/3 complex captures fragments of DNA - A loop in the RecA1 domain controls access to the Cas3 nuclease active site - Foreign DNA binding allosterically regulates access to additional DNA binding sites - Distortion of the CRISPR repeat sequence licenses complete foreign DNA integration

molecular biology↗

bioRxiv2024

Multi-species cryoEM calibration and workflow verification standard

Cryogenic electron microscopy (cryoEM) is a rapidly growing structural biology modality that has been successful in revealing molecular details of biological systems. However, unlike established biophysical and analytical techniques with calibration standards, cryoEM has lacked comprehensive biological test samples. We introduce a cryoEM calibration sample that is a mixture of compatible macromolecules that can be used not only for resolution optimization but also provides multiple reference points for evaluating instrument performance, data quality, and image processing workflows in a single experiment. This combined test specimen provides researchers a reference point for validating their cryoEM pipeline, benchmarking their methodologies, and testing new algorithms.

biochemistry↗

bioRxiv2024

Early intermediates in bacterial RNA polymerase promoter melting visualized by time-resolved cryo-electron microscopy

During formation of the transcription-competent open complex (RPo) by bacterial RNA polymerases (RNAP), transient intermediates pile up before overcoming a rate-limiting step. Structural descriptions of these interconversions in real time are unavailable. To address this gap, time-resolved cryo-electron microscopy (cryo-EM) was used to capture four intermediates populated 120 or 500 milliseconds (ms) after mixing Escherichia coli {sigma}70-RNAP and the PR promoter. Cryo-EM snapshots revealed the upstream edge of the transcription bubble unpairs rapidly, followed by stepwise insertion of two conserved nontemplate strand (nt-strand) bases into RNAP pockets. As nt-strand "read-out" extends, the RNAP clamp closes, expelling an inhibitory {sigma}70 domain from the active-site cleft. The template strand is fully unpaired by 120 ms but remains dynamic, indicating yet unknown conformational changes load it in subsequent steps. Because these events likely describe DNA opening at many bacterial promoters, this study provides needed insights into how DNA sequence regulates steps of RPo formation.

biophysics↗

bioRxiv2024

CryoCycle your grids: Plunge vitrifying and reusing clipped grids to advance cryoEM democratization

CryoEM democratization is hampered by access to costly plunge-freezing supplies. We introduce methods, called CryoCycle, for reliably blotting, vitrifying, and reusing clipped cryoEM grids. We demonstrate that vitreous ice may be produced by plunging clipped grids with purified proteins into liquid ethane and that clipped grids may be reused several times for different protein samples. Furthermore, we demonstrate the vitrification of thin areas of cells prepared on gold-coated, pre-clipped grids.

molecular biology↗

bioRxiv2023

Smart Leginon enables cryoEM screening automation across multiple grids

SummaryCryoEM multi-grid screening is often a tedious process that demands hours of attention. Here, this protocol shows how to set up standard Leginon collection and Smart Leginon Autoscreen to automate this process. This protocol can be applied to the majority of cryoEM holey foil grids. Advancements in cryo-electron microscopy (cryoEM) techniques over the past decade have allowed structural biologists to routinely resolve macromolecular protein complexes to near-atomic resolution. The general workflow of the entire cryoEM pipeline involves iterating between sample preparation, cryoEM grid preparation, and sample/grid screening before moving on to high-resolution data collection. Iterating between sample/grid preparation and screening is typically a major bottleneck for researchers, as every iterative experiment must optimize for sample concentration, buffer conditions, grid material, grid hole size, ice thickness, and protein particle behavior in the ice, amongst other variables. Furthermore, once these variables are satisfactorily determined, grids prepared under identical conditions vary widely in whether they are ready for data collection, so additional screening sessions prior to selecting optimal grids for high-resolution data collection are recommended. This sample/grid preparation and screening process often consumes several dozen grids and days of operator time at the microscope. Furthermore, the screening process is limited to operator/microscope availability and microscope accessibility. Here, we demonstrate how to use Leginon and Smart Leginon Autoscreen to automate the majority of cryoEM grid screening. Autoscreen combines machine learning, computer vision algorithms, and microscope-handling algorithms to remove the need for constant manual operator input. Autoscreen can autonomously load and image grids with multi-scale imaging using an automated specimen-exchange cassette system, resulting in unattended grid screening for an entire cassette. As a result, operator time for screening 12 grids may be reduced to [~]10 minutes with Autoscreen compared to [~]6 hours using previous methods which are hampered by their inability to account for high variability between grids. This protocol and video tutorial first introduces basic Leginon setup and functionality, then demonstrates Autoscreen functionality step-by-step from the creation of a template session to the end of a 12 grid automated screening session.

