Molecular plasticity of herpesvirus nuclear egress analysed in situ.

The viral nuclear egress complex (NEC) allows herpesvirus capsids to escape from the nucleus without compromising the nuclear envelope integrity. The NEC lattice assembles on the inner nuclear membrane and mediates the budding of nascent nucleocapsids into the perinuclear space and their subsequent release into the cytosol. Its essential role makes it a potent antiviral target, necessitating structural information in the context of a cellular infection. Here we determined structures of NEC-capsid interfaces in situ using electron cryo-tomography, showing a substantial structural heterogeneity. In addition, while the capsid is associated with budding initiation, it is not required for curvature formation. By determining the NEC structure in several conformations, we show that curvature arises from an asymmetric assembly of disordered and hexagonally ordered lattice domains independent of pUL25 or other viral capsid vertex components. Our results advance our understanding of the mechanism of nuclear egress in the context of a living cell.

Dynamics of the Herpes simplex virus DNA polymerase holoenzyme during DNA synthesis and proof-reading revealed by Cryo-EM.

Herpes simplex virus 1 (HSV-1), a double-stranded DNA virus, replicates using seven essential proteins encoded by its genome. Among these, the UL30 DNA polymerase, complexed with the UL42 processivity factor, orchestrates leading and lagging strand replication of the 152 kb viral genome. UL30 polymerase is a prime target for antiviral therapy, and resistance to current drugs can arise in immunocompromised individuals. Using electron cryo-microscopy (cryo-EM), we unveil the dynamic changes of the UL30/UL42 complex with DNA in three distinct states. First, a pre-translocation state with an open fingers domain ready for nucleotide incorporation. Second, a halted elongation state where the fingers close, trapping dATP in the dNTP pocket. Third, a DNA-editing state involving significant conformational changes to allow DNA realignment for exonuclease activity. Additionally, the flexible UL30 C-terminal domain interacts with UL42, forming an extended positively charged surface binding to DNA, thereby enhancing processive synthesis. These findings highlight substantial structural shifts in the polymerase and its DNA interactions during replication, offering insights for future antiviral drug development.

Structural snapshots of phenuivirus cap-snatching and transcription.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a human pathogen that is now endemic to several East Asian countries. The viral large (L) protein catalyzes viral transcription by stealing host mRNA caps via a process known as cap-snatching. Here, we establish an in vitro cap-snatching assay and present three high-quality electron cryo-microscopy (cryo-EM) structures of the SFTSV L protein in biologically relevant, transcription-specific states. In a priming-state structure, we show capped RNA bound to the L protein cap-binding domain (CBD). The L protein conformation in this priming structure is significantly different from published replication-state structures, in particular the N- and C-terminal domains. The capped-RNA is positioned in a way that it can feed directly into the RNA-dependent RNA polymerase (RdRp) ready for elongation. We also captured the L protein in an early-elongation state following primer-incorporation demonstrating that this priming conformation is retained at least in the very early stages of primer extension. This structural data is complemented by in vitro biochemical and cell-based assays. Together, these insights further our mechanistic understanding of how SFTSV and other bunyaviruses incorporate stolen host mRNA fragments into their viral transcripts thereby allowing the virus to hijack host cell translation machinery.

The palisade layer of the poxvirus core is composed of flexible A10 trimers.

Due to its asymmetric shape, size and compactness, the structure of the infectious mature virus (MV) of vaccinia virus (VACV), the best-studied poxvirus, remains poorly understood. Instead, subviral particles, in particular membrane-free viral cores, have been studied with cryo-electron microscopy. Here, we compared viral cores obtained by detergent stripping of MVs with cores in the cellular cytoplasm, early in infection. We focused on the prominent palisade layer on the core surface, combining cryo-electron tomography, subtomogram averaging and AlphaFold2 structure prediction. We showed that the palisade is composed of densely packed trimers of the major core protein A10. Trimers display a random order and their classification indicates structural flexibility. A10 on cytoplasmic cores is organized in a similar manner, indicating that the structures obtained in vitro are physiologically relevant. We discuss our results in the context of the VACV replicative cycle, and the assembly and disassembly of the infectious MV.

Modeling Flexible Protein Structure With AlphaFold2 and Crosslinking Mass Spectrometry.

