Solution Structure of the C-terminal Domain of A20, the Missing Brick for the Characterization of the Interface between Vaccinia Virus DNA Polymerase and its Processivity Factor

Structural basis for the inhibition mechanism of the DNA polymerase holoenzyme from mpox virus

There are three key components at the core of the mpox virus (MPXV) DNA polymerase holoenzyme: DNA polymerase F8, processivity factors A22, and the Uracil-DNA glycosylase E4. The holoenzyme is recognized as a vital antiviral target because MPXV replicates in the cytoplasm of host cells. Nucleotide analogs such as cidofovir and cytarabine (Ara-C) have shown potential in curbing MPXV replication and they also display promise against other poxviruses. However, the mechanism behind their inhibitory effects remains unclear. Here, we present the cryo-EM structure of the DNA polymerase holoenzyme F8/A22/E4 bound with its competitive inhibitor Ara-C-derived cytarabine triphosphate (Ara-CTP) at an overall resolution of 3.0 Å and reveal its inhibition mechanism. Ara-CTP functions as a direct chain terminator in proximity to the deoxycytidine triphosphate (dCTP)-binding site. The extra hydrogen bond formed with Asn665 makes it more potent in binding than dCTP. Asn665 is conserved among eukaryotic B-family polymerases.

Structural insight into the assembly and working mechanism of helicase-primase D5 from Mpox virus

The Mpox pandemic, caused by the Mpox virus (or monkeypox virus, MPXV), has gained global attention. The D5 protein, a putative helicase-primase found in MPXV, plays a vital role in viral replication and genome uncoating. Here we determined multiple cryo-EM structures of full-length hexameric D5 in diverse states. These states were captured during ATP hydrolysis while moving along the single-stranded DNA (ssDNA) track. Through comprehensive structural analysis combined with the helicase activity system, we revealed that when the primase domain is truncated or the interaction between the primase and helicase domains is disrupted, the double-stranded DNA (dsDNA) unwinds into ssDNA, suggesting a critical regulatory role of the primase domain. Two transition states bound with ssDNA substrate during unwinding reveals that two ATP molecules were consumed to drive DNA moving forward two nucleotides. Collectively, our findings shed light on the molecular mechanism that links ATP hydrolysis to the DNA unwinding in poxviruses.

Structural insights into the assembly and mechanism of mpox virus DNA polymerase complex F8-A22-E4-H5

The DNA replication of mpox virus is performed by the viral polymerase F8 and also requires other viral factors, including processivity factor A22, uracil DNA glycosylase E4, and phosphoprotein H5. However, the molecular roles of these viral factors remain unclear. Here, we characterize the structures of F8-A22-E4 and F8-A22-E4-H5 complexes in the presence of different primer-template DNA substrates. E4 is located upstream of F8 on the template single-stranded DNA (ssDNA) and is catalytically active, highlighting a functional coupling between DNA base-excision repair and DNA synthesis. Moreover, H5, in the form of tetramer, binds to the double-stranded DNA (dsDNA) region downstream of F8 in a similar position as PCNA (proliferating cell nuclear antigen) does in eukaryotic polymerase complexes. Omission of H5 or disruption of its DNA interaction showed a reduced synthesis of full-length DNA products. These structures provide snapshots for the working cycle of the polymerase and generate insights into the mechanisms of these essential factors in viral DNA replication.

Transcriptome Changes in Glioma Cells upon Infection with the Oncolytic Virus VV-GMCSF-Lact

