The SUMO E3 ligase activity of ORF45 determines KSHV lytic replication

Development and Characterization of an Inducible Bacterial Artificial Chromosome System for Studying Lytic Replication and Pathogenesis of Kaposi's Sarcoma-Associated Herpesvirus

Bacterial artificial chromosome (BAC) is widely used to manipulate herpesvirus genome and generate recombinant virus. Here, we developed a new KSHV BACmid, namely as iBAC, by replacing the EGFP with TET3G transactivator under EF1α promoter and inserted Tet response elements in the promoter of RTA in the original KSHV BAC16 clone and characterized KSHV lytic replication in SLK-iBAC cells. SLK-iBAC cells developed more efficient lytic replication and generated more progeny virus than iSLK-BAC16 cells upon the same conditions of doxycycline treatment. Since SLK-iBAC cells only occupied hygromycin selection marker, it is convenient to generate cellular gene knockout via lentivirus-mediated CRISPR-Cas9 or stably express viral or cellular gene via lentivirus followed by antibiotic selection, making iBAC system a better tool to identify cellular targets of viral proteins in the context of virus infection or study the role of viral or cellular genes for KSHV lytic replication and pathogenesis. In addition, iBAC is color-free and can be utilized to track subcellular localization of viral proteins or colocalization between different viral proteins by introducing fusing fluorescent proteins into the BAC backbone. Therefore, the new KSHV iBAC is a powerful inducible tool to study KSHV lytic replication and pathogenesis in cell model.

Viral pseudo-enzyme facilitates KSHV lytic replication via suppressing PFAS-mediated RTA deamidation

Deamidation, a type of post-translational modification commonly considered a hallmark of protein "aging" and function decay, is increasingly recognized for its pivotal role in regulating biological processes and viral infection. Our previous study has demonstrated that the deamidation of replication and transcription activator (RTA), a master regulator of ubiquitous and oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV), mediated by phosphoribosylformylglycinamidine synthetase (PFAS), hinders its nuclear import and transcriptional activity. Here we report that the viral glutamine amidotransferase (vGAT) pseudo-enzyme was exploited to facilitate KSHV lytic infection by inhibiting RTA deamidation. To be more specific, vGAT interacted with both RTA and cellular PFAS, and inhibited PFAS-mediated RTA deamidation, thus facilitating RTA nuclear localization and suppressing nuclear factor-kappa B (NF-κB) signaling activation, as well as augmenting RTA-mediated transcriptional activation of viral open reading frames (ORFs). In addition, vGAT appeared to regulate the deamidation process of several viral ORFs of KSHV. Collectively, these findings unveil that a viral pseudo-enzyme was exploited to enhance viral infection via deamidation regulation.

The crosstalk between SUMOylation and immune system in host-pathogen interactions

Pathogens can not only cause infectious diseases, immune system diseases, and chronic diseases, but also serve as potential triggers or initiators for certain tumors. They directly or indirectly damage human health and are one of the leading causes of global deaths. Small ubiquitin-like modifier (SUMO) modification, a type of protein post-translational modification (PTM) that occurs when SUMO groups bond covalently to particular lysine residues on substrate proteins, plays a crucial role in both innate and adaptive immunologic responses, as well as pathogen-host immune system crosstalk. SUMOylation participates in the host's defense against pathogens by regulating immune responses, while numerically vast and taxonomically diverse pathogens have evolved to exploit the cellular SUMO modification system to break through innate defenses. Here, we describe the characteristics and multiple functions of SUMOylation as a pivotal PTM mechanism, the tactics employed by various pathogens to counteract the immune system through targeting host SUMOylation, and the character of the SUMOylation system in the fight between pathogens and the host immune system. We have also included a summary of the potential anti-pathogen SUMO enzyme inhibitors. This review serves as a reference for basic research and clinical practice in the diagnosis, prognosis, and treatment of pathogenic microorganism-caused disorders.