We propose a pipeline that combines AlphaFold2 (AF2) and crosslinking mass spectrometry (XL-MS) to model the structure of proteins with multiple conformations. The pipeline consists of two main steps: ensemble generation using AF2 and conformer selection using XL-MS data. For conformer selection, we developed two scores-the monolink probability score (MP) and the crosslink probability score (XLP)-both of which are based on residue depth from the protein surface. We benchmarked MP and XLP on a large dataset of decoy protein structures and showed that our scores outperform previously developed scores. We then tested our methodology on three proteins having an open and closed conformation in the Protein Data Bank: Complement component 3 (C3), luciferase, and glutamine-binding periplasmic protein, first generating ensembles using AF2, which were then screened for the open and closed conformations using experimental XL-MS data. In five out of six cases, the most accurate model within the AF2 ensembles-or a conformation within 1 Å of this model-was identified using crosslinks, as assessed through the XLP score. In the remaining case, only the monolinks (assessed through the MP score) successfully identified the open conformation of glutamine-binding periplasmic protein, and these results were further improved by including the "occupancy" of the monolinks. This serves as a compelling proof-of-concept for the effectiveness of monolinks. In contrast, the AF2 assessment score was only able to identify the most accurate conformation in two out of six cases. Our results highlight the complementarity of AF2 with experimental methods like XL-MS, with the MP and XLP scores providing reliable metrics to assess the quality of the predicted models. The MP and XLP scoring functions mentioned above are available at https://gitlab.com/topf-lab/xlms-tools.

Targeted mutagenesis of the herpesvirus fusogen central helix captures transition states.

Herpesviruses remain a burden for animal and human health, including the medically important varicella-zoster virus (VZV). Membrane fusion mediated by conserved core glycoproteins, the fusogen gB and the heterodimer gH-gL, enables herpesvirus cell entry. The ectodomain of gB orthologs has five domains and is proposed to transition from a prefusion to postfusion conformation but the functional relevance of the domains for this transition remains poorly defined. Here we describe structure-function studies of the VZV gB DIII central helix targeting residues 526EHV528. Critically, a H527P mutation captures gB in a prefusion conformation as determined by cryo-EM, a loss of membrane fusion in a virus free assay, and failure of recombinant VZV to spread in cell monolayers. Importantly, two predominant cryo-EM structures of gB[H527P] are identified by 3D classification and focused refinement, suggesting they represented gB conformations in transition. These studies reveal gB DIII as a critical element for herpesvirus gB fusion function.

Structural and functional characterization of the Sin Nombre virus L protein.

The Bunyavirales order is a large and diverse group of segmented negative-strand RNA viruses. Several virus families within this order contain important human pathogens, including Sin Nombre virus (SNV) of the Hantaviridae. Despite the high epidemic potential of bunyaviruses, specific medical countermeasures such as vaccines or antivirals are missing. The multifunctional ~250 kDa L protein of hantaviruses, amongst other functional domains, harbors the RNA-dependent RNA polymerase (RdRp) and an endonuclease and catalyzes transcription as well as replication of the viral RNA genome, making it a promising therapeutic target. The development of inhibitors targeting these key processes requires a profound understanding of the catalytic mechanisms. Here, we established expression and purification protocols of the full-length SNV L protein bearing the endonuclease mutation K124A. We applied different biochemical in vitro assays to provide an extensive characterization of the different enzymatic functions as well as the capacity of the hantavirus L protein to interact with the viral RNA. By using single-particle cryo-EM, we obtained a 3D model including the L protein core region containing the RdRp, in complex with the 5' promoter RNA. This first high-resolution model of a New World hantavirus L protein shows striking similarity to related bunyavirus L proteins. The interaction of the L protein with the 5' RNA observed in the structural model confirms our hypothesis of protein-RNA binding based on our biochemical data. Taken together, this study provides an excellent basis for future structural and functional studies on the hantavirus L protein and for the development of antiviral compounds.

Variable microtubule architecture in the malaria parasite.

Microtubules are a ubiquitous eukaryotic cytoskeletal element typically consisting of 13 protofilaments arranged in a hollow cylinder. This arrangement is considered the canonical form and is adopted by most organisms, with rare exceptions. Here, we use in situ electron cryo-tomography and subvolume averaging to analyse the changing microtubule cytoskeleton of Plasmodium falciparum, the causative agent of malaria, throughout its life cycle. Unexpectedly, different parasite forms have distinct microtubule structures coordinated by unique organising centres. In merozoites, the most widely studied form, we observe canonical microtubules. In migrating mosquito forms, the 13 protofilament structure is further reinforced by interrupted luminal helices. Surprisingly, gametocytes contain a wide distribution of microtubule structures ranging from 13 to 18 protofilaments, doublets and triplets. Such a diversity of microtubule structures has not been observed in any other organism to date and is likely evidence of a distinct role in each life cycle form. This data provides a unique view into an unusual microtubule cytoskeleton of a relevant human pathogen.