molecular biology↗

bioRxiv2023

Protein-mediated genome folding allosterically enhances site-specific integration of foreign DNA into CRISPRs

Bacteria and archaea acquire resistance to viruses and plasmids by integrating fragments of foreign DNA into the first repeat of a CRISPR array. However, the mechanism of site-specific integration remains poorly understood. Here, we determine a 560 kDa integration complex structure that explains how Cas (Cas1-2/3) and non-Cas proteins (IHF) fold 150 base-pairs of host DNA into a U-shaped bend and a loop that protrude from Cas1-2/3 at right angles. The U-shaped bend traps foreign DNA on one face of the Cas1-2/3 integrase, while the loop places the first CRISPR repeat in the Cas1 active site. Both Cas3s rotate 100-degrees to expose DNA binding sites on either side of the Cas2 homodimer, that each bind an inverted repeat motif in the leader. Leader sequence motifs direct Cas1-2/3-mediated integration to diverse repeat sequences that have a 5-GT.

biochemistry↗

bioRxiv2022

Fully Automated Multi-Grid Cryo-EM Screening using Smart Leginon

Single particle cryo-electron microscopy (cryoEM) is a swiftly growing method for understanding protein structure. With increasing demand for high-throughput, high-resolution cryoEM services comes greater demand for rapid and automated cryoEM grid and sample screening. During screening, optimal grids and sample conditions are identified for subsequent high-resolution data collection. Screening is a major bottleneck for new cryoEM projects because grids must be optimized over several factors, including grid type, grid hole size, sample concentration, buffer conditions, ice thickness, and particle behaviors. Even for mature projects, multiple grids are commonly screened to select a subset for high-resolution data collection. Here, machine learning and novel, purpose-built image processing and microscope-handling algorithms are incorporated into the automated data collection software, Leginon, to provide an open-source solution for fully automated, high-throughput grid screening. This new version, broadly called Smart Leginon, emulates the actions of an operator in identifying areas on the grid to explore as potentially useful for data collection. Smart Leginon Autoscreen sequentially loads and examines grids from an automated specimen exchange system to provide completely unattended grid screening across a set of grids. Comparisons between a multi-grid Autoscreen session and conventional manual screening by five expert microscope operators are presented. On average, Autoscreen reduces operator time from [~]6 hours to <10 minutes and provides a comparable percentage of suitable images for evaluation as the best operator. Smart Leginons ability to target holes that are particularly difficult to identify is analyzed. Finally, Smart Leginons utility is illustrated with three real-world multi-grid user screening/collection sessions, demonstrating the efficiency and flexibility of the software package. Smart Leginons fully automated functionality significantly reduces the burden on operator screening time, improves the throughput of screening, and recovers idle microscope time, thereby improving availability of cryoEM services.

biophysics↗

bioRxiv2022

Structural basis for substrate selection by the SARS-CoV-2 replicase

The SARS-CoV-2 RNA-dependent RNA polymerase coordinates viral RNA synthesis as part of an assembly known as the replication-transcription complex (RTC)1. Accordingly, the RTC is a target for clinically approved antiviral nucleoside analogs, including remdesivir2. Faithful synthesis of viral RNAs by the RTC requires recognition of the correct nucleotide triphosphate (NTP) for incorporation into the nascent RNA. To be effective inhibitors, antiviral nucleoside analogs must compete with the natural NTPs for incorporation. How the SARS-CoV-2 RTC discriminates between the natural NTPs, and how antiviral nucleoside analogs compete, has not been discerned in detail. Here, we use cryo-electron microscopy to visualize the RTC bound to each of the natural NTPs in states poised for incorporation. Furthermore, we investigate the RTC with the active metabolite of remdesivir, remdesivir triphosphate (RDV-TP), highlighting the structural basis for the selective incorporation of RDV-TP over its natural counterpart ATP3,4. Our results elucidate the suite of interactions required for NTP recognition, informing the rational design of antivirals. Our analysis also yields insights into nucleotide recognition by the nsp12 NiRAN, an enigmatic catalytic domain essential for viral propagation5. The NiRAN selectively binds GTP, strengthening proposals for the role of this domain in the formation of the 5 RNA cap6.

biophysics↗

bioRxiv2021

Ensemble cryo-electron microscopy reveals conformational states of the nsp13 helicase in the SARS-CoV-2 helicase replication-transcription complex

The SARS-CoV-2 nonstructural proteins coordinate genome replication and gene expression. Structural analyses revealed the basis for coupling of the essential nsp13 helicase with the RNA dependent RNA polymerase (RdRp) where the holo-RdRp and RNA substrate (the replication-transcription complex, or RTC) associated with two copies of nsp13 (nsp132-RTC). One copy of nsp13 interacts with the template RNA in an opposing polarity to the RdRp and is envisaged to drive the RdRp backwards on the RNA template (backtracking), prompting questions as to how the RdRp can efficiently synthesize RNA in the presence of nsp13. Here, we use cryo-electron microscopy and molecular dynamics simulations to analyze the nsp132-RTC, revealing four distinct conformational states of the helicases. The results suggest a mechanism for the nsp132-RTC to turn backtracking on and off, using an allosteric mechanism to switch between RNA synthesis or backtracking in response to stimuli at the RdRp active site.

biophysics↗

bioRxiv2021

System Environmental Metrics Collector for EM facilities

Recent developments in cryo-electron microscopy (cryoEM) have led to the routine determination of structures at near atomic resolution and greatly increased the number of biomedical researchers wanting access to high-end cryoEM instrumentation. The high costs and long wait times for gaining access encourages facilities to maximize instrument uptime for data collection. To support these goals, we developed a System Environmental Metrics Collector for facilities (SEMCf) that serves as a laboratory performance and management tool. SEMCf consists of an architecture of automated and robust sensors that track, organize and report key facility metrics. The individual sensors are connected to Raspberry Pi (RPi) single board computers installed in close proximity to the input metrics being measured. The system is controlled by a central server that may be installed on a RPi or existing microscope support PC. Tracking the system and the environment provides feedback of imminent issues, suggestions for interventions that are needed to optimize data production, and indications as to when preventative maintenance should be scheduled. The sensor components are relatively inexpensive and widely commercially available, and the open-source design and software enables straightforward implementation, customization, and optimization by any facility that would benefit from real time environmental monitoring and reporting.

biophysics↗

bioRxiv2021

Structural origins of Escherichia coli RNA polymerase open promoter complex stability

The first step of gene expression in all organisms requires opening the DNA duplex to expose one strand for templated RNA synthesis. In Escherichia coli, promoter DNA sequence fundamentally determines how fast the RNA polymerase (RNAP) forms "open" complexes (RPo), whether RPo persists for seconds or hours, and how quickly RNAP transitions from initiation to elongation. These rates control promoter strength in vivo but their structural origins remain largely unknown. Here we use cryo-electron microscopy to determine structures of RPo formed de novo at three promoters with widely differing lifetimes at 37{degrees}C: {lambda}PR (t1/2 [~] 10 hours), T7A1 (t1/2 [~] 4 minutes), and a point mutant in {lambda}PR ({lambda}PR-5C) (t1/2 [~] 2 hours). Two distinct RPo conformers are populated at {lambda}PR, likely representing productive and unproductive forms of RPo observed in solution studies. We find that changes in the sequence and length of DNA in the transcription bubble just upstream of the start site (+1) globally alter the network of DNA-RNAP interactions, base stacking, and strand order in the single-stranded DNA of the transcription bubble; these differences propagate beyond the bubble to upstream and downstream DNA. After expanding the transcription bubble by one base (T7A1), the nontemplate-strand "scrunches" inside the active site cleft; the template-strand bulges outside the cleft at the upstream edge of the bubble. The structures illustrate how limited sequence changes trigger global alterations in the transcription bubble that modulate RPo lifetime and affect the subsequent steps of the transcription cycle.