Oncolytic virotherapy is a rapidly evolving approach that aims to selectively kill cancer cells. We designed a promising recombinant vaccinia virus, VV-GMCSF-Lact, for the treatment of solid tumors, including glioma. We assessed how VV-GMCSF-Lact affects human cells using immortalized and patient-derived glioma cultures and a non-malignant brain cell culture. Studying transcriptome changes in cells 12 h or 24 h after VV-GMCSF-Lact infection, we detected the common activation of histone genes. Additionally, genes associated with the interferon-gamma response, NF-kappa B signaling pathway, and inflammation mediated by chemokine and cytokine signaling pathways showed increased expression. By contrast, genes involved in cell cycle progression, including spindle organization, sister chromatid segregation, and the G2/M checkpoint, were downregulated following virus infection. The upregulation of genes responsible for Golgi vesicles, protein transport, and secretion correlated with reduced sensitivity to the cytotoxic effect of VV-GMCSF-Lact. Higher expression of genes encoding proteins, which participate in the maturation of pol II nuclear transcripts and mRNA splicing, was associated with an increased sensitivity to viral cytotoxicity. Genes whose expression correlates with the sensitivity of cells to the virus are important for increasing the effectiveness of cancer virotherapy. Overall, the results highlight molecular markers, biological pathways, and gene networks influencing the response of glioma cells to VV-GMCSF-Lact.

Structural basis for the assembly of the DNA polymerase holoenzyme from a monkeypox virus variant

The ongoing global pandemic caused by a variant of the monkeypox (or mpox) virus (MPXV) has prompted widespread concern. The MPXV DNA polymerase holoenzyme, consisting of F8, A22, and E4, is vital for replicating the viral genome and represents a crucial target for the development of antiviral drugs. However, the assembly and working mechanism for the DNA polymerase holoenzyme of MPXV remains elusive. Here, we present the cryo-electron microscopy (cryo-EM) structure of the DNA polymerase holoenzyme at an overall resolution of 3.5 Å. Unexpectedly, the holoenzyme is assembled as a dimer of heterotrimers, of which the extra interface between the thumb domain of F8 and A22 shows a clash between A22 and substrate DNA, suggesting an autoinhibition state. Addition of exogenous double-stranded DNA shifts the hexamer into trimer exposing DNA binding sites, potentially representing a more active state. Our findings provide crucial steps toward developing targeted antiviral therapies for MPXV and related viruses.

Structure of monkeypox virus DNA polymerase holoenzyme

The World Health Organization declared mpox (or monkeypox) a public health emergency of international concern in July 2022, and prophylactic and therapeutic measures are in urgent need. The monkeypox virus (MPXV) has its own DNA polymerase F8, together with the processive cofactors A22 and E4, constituting the polymerase holoenzyme for genome replication. Here, we determined the holoenzyme structure in complex with DNA using cryo-electron microscopy at the global resolution of ~2.8 angstroms. The holoenzyme possesses an architecture that suggests a "forward sliding clamp" processivity mechanism for viral DNA replication. MPXV polymerase has a DNA binding mode similar to that of other B-family DNA polymerases from different species. These findings reveal the mechanism of the MPXV genome replication and may guide the development of anti-poxvirus drugs.

Mutations in the monkeypox virus replication complex: Potential contributing factors to the 2022 outbreak

Attributes contributing to the current monkeypox virus (MPXV) outbreak remain unknown. It has been established that mutations in viral proteins may alter phenotype and pathogenicity. To assess if mutations in the MPXV DNA replication complex (RC) contribute to the outbreak, we conducted a temporal analysis of available MPXV sequences to identify mutations, generated a DNA replication complex (RC) using structures of related viral and eukaryotic proteins, and structure prediction method AlphaFold. Ten mutations within the RC were identified and mapped onto the RC to infer role of mutations. Two mutations in F8L (RC catalytic subunit), and two in G9R (a processivity factor) were ∼100% prevalent in the 2022 sequences. F8L mutation L108F emerged in 2022, whereas W411L emerged in 2018, and persisted in 2022. L108 is topologically located to enhance DNA binding affinity of F8L. Therefore, mutation L108F can change the fidelity, sensitivity to nucleoside inhibitors, and processivity of F8L. Surface exposed W411L likely affects the binding of regulatory factor(s). G9R mutations S30L and D88 N in G9R emerged in 2022, and may impact the interaction of G9R with E4R (uracil DNA glycosylase). The remaining six mutations that appeared in 2001, reverted to the first (1965 Rotterdam) isolate. Two nucleoside inhibitors brincidofovir and cidofovir have been approved for MPXV treatment. Cidofovir resistance in vaccinia virus is achieved by A314T and A684V mutations. Both A314 and A684 are conserved in MPXV. Therefore, resistance to these drugs in MPXV may arise through similar mechanisms.