KSHV hijacks the antiviral kinase IKKε to initiate lytic replication

IKKε is a traditional antiviral kinase known for positively regulating the production of type I interferon (IFN) and the expression of IFN-stimulated genes (ISGs) during various virus infections. However, through an inhibitor screen targeting cellular kinases, we found that IKKε plays a crucial role in the lytic replication of Kaposi's sarcoma-associated herpesvirus (KSHV). Mechanistically, during KSHV lytic replication, IKKε undergoes significant SUMOylation at both Lys321 and Lys549 by the viral SUMO E3 ligase ORF45. This SUMOylation event leads to the association of IKKε with PML, resulting in the disruption of PML nuclear bodies (PML NBs) and subsequent increase in lytic replication of KSHV. Notably, IKKε does not affect the total expression level of PML but facilitates the translocation of PML from the nucleus to the cytoplasm during KSHV lytic replication. Further experiments utilizing mutations on the SUMOylation sites of IKKε or inhibiting IKKε using BAY-985 showed that these actions no longer impact PML NBs and completely suppress the lytic replication of KSHV. These findings not only emphasize the essential role of IKKε in the life cycle of KSHV but also illustrate how KSHV exploits IKKε through SUMOylation modification to enhance its own replication process.

Conserved linear motif within the immediate early protein ORF45 promotes its engagement with KSHV lytic cycle-promoting forkhead transcription factors, FOXK1 and FOXK2

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus that can cause several cancers, such as Kaposi sarcoma and primary effusion lymphoma (PEL). We and others have recently demonstrated that Forkhead box (FOX) transcription factors can be dysregulated by KSHV, and they can affect KSHV infection. Herein, we focus on dissecting the role of two FOXK subfamily members, FOXK1 and FOXK2, in the KSHV life cycle. FOXK proteins are key host regulators of cellular functions, yet their role in KSHV infection remains unknown. Here, we demonstrated that both FOXK proteins are essential for efficient KSHV lytic reactivation in PEL cells. FOXK1 and FOXK2 are unique as they are the only FOX proteins that contain a Forkhead-associated (FHA) domain. The FHA domain is a specialized protein binding domain that recognizes a short linear serine/threonine-rich (S/T) motif. Through an unbiased motif survey, we found that KSHV viral protein ORF45 and its gammaherpesvirus homologs contain a putative FHA-binding motif. ORF45 is an immediate early tegument protein, vital for lytic reactivation and virus production. We demonstrated that ORF45 uses its novel conserved motif to interact with the FHA domain containing FOXK factors in the nucleus of infected cells. A single-point mutation of the conserved threonine residue in the motif within ORF45 abolished the ORF45-FOXK1/2 interaction. Our data indicates that FOXK proteins interact with ORF45 homologs encoded by murine gammaherpesvirus 68 (MHV68) and Rhesus macaque rhadinovirus (RRV), and that the FHA domains of FOXK proteins are sufficient for their interactions, highlighting a conserved mechanism.IMPORTANCEThe dysregulation of Forkhead transcription factors contributes to many different human diseases, including cancers, but their impact on herpesvirus lifecycle and pathogenesis is less understood. Our study uncovers a critical pro-lytic function of the FOXK subfamily and its requirement for KSHV lytic reactivation in PEL. We found that FOXK proteins bind to a key immediate early KSHV protein ORF45 using its novel short linear S/T motif. Notably, even though ORF45 homologs in gammaherpesviruses are highly diverse, we identified a similar S/T short linear motif in ORF45 homologs and also showed an evolutionary conserved interaction between FOXK proteins and ORF45 homologs of MHV68 and RRV. Our study provides a basis for future studies in animal models to evaluate the role of FOXK proteins and the impact of their interactions with ORF45 in gammaherpesvirus infection and pathogenesis. Targeting these interactions could allow a novel way to limit gammaherpesvirus infections.