biophysics↗

bioRxiv2021

Structural basis for backtracking by the SARS-CoV-2 replication-transcription complex

Backtracking, the reverse motion of the transcriptase enzyme on the nucleic acid template, is a universal regulatory feature of transcription in cellular organisms but its role in viruses is not established. Here we present evidence that backtracking extends into the viral realm, where backtracking by the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) may aid viral transcription and replication. Structures of SARS-CoV-2 RdRp bound to the essential nsp13 helicase and RNA suggested the helicase facilitates backtracking. We use cryo-electron microscopy, RNA-protein crosslinking, and unbiased molecular dynamics simulations to characterize SARS-CoV-2 RdRp backtracking. The results establish that the single-stranded 3-segment of the product-RNA generated by backtracking extrudes through the RdRp NTP-entry tunnel, that a mismatched nucleotide at the product-RNA 3-end frays and enters the NTP-entry tunnel to initiate backtracking, and that nsp13 stimulates RdRp backtracking. Backtracking may aid proofreading, a crucial process for SARS-CoV-2 resistance against antivirals. Significance StatementThe COVID-19 pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The SARS-CoV-2 genome is replicated and transcribed by its RNA-dependent RNA polymerase (RdRp), which is the target for antivirals such as remdesivir. We use a combination of approaches to show that backtracking (backwards motion of the RdRp on the template RNA) is a feature of SARS-CoV-2 replication/transcription. Backtracking may play a critical role in proofreading, a crucial process for SARS-CoV-2 resistance against many antivirals.

biochemistry↗

bioRxiv2020

Waffle method: A general and flexible approach for FIB-milling small and anisotropically oriented samples

Cryo-FIB/SEM combined with cryo-ET has emerged from within the field of cryo-EM as the method for obtaining the highest resolution structural information of complex biological samples in-situ in native and non-native environments. However, challenges remain in conventional cryo-FIB/SEM workflows, including milling thick specimens that do not vitrify well, specimens with preferred orientation, low-throughput when milling small and/or low concentration specimens, and cellular specimens that distribute poorly across grid squares. Here we present a general approach we call the Waffle Method which leverages high-pressure freezing to address these challenges. We illustrate the mitigation of these challenges by applying the Waffle Method and cryo-ET to reveal the macrostructure of the polar tube in microsporidian spores in multiple complementary orientations, which was previously not possible due to preferred orientation of the spores on the grid. We demonstrate the broadness of the Waffle Method by applying it to three additional cellular samples and a single particle sample using a variety of cryo-FIB-milling hardware, with both manual and automated approaches. We also present a unique and critical stress-relief gap designed specifically for waffled lamellae. Additionally, we describe applications of the Waffle Method which are currently being explored. We propose the Waffle Method as a way to achieve many of the advantages of cryo-liftout on the specimen grid while avoiding the long, challenging, and technically-demanding process required for cryo-liftout.

microbiology↗

bioRxiv2020

Characterization of the SARS-CoV-2 S Protein: Biophysical, Biochemical, Structural, and Antigenic Analysis

Coronavirus disease 2019 (COVID-19) is a global health crisis caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and there is a critical need to produce large quantities of high-quality SARS-CoV-2 Spike (S) protein for use in both clinical and basic science settings. To address this need, we have evaluated the expression and purification of two previously reported S protein constructs in Expi293F and ExpiCHO-S cells, two different cell lines selected for increased expression of secreted glycoproteins. We show that ExpiCHO-S cells produce enhanced yields of both SARS-CoV-2 S proteins. Biochemical, biophysical, and structural (cryo-EM) characterization of the SARS-CoV-2 S proteins produced in both cell lines demonstrate that the reported purification strategy yields high quality S protein (non-aggregated, uniform material with appropriate biochemical and biophysical properties). Importantly, we show that multiple preparations of these two recombinant S proteins from either cell line exhibit identical behavior in two different serology assays. We also evaluate the specificity of S protein-mediated host cell binding by examining interactions with proposed binding partners in the human secretome. In addition, the antigenicity of these proteins is demonstrated by standard ELISAs, and in a flexible protein microarray format. Collectively, we establish an array of metrics for ensuring the production of high-quality S protein to support clinical, biological, biochemical, structural and mechanistic studies to combat the global pandemic caused by SARS-CoV-2.