Structural insights into viral genome replication by the severe fever with thrombocytopenia syndrome virus L protein.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a phenuivirus that has rapidly become endemic in several East Asian countries. The large (L) protein of SFTSV, which includes the RNA-dependent RNA polymerase (RdRp), is responsible for catalysing viral genome replication and transcription. Here, we present 5 cryo-electron microscopy (cryo-EM) structures of the L protein in several states of the genome replication process, from pre-initiation to late-stage elongation, at a resolution of up to 2.6 Å. We identify how the L protein binds the 5' viral RNA in a hook-like conformation and show how the distal 5' and 3' RNA ends form a duplex positioning the 3' RNA terminus in the RdRp active site ready for initiation. We also observe the L protein stalled in the early and late stages of elongation with the RdRp core accommodating a 10-bp product-template duplex. This duplex ultimately splits with the template binding to a designated 3' secondary binding site. The structural data and observations are complemented by in vitro biochemical and cell-based mini-replicon assays. Altogether, our data provide novel key insights into the mechanism of viral genome replication by the SFTSV L protein and will aid drug development against segmented negative-strand RNA viruses.

Protocol for live-cell fluorescence-guided cryoFIB-milling and electron cryo-tomography of virus-infected cells.

Here, we present a protocol for assessing virus-infected cells using electron cryo-tomography (cryoET). It includes the basic workflows of seeding cells, plunge-freezing, clipping, cryo-focused ion beam milling (cryoFIB-milling), and cryoET, as well as two optional modules: micropatterning and live-cell fluorescence microscopy. We use an A549 human cell line and the virus HAdV5-pIX-mcherry in this protocol, but the comprehensive workflow can be easily transferred to other cell types and different types of virus infection or treatment. For complete details on the use and execution of this protocol, please refer to Pfitzner et al. (2021).

Intermittent bulk release of human cytomegalovirus.

Human Cytomegalovirus (HCMV) can infect a variety of cell types by using virions of varying glycoprotein compositions. It is still unclear how this diversity is generated, but spatio-temporally separated envelopment and egress pathways might play a role. So far, one egress pathway has been described in which HCMV particles are individually enveloped into small vesicles and are subsequently exocytosed continuously. However, some studies have also found enveloped virus particles inside multivesicular structures but could not link them to productive egress or degradation pathways. We used a novel 3D-CLEM workflow allowing us to investigate these structures in HCMV morphogenesis and egress at high spatio-temporal resolution. We found that multiple envelopment events occurred at individual vesicles leading to multiviral bodies (MViBs), which subsequently traversed the cytoplasm to release virions as intermittent bulk pulses at the plasma membrane to form extracellular virus accumulations (EVAs). Our data support the existence of a novel bona fide HCMV egress pathway, which opens the gate to evaluate divergent egress pathways in generating virion diversity.

The Cryo-EM structures of two amphibian antimicrobial cross-ß amyloid fibrils.

The amyloid-antimicrobial link hypothesis is based on antimicrobial properties found in human amyloids involved in neurodegenerative and systemic diseases, along with amyloidal structural properties found in antimicrobial peptides (AMPs). Supporting this hypothesis, we here determined the fibril structure of two AMPs from amphibians, uperin 3.5 and aurein 3.3, by cryogenic electron microscopy (cryo-EM), revealing amyloid cross-ß fibrils of mated ß-sheets at atomic resolution. Uperin 3.5 formed a 3-blade symmetrical propeller of nine peptides per fibril layer including tight ß-sheet interfaces. This cross-ß cryo-EM structure complements the cross-α fibril conformation previously determined by crystallography, substantiating a secondary structure switch mechanism of uperin 3.5. The aurein 3.3 arrangement consisted of six peptides per fibril layer, all showing kinked ß-sheets allowing a rounded compactness of the fibril. The kinked ß-sheets are similar to LARKS (Low-complexity, Amyloid-like, Reversible, Kinked Segments) found in human functional amyloids.

The giant mimivirus 1.2 Mb genome is elegantly organized into a 30-nm diameter helical protein shield.