Porcine circovirus type 2 infection promotes the SUMOylation of nucleophosmin-1 to facilitate the viral circular single-stranded DNA replication

The mechanism of genome DNA replication in circular single-stranded DNA viruses is currently a mystery, except for the fact that it undergoes rolling-circle replication. Herein, we identified SUMOylated porcine nucleophosmin-1 (pNPM1), which is previously reported to be an interacting protein of the viral capsid protein, as a key regulator that promotes the genome DNA replication of porcine single-stranded DNA circovirus. Upon porcine circovirus type 2 (PCV2) infection, SUMO2/3 were recruited and conjugated with the K263 site of pNPM1's C-terminal domain to SUMOylate pNPM1, subsequently, the SUMOylated pNPM1 were translocated in nucleoli to promote the replication of PCV2 genome DNA. The mutation of the K263 site reduced the SUMOylation levels of pNPM1 and the nucleolar localization of pNPM1, resulting in a decrease in the level of PCV2 DNA replication. Meanwhile, the mutation of the K263 site prevented the interaction of pNPM1 with PCV2 DNA, but not the interaction of pNPM1 with PCV2 Cap. Mechanistically, PCV2 infection increased the expression levels of Ubc9, the only E2 enzyme involved in SUMOylation, through the Cap-mediated activation of ERK signaling. The upregulation of Ubc9 promoted the interaction between pNPM1 and TRIM24, a potential E3 ligase for SUMOylation, thereby facilitating the SUMOylation of pNPM1. The inhibition of ERK activation could significantly reduce the SUMOylation levels and the nucleolar localization of pNPM1, as well as the PCV2 DNA replication levels. These results provide new insights into the mechanism of circular single-stranded DNA virus replication and highlight NPM1 as a potential target for inhibiting PCV2 replication.

Characterization of BoHV-4 ORF45

Bovine herpesvirus 4 (BoHV-4) is a Gammaherpesvirus belonging to the Rhadinovirus genus. The bovine is BoHV-4's natural host, and the African buffalo is BoHV-4's natural reservoir. In any case, BoHV-4 infection is not associated with a specific disease. Genome structure and genes are well-conserved in Gammaherpesvirus, and the orf 45 gene and its product, ORF45, are one of those. BoHV-4 ORF45 has been suggested to be a tegument protein; however, its structure and function have not yet been experimentally characterized. The present study shows that BoHV-4 ORF45, despite its poor homology with other characterized Rhadinovirus ORF45s, is structurally related to Kaposi's sarcoma-associated herpesvirus (KSHV), is a phosphoprotein, and localizes in the host cell nuclei. Through the generation of an ORF45-null mutant BoHV-4 and its pararevertant, it was possible to demonstrate that ORF45 is essential for BoHV-4 lytic replication and is associated with the viral particles, as for the other characterized Rhadinovirus ORF45s. Finally, the impact of BoHV-4 ORF45 on cellular transcriptome was investigated, an aspect poorly explored or not at all for other Gammaherpesvirus. Many cellular transcriptional pathways were found to be altered, mainly those involving p90 ribosomal S6 kinase (RSK) and signal-regulated kinase (ERK) complex (RSK/ERK). It was concluded that BoHV-4 ORF45 has similar characteristics to those of KSHV ORF45, and its unique and incisive impact on the cell transcriptome paves the way for further investigations.

The ORF45 Protein of Kaposi's Sarcoma-Associated Herpesvirus and Its Critical Role in the Viral Life Cycle

Kaposi's sarcoma-associated herpesvirus (KSHV) protein ORF45 is a virion-associated tegument protein that is unique to the gammaherpesvirus family. Generation of KSHV ORF45-knockout mutants and their subsequent functional analyses have permitted a better understanding of ORF45 and its context-specific and vital role in the KSHV lytic cycle. ORF45 is a multifaceted protein that promotes infection at both the early and late phases of the viral life cycle. As an immediate-early protein, ORF45 is expressed within hours of KSHV lytic reactivation and plays an essential role in promoting the lