Biology subjects

Didychuk, A. L.

Publications and source records attributed to Didychuk, A. L..

9 recordsLinked to original sources

bioRxiv2026

Kaposi's Sarcoma-associated herpesvirus uses a novel protein fold to hijack RNA Polymerase II for viral late gene transcription

Herpesviruses, including the oncogenic human pathogen Kaposis Sarcoma-associated herpesvirus (KSHV), rely on cellular RNA Polymerase II (RNAPII) for expression of their protein-coding genes. Late in the lytic cycle, transcription initiation on a subset of KSHV genes depends on ORF24, a viral transcriptional activator that binds to viral late promoters through structural mimicry of a host transcription factor. Here, we present a structure of the N-terminal domain of ORF24, and show that it adopts a novel fold that facilitates direct binding of the RNAPII C-terminal domain (CTD). We further show that ORF24 is highly sensitive to the phosphorylation state of the RNAPII CTD, allowing for selective binding of the hypophosphorylated form to viral preinitiation complexes. By combining promoter binding and polymerase recruitment into a single protein, KSHV has streamlined essential steps in transcription complex assembly. Our work lays the groundwork for mechanistic dissection of a viral preinitiation complex.

microbiology↗

bioRxiv2026

The Kaposi's sarcoma-associated herpesvirus viral genome packaging accessory factor ORF68 forms cytoplasmic puncta dependent on the viral tyrosine kinase

Assembly of Kaposis sarcoma-associated herpesvirus (KSHV) virions is dependent on ORF68, a viral genome packaging accessory factor of unknown function. We used confocal fluorescence imaging to monitor ORF68 localization throughout the KSHV lytic cycle. ORF68 localizes to nuclear viral replication compartments, the site of viral genome packaging. Unexpectedly, ORF68 also localized to cytoplasmic puncta. Using proximity labeling mass spectrometry during infection, we identified ORF68 interaction partners, including the viral tyrosine kinase ORF21. We show that ORF21 colocalizes with ORF68 in cytoplasmic puncta and draws ORF68 to the cytoplasm during infection. This interaction is mediated by the disordered N-terminal region of ORF21. We propose that ORF68 possesses a novel secondary function independent from virion assembly. IMPORTANCEKaposis sarcoma-associated herpesvirus (KSHV) is the underlying cause of multiple human malignancies including Kaposis sarcoma, primary effusion lymphoma, and multicentric Castlemans disease. Encapsidation of the viral genome is necessary to produce new infectious virus. Viral genome packaging can be targeted with small molecules in human cytomegalovirus, a related herpesvirus, but we lack a sufficiently detailed mechanistic understanding to develop additional therapeutics. The molecular role of the essential packaging accessory factor, encoded by ORF68 in KSHV, has remained unclear. We studied the interactions and localization of ORF68 during KSHV infection and find that, in addition to its nuclear role in packaging, ORF68 forms cytoplasmic puncta through its interaction with the viral tyrosine kinase ORF21. This study demonstrates how conserved, essential herpesvirus proteins can play multiple roles in distinct subcellular compartments during viral infection, with secondary functions that may be herpesvirus species-specific.

microbiology↗

bioRxiv2026

Conserved assembly architecture of the essential herpesvirus packaging accessory factor

To create a new wave of infectious virions, all herpesviruses require an accessory factor of unknown function to package their viral genomes into nascent capsids. Here, we present cryo-EM structures of the packaging accessory factor from the -herpesvirus herpes simplex virus type 1 (HSV-1, UL32) and the {beta}-herpesvirus human cytomegalovirus (HCMV, UL52). Unlike homologs from the {gamma}-herpesviruses, neither UL32 nor UL52 form stable homopentameric rings. UL52 forms incomplete pentameric rings lacking one or two protomers. UL32 does not form stable higher-order species, but stabilization through chemical crosslinking revealed a novel quaternary structure where three pentameric rings assemble into a "tripentamer." Our results reveal that herpesvirus packaging accessory factors adopt distinct oligomeric states but are constrained to pentameric symmetry. Assembly of protomers into a ring creates a positively charged central channel that we show is critical for infectious virus production in HSV-1. Taken together, our study points to a structurally conserved, essential function of packaging accessory factors across the Herpesviridae. AUTHOR SUMMARYHerpesviruses are a diverse family of pathogens that drive human disease. At the end of their life cycle, all herpesviruses must actively package their genomes into newly formed capsids. Although some aspects of packaging are conserved with tailed bacteriophages, herpesviruses require an additional packaging accessory factor of unknown function. Here, we present two high-resolution structures of the packaging accessory factor from herpes simplex virus and human cytomegalovirus and identify pentameric symmetry as a unifying structural feature. We propose that the packaging accessory factor oligomerizes to concentrate positively charges in a central channel critical for packaging progression.

microbiology↗

bioRxiv2025

The Kaposi's sarcoma-associated herpesvirus TBP mimic uses a non-canonical DNA binding mode to promote viral late gene transcription

Kaposis sarcoma-associated herpesvirus (KSHV) orchestrates late gene transcription through viral transcriptional activators that hijack host RNA polymerase II machinery, maintaining selectivity for viral promoters. Among these, the KSHV protein ORF24 serves as a TATA-binding protein (TBP) mimic essential for recognizing viral late promoters, although the molecular mechanisms underlying its function remain poorly characterized. Here, we used AlphaFold3 to predict the structure of ORF24 in complex with DNA and validated key features in both transfected cells and during KSHV lytic replication. Structural modeling revealed that ORF24 employs a non-canonical DNA binding mode where the C-terminal domain (CTD) makes critical DNA contacts beyond the canonical TBP fold. Targeted mutagenesis confirmed that ORF24 requires conserved TBP-like phenylalanines alongside a polar-rich binding interface distinct from cellular TBP. During infection, both the TBP-like domain and CTD are essential for ORF24 occupancy at viral late promoters. Most surprisingly, we discovered that ORF24 pre-assembles with RNA polymerase II and the viral protein ORF34 to achieve stable promoter binding. This cooperative assembly mechanism represents a fundamental departure from stepwise eukaryotic transcription initiation, resembling a prokaryotic strategy within the eukaryotic nucleus. Summary Bullet pointsO_LIThe structure of the KSHV TBP mimic ORF24 binding DNA was modeled and experimentally tested. C_LIO_LIKSHV ORF24 uses an extended DNA-binding interface beyond the canonical TBP fold. C_LIO_LIORF24 requires cooperative pre-assembly with transcriptional machinery before DNA engagement C_LI

molecular biology↗

bioRxiv2023

The viral packaging motor potentiates late gene expression in Kaposi's sarcoma-associated herpesvirus

{beta}- and {gamma}-herpesviruses transcribe their late genes in a manner distinct from host transcription. This process is directed by a complex of viral transcriptional activator proteins that hijack cellular RNA polymerase II and an unknown set of additional factors. We employed proximity labeling coupled with mass spectrometry, followed by CRISPR and siRNA screening to identify proteins functionally associated with the Kaposis sarcoma-associated herpesvirus (KSHV) late gene transcriptional complex. These data revealed that the catalytic subunit of the viral DNA packaging motor, ORF29, is both dynamically associated with the viral transcriptional activator complex and potentiates late gene expression. Through genetic mutation and deletion of ORF29, we establish that its catalytic activity potentiates viral transcription and is required for robust accumulation of essential late proteins during infection. Thus, we propose an expanded role for ORF29 that encompasses its established function in viral packaging and its newly discovered contributions to viral transcription and late gene expression in KSHV. Author summary{beta}- and {gamma}-herpesviruses express a class of genes essential for completion of the viral life cycle late during infection. A specialized complex of viral transcriptional activator proteins drives expression of these late genes in a manner dependent on viral genome replication, although the mechanisms and regulation of this process are largely unknown. Here, we identified factors that physically and functionally associate with the late gene transcription complex and unexpectedly found that the viral DNA packaging motor in Kaposis sarcoma-associated herpesvirus (KSHV) contributes to late gene expression. We show that the catalytic activity of this protein is not only required for genomic packaging but also for the robust expression of late genes to ensure the successful production of progeny virions. Thus, late gene transcription is mechanistically linked to the conserved processes of viral genome replication and packaging.

molecular biology↗

bioRxiv2021

A Two-tiered Functional Screen Identifies Herpesviral Transcriptional Modifiers and their Essential Domains

While traditional methods for studying large DNA viruses allow the creation of individual mutants, CRISPR/Cas9 can be used to rapidly create thousands of mutant dsDNA viruses in parallel. Here, we used this approach to study the human oncogenic Kaposis sarcoma-associated herpesvirus (KSHV). We designed a sgRNA library containing all possible ~22,000 guides targeting the genome of KSHV - one cut site approximately every 8 base pairs - enabling the pooled screening of the entire genome. We used this tool to phenotype all possible Cas9-targeted viruses for transcription of KSHV late genes, which is required to produce structural components of the viral capsid. By performing targeted deep sequencing of the viral genome to distinguish between knock-out and in-frame alleles created by Cas9, we discovered a novel hit, ORF46 - and more specifically its DNA binding domain - is required for viral DNA replication. Our pooled Cas9 tiling screen followed by targeted deep viral sequencing represents a two-tiered screening paradigm that may be widely applicable to dsDNA viruses.

genomics↗

bioRxiv2020

A pentameric protein ring with novel architecture is required for herpesviral packaging

Genome packaging in large double-stranded DNA viruses requires a powerful molecular motor to force the viral genome into nascent capsids. This process appears mechanistically similar in two evolutionarily distant viruses, the herpesviruses and the tailed bacteriophages, which infect different kingdoms of life. While the motor and mechanism as a whole are thought to be conserved, accessory factors that influence packaging are divergent and poorly understood, despite their essential roles. An accessory factor required for herpesviral packaging is encoded by ORF68 in the oncogenic virus Kaposis sarcoma-associated herpesvirus (KSHV), whose homolog in Epstein Barr Virus (EBV) is BFLF1. Here, we present structures of both KSHV ORF68 and EBV BFLF1, revealing that these proteins form a highly similar homopentameric ring. The central channel of this ring is positively charged, and we demonstrate that this region of KSHV ORF68 binds double-stranded DNA. Mutation of individual positively charged residues within but not outside the channel ablates DNA binding, and in the context of KSHV infection these mutants fail to package the viral genome or produce progeny virions. Thus, we propose a model in which ORF68 facilitates the transfer of newly replicated viral genomes to the packaging motor.

microbiology↗

bioRxiv2020

The γ-herpesviral TATA box binding protein directly interacts with RNA Polymerase II to direct late gene transcription

{beta}- and {gamma}-herpesviruses include the oncogenic human viruses Kaposis sarcoma-associated virus (KSHV) and Epstein-Barr virus (EBV), and human cytomegalovirus (HCMV), which is a significant cause of congenital disease. Near the end of their replication cycle, these viruses transcribe their late genes in a manner distinct from host transcription. Late gene transcription requires six virally-encoded proteins, one of which is a functional mimic of host TATA-box-binding protein (TBP) that is also involved in recruitment of RNA polymerase II (Pol II) via unknown mechanisms. Here, we applied biochemical protein interaction studies together with electron microscopy-based imaging of a reconstituted human preinitiation complex to define the mechanism underlying Pol II recruitment. These data revealed that the herpesviral TBP, encoded by ORF24 in KSHV, makes a direct protein-protein contact with the C-terminal domain of host RNA polymerase II (Pol II), which is a unique feature that functionally distinguishes viral from cellular TBP. The interaction is mediated by the N-terminal domain (NTD) of ORF24 through a conserved motif that is shared in its {beta}- and {gamma}-herpesvirus homologs. Thus, these herpesviruses employ an unprecedented strategy in eukaryotic transcription, wherein promoter recognition and polymerase recruitment are facilitated by a single transcriptional activator with functionally distinct domains. SIGNIFICANCE STATEMENTThe {beta}- and {gamma}-herpesviruses mediate their late gene transcription through a set of viral transcriptional activators (vTAs). One of these vTAs, ORF24 in Kaposis sarcoma-associated herpesvirus (KSHV), is a mimic of host TATA-box-binding protein (TBP). We demonstrate that the N-terminal domain of ORF24 and its homologs from other {beta}- and {gamma}-herpesviruses directly bind the unstructured C-terminal domain (CTD) of RNA Pol II. This functionally distinguishes the viral TBP mimic from cellular TBP, which does not bind Pol II. Thus, herpesviruses encode a transcription factor that has the dual ability to directly interact with promoter DNA and the polymerase, a property which is unique in eukaryotic transcription and is conceptually akin to prokaryotic transcription factors.

molecular biology↗

bioRxiv2019

Conserved CxnC motifs in Kaposi’s sarcoma-associated herpesvirus ORF66 are required for viral late gene expression and mediate its interaction with ORF34

Late gene transcription in the beta- and gammaherpesviruses depends on a set of virally-encoded transcriptional activators (vTAs) that hijack the host transcriptional machinery and direct it to a subset of viral genes that are required for completion of the viral replication cycle and capsid assembly. In Kaposis sarcoma-associated herpesvirus (KSHV), these vTAs are encoded by ORF18, ORF24, ORF30, ORF31, ORF34, ORF66. Assembly of the vTAs into a complex is critical for late gene transcription, and thus deciphering the architecture of the complex is central to understanding its transcriptional regulatory activity. Here, we generated an ORF66-null virus and confirmed that it fails to produce late genes and infectious virions. We show that ORF66 is incorporated into the vTA complex primarily through its interaction with ORF34, which is mediated by a set of four conserved cysteine-rich motifs in the C-terminal domain of ORF66. While both ORF24 and ORF66 occupy the canonical K8.1 late gene promoter, their promoter occupancy requires the presence of the other vTAs, suggesting that sequence-specific, stable binding requires assembly of the entire complex on the promoter. Additionally, we find that ORF24 expression is impaired in the absence of a stable vTA complex. This work extends our knowledge about the architecture of the KSHV vPIC and suggests that it functions as a complex to recognize late gene promoters.\n\nIMPORTANCEKaposis sarcoma-associated herpesvirus (KSHV; human herpesvirus 8) is an oncogenic gammaherpesvirus that is the causative agent of multiple human cancers. Release of infectious virions requires production of capsid proteins and other late genes, whose production are transcriptionally controlled by a complex of six virally-encoded proteins that hijack the host transcription machinery. It is poorly understood how this complex assembles or what function five of its six components play in transcription. Here, we demonstrate that ORF66 is an essential component of this complex in KSHV and that its inclusion in the complex is mediated through its C-terminal domain, which contains highly conserved cysteine-rich motifs reminiscent of zinc finger motifs. Additionally, we examine assembly of the viral pre-initiation complex at late gene promoters and find that while sequence-specific binding of late gene promoters requires ORF24, it additionally requires a fully assembled viral pre-initation complex.

molecular biology↗