Mimivirus is the prototype of the Mimiviridae family of giant dsDNA viruses. Little is known about the organization of the 1.2 Mb genome inside the membrane-limited nucleoid filling the ~0.5 µm icosahedral capsids. Cryo-electron microscopy, cryo-electron tomography, and proteomics revealed that it is encased into a ~30-nm diameter helical protein shell surprisingly composed of two GMC-type oxidoreductases, which also form the glycosylated fibrils decorating the capsid. The genome is arranged in 5- or 6-start left-handed super-helices, with each DNA-strand lining the central channel. This luminal channel of the nucleoprotein fiber is wide enough to accommodate oxidative stress proteins and RNA polymerase subunits identified by proteomics. Such elegant supramolecular organization would represent a remarkable evolutionary strategy for packaging and protecting the genome, in a state ready for immediate transcription upon unwinding in the host cytoplasm. The parsimonious use of the same protein in two unrelated substructures of the virion is unexpected for a giant virus with thousand genes at its disposal.

Human cytomegalovirus forms phase-separated compartments at viral genomes to facilitate viral replication.

Human cytomegalovirus (HCMV) replicates its DNA genome in specialized replication compartments (RCs) in the host cell nucleus. These membrane-less organelles originate as spherical structures and grow in size over time. However, the mechanism of RC biogenesis has remained understudied. Using live-cell imaging and photo-oligomerization, we show that a central component of RCs, the UL112-113 proteins, undergo liquid-liquid phase separation (LLPS) to form RCs in the nucleus. We show that the self-interacting domain and large intrinsically disordered regions of UL112-113 are required for LLPS. Importantly, viral DNA induces local clustering of these proteins and lowers the threshold for phase separation. The formation of phase-separated compartments around viral genomes is necessary to recruit the viral DNA polymerase for viral genome replication. Thus, HCMV uses its UL112-113 proteins to generate RCs around viral genomes by LLPS to ensure the formation of a pro-replicative environment.

Caldendrin and myosin V regulate synaptic spine apparatus localization via ER stabilization in dendritic spines.

Excitatory synapses of principal hippocampal neurons are frequently located on dendritic spines. The dynamic strengthening or weakening of individual inputs results in structural and molecular diversity of dendritic spines. Active spines with large calcium ion (Ca2+ ) transients are frequently invaded by a single protrusion from the endoplasmic reticulum (ER), which is dynamically transported into spines via the actin-based motor myosin V. An increase in synaptic strength correlates with stable anchoring of the ER, followed by the formation of an organelle referred to as the spine apparatus. Here, we show that myosin V binds the Ca2+ sensor caldendrin, a brain-specific homolog of the well-known myosin V interactor calmodulin. While calmodulin is an essential activator of myosin V motor function, we found that caldendrin acts as an inhibitor of processive myosin V movement. In mouse and rat hippocampal neurons, caldendrin regulates spine apparatus localization to a subset of dendritic spines through a myosin V-dependent pathway. We propose that caldendrin transforms myosin into a stationary F-actin tether that enables the localization of ER tubules and formation of the spine apparatus in dendritic spines.

Conformational changes in Lassa virus L protein associated with promoter binding and RNA synthesis activity.

Lassa virus is endemic in West Africa and can cause severe hemorrhagic fever. The viral L protein transcribes and replicates the RNA genome via its RNA-dependent RNA polymerase activity. Here, we present nine cryo-EM structures of the L protein in the apo-, promoter-bound pre-initiation and active RNA synthesis states. We characterize distinct binding pockets for the conserved 3' and 5' promoter RNAs and show how full-promoter binding induces a distinct pre-initiation conformation. In the apo- and early elongation states, the endonuclease is inhibited by two distinct L protein peptides, whereas in the pre-initiation state it is uninhibited. In the early elongation state, a template-product duplex is bound in the active site cavity together with an incoming non-hydrolysable nucleotide and the full C-terminal region of the L protein, including the putative cap-binding domain, is well-ordered. These data advance our mechanistic understanding of how this flexible and multifunctional molecular machine is activated.

Human Adenovirus Type 5 Infection Leads to Nuclear Envelope Destabilization and Membrane Permeability Independently of Adenovirus Death Protein.

The human adenovirus type 5 (HAdV5) infects epithelial cells of the upper and lower respiratory tract. The virus causes lysis of infected cells and thus enables spread of progeny virions to neighboring cells for the next round of infection. The mechanism of adenovirus virion egress across the nuclear barrier is not known. The human adenovirus death protein (ADP) facilitates the release of virions from infected cells and has been hypothesized to cause membrane damage. Here, we set out to answer whether ADP does indeed increase nuclear membrane damage. We analyzed the nuclear envelope morphology using a combination of fluorescence and state-of-the-art electron microscopy techniques, including serial block-face scanning electron microscopy and electron cryo-tomography of focused ion beam-milled cells. We report multiple destabilization phenotypes of the nuclear envelope in HAdV5 infection. These include reduction of lamin A/C at the nuclear envelope, large-scale membrane invaginations, alterations in double membrane separation distance and small-scale membrane protrusions. Additionally, we measured increased nuclear membrane permeability and detected nuclear envelope lesions under cryoconditions. Unexpectedly, and in contrast to previous hypotheses, ADP did not have an effect on lamin A/C reduction or nuclear permeability.