Efficient Method for Generating Point Mutations in the Vaccinia Virus Genome Using CRISPR/Cas9

The vaccinia virus (VACV) was previously used as a vaccine for smallpox eradication. Nowadays, recombinant VACVs are developed as vaccine platforms for infectious disease prevention and cancer treatment. The conventional method for genome editing of the VACV is based on homologous recombination, which is poorly efficient. Recently, the use of CRISPR/Cas9 technology was shown to greatly improve the speed and efficiency of the production of recombinant VACV expressing a heterologous gene. However, the ability to rapidly recover viruses bearing single nucleotide substitutions is still challenging. Notwithstanding, ongoing studies on the VACV and its interaction with the host cell could benefit from viral gene targeted mutagenesis. Here, we present a modified version of the CRISPR/Cas9 system for the rapid selection of mutant VACV carrying point mutations. For this purpose, we introduced a silent mutation into the donor gene (which will replace the wildtype gene) that serves a double function: it is located in the PAM (NGG) sequence, which is essential for Cas9 cleavage, and it alters a restriction site. This silent mutation, once introduced into the VACV genome, allows for rapid selection and screening of mutant viruses carrying a mutation of interest in the targeted gene. As a proof of concept, we produced several recombinant VACVs, with mutations in the E9L gene, upon which, phenotypic analysis was performed.

The Life Cycle of the Vaccinia Virus Genome

Poxviruses, of which vaccinia virus is the prototype, are a large family of double-stranded DNA viruses that replicate exclusively in the cytoplasm of infected cells. This physical and genetic autonomy from the host cell nucleus necessitates that these viruses encode most, if not all, of the proteins required for replication in the cytoplasm. In this review, we follow the life of the viral genome through space and time to address some of the unique challenges that arise from replicating a 195-kb DNA genome in the cytoplasm. We focus on how the genome is released from the incoming virion and deposited into the cytoplasm; how the endoplasmic reticulum is reorganized to form a replication factory, thereby compartmentalizing and helping to protect the replicating genome from immune sensors; how the cellular milieu is tailored to support high-fidelity replication of the genome; and finally, how newly synthesized genomes are faithfully and specifically encapsidated into new virions. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

New system for archiving integrative structures

Structures of many complex biological assemblies are increasingly determined using integrative approaches, in which data from multiple experimental methods are combined. A standalone system, called PDB-Dev, has been developed for archiving integrative structures and making them publicly available. Here, the data standards and software tools that support PDB-Dev are described along with the new and updated components of the PDB-Dev data-collection, processing and archiving infrastructure. Following the FAIR (Findable, Accessible, Interoperable and Reusable) principles, PDB-Dev ensures that the results of integrative structure determinations are freely accessible to everyone.

Assessment of prediction methods for protein structures determined by NMR in CASP14: Impact of AlphaFold2

NMR studies can provide unique information about protein conformations in solution. In CASP14, three reference structures provided by solution NMR methods were available (T1027, T1029, and T1055), as well as a fourth data set of NMR‐derived contacts for an integral membrane protein (T1088). For the three targets with NMR‐based structures, the best prediction results ranged from very good (GDT_TS = 0.90, for T1055) to poor (GDT_TS = 0.47, for T1029). We explored the basis of these results by comparing all CASP14 prediction models against experimental NMR data. For T1027, NMR data reveal extensive internal dynamics, presenting a unique challenge for protein structure prediction methods. The analysis of T1029 motivated exploration of a novel method of “inverse structure determination,” in which an AlphaFold2 model was used to guide NMR data analysis. NMR data provided to CASP predictor groups for target T1088, a 238‐residue integral membrane porin, was also used to assess several NMR‐assisted prediction methods. Most groups involved in this exercise generated similar beta‐barrel models, with good agreement with the experimental data. However, as was also observed in CASP13, some pure prediction groups that did not use any NMR data generated models for T1088 that better fit the NMR data than the models generated using these experimental data. These results demonstrate the remarkable power of modern methods to predict structures of proteins with accuracies rivaling solution NMR structures, and that it is now possible to reliably use prediction models to guide and complement experimental NMR data analysis.