biochemistry↗

bioRxiv2020

Time-resolved cryoEM using Spotiton

We present an approach for preparing cryoEM grids to study short-lived molecular states. Using piezo electric dispensing, two independent streams of ~50 pL sample drops are deposited within 10 ms of each other onto a nanowire EM grid surface, and the mixing reaction stops when the grid is vitrified in liquid ethane, on the order of ~100 ms later. We demonstrate the utility of this approach for four biological systems where short-lived states are of high interest.

biophysics↗

bioRxiv2018

The impact of data reduction and lossy image formats on electron microscope images

Recent advances in instrumentation and software for cryoEM have increased the applicability and utility of this method. Coupled with the adoption of automated pipelines, significant infrastructure support is required to sustain high throughput workflows. In particular, data generation rates may outpace the ability to deploy data storage and archival solutions. We have investigated what effects data compression and conversion to different file formats have on the ability to obtain high resolution cryoEM reconstructions. Standard lossless data compression strategies have a high impact on reducing the size of direct detector electron counting movie stacks, but provide more modest gains for aligned summed images. We show that EM images can be compressed using standard lossy methods to reduce file storage to 5-10% of the size of the original aligned sum or movie stack file and yet still retain enough information such that modern image processing pipelines will provide sub-2[A] reconstructions from the compressed data.

biophysics↗

bioRxiv2018

Benchmarking cryo-EM single particle analysis workflow

Cryo electron microscopy facilities running multiple instruments and serving users with varying skill levels need a robust and reliable method for benchmarking both the hardware and software components of their single particle analysis workflow. The workflow is complex, with many bottlenecks existing at the specimen preparation, data collection and image analysis steps; the samples and grid preparation can be of unpredictable quality, there are many different protocols for microscope and camera settings, and there is a myriad of software programs for analysis that can depend on dozens of settings chosen by the user. For this reason, we believe it is important to benchmark the entire workflow, using a standard sample and standard operating procedures, on a regular basis. This provides confidence that all aspects of the pipeline are capable of producing maps to high resolution. Here we describe benchmarking procedures using a test sample, rabbit muscle aldolase.

molecular biology↗

bioRxiv2018

High Resolution Single Particle Cryo-Electron Microscopy using Beam-Image Shift

Automated data acquisition is now used widely for the single-particle averaging approach to reconstruction of three-dimensional (3D) volumes of biological complexes preserved in vitreous ice and imaged in a transmission electron microscope (cryo-EM). Automation has become integral to this method because of the very large number of particle images required to be averaged in order to overcome the typically low signal-to-noise ratio of these images.\n\nFor optimal efficiency, all automated data acquisition software packages employ some degree of beam-image shift because this method is fast and accurate (+/- 0.1 m). Relocation to a targeted area under low-dose conditions can only be achieved using stage movements in combination with multiple iterations or long relaxation times, both reducing efficiency. It is, however, well known that applying beam-image shift induces beam-tilt and hence structure phase error. A {pi}/4 phase error is considered as the worst that can be accepted, and is used as an argument against the use of any beam-image shift for high resolution data collection.\n\nIn this study, we performed cryo-EM single-particle reconstructions on a T20S proteasome sample using applied beam-image shifts corresponding to beam tilts from 0 to 10 mrad. To evaluate the results we compared the FSC values, and examined the water density peaks in the 3D map. We conclude that the {pi}/4 phase error does not limit the validity of the 3D reconstruction from single-particle averaging beyond the {pi}/4 resolution limit.

biophysics↗