In Vitro Viral Evolution Identifies a Critical Residue in the Alphaherpesvirus Fusion Glycoprotein B Ectodomain That Controls gH/gL-Independent Entry.

Herpesvirus entry and spread requires fusion of viral and host cell membranes, which is mediated by the conserved surface glycoprotein B (gB). Upon activation, gB undergoes a major conformational change and transits from a metastable prefusion to a stable postfusion conformation. Although gB is a structural homolog of low-pH-triggered class III fusogens, its fusion activity depends strictly on the presence of the conserved regulatory gH/gL complex and nonconserved receptor binding proteins, which ensure that fusion occurs at the right time and space. How gB maintains its prefusion conformation and how gB fusogenicity is controlled remain poorly understood. Here, we report the isolation and characterization of a naturally selected pseudorabies virus (PrV) gB able to mediate efficient gH/gL-independent virus-cell and cell-cell fusion. We found that the control exerted on gB by the accompanying viral proteins is mediated via its cytosolic domain (CTD). Whereas gB variants lacking the CTD are inactive, a single mutation of a conserved asparagine residue in an alpha-helical motif of the ectodomain recently shown to be at the core of the gB prefusion trimer compensated for CTD absence and uncoupled gB from regulatory viral proteins, resulting in a hyperfusion phenotype. This phenotype was transferred to gB homologs from different alphaherpesvirus genera. Overall, our data propose a model in which the central helix acts as a molecular switch for the gB pre-to-postfusion transition by conveying the structural status of the endo- to the ectodomain, thereby governing their cross talk for fusion activation, providing a new paradigm for herpesvirus fusion regulation.IMPORTANCE The class III fusion protein glycoprotein B (gB) drives membrane fusion during entry and spread of herpesviruses. To mediate fusion, gB requires activation by the conserved gH/gL complex by a poorly defined mechanism. A detailed molecular-level understanding of herpesvirus membrane fusion is of fundamental virological interest and has considerable potential for the development of new therapeutics blocking herpesvirus cell invasion and spread. Using in vitro evolution and targeted mutagenesis of three different animal alphaherpesviruses, we identified a single conserved amino acid in a regulatory helix in the center of the gB ectodomain that enables efficient gH/gL-independent entry and plays a crucial role in the pre-to-postfusion transition of gB. Our results propose that the central helix is a key regulatory element involved in the intrastructural signal transduction between the endo- and ectodomain for fusion activation. This study expands our understanding of herpesvirus membrane fusion and uncovers potential targets for therapeutic interventions.

Correlative super-resolution fluorescence and electron cryo-microscopy based on cryo-SOFI.

The combination of super-resolution fluorescence microscopy and electron microscopy at ambient temperatures has become an established technique and a broad variety of modalities are now available to the cell biology community. In contrast, correlative cryogenic super-resolution fluorescence and electron microscopy (super-resolution cryo-CLEM) is just emerging. Aside from technical challenges, one of the major issues is the risk of devitrification of the specimen caused by the laser intensities required for super-resolution imaging. Cryo-SOFI (cryogenic super-resolution optical fluctuation imaging) allows the reconstruction of super-resolution images at particularly low laser intensities. It is fully compatible with the standard sample preparation for cryogenic electron microscopy (cryo-EM) and fairly easy to implement in any standard cryogenic fluorescence microscope.

DNA origami signposts for identifying proteins on cell membranes by electron cryotomography.

Electron cryotomography (cryoET), an electron cryomicroscopy (cryoEM) modality, has changed our understanding of biological function by revealing the native molecular details of membranes, viruses, and cells. However, identification of individual molecules within tomograms from cryoET is challenging because of sample crowding and low signal-to-noise ratios. Here, we present a tagging strategy for cryoET that precisely identifies individual protein complexes in tomograms without relying on metal clusters. Our method makes use of DNA origami to produce "molecular signposts" that target molecules of interest, here via fluorescent fusion proteins, providing a platform generally applicable to biological surfaces. We demonstrate the specificity of signpost origami tags (SPOTs) in vitro as well as their suitability for cryoET of membrane vesicles, enveloped viruses, and the exterior of intact mammalian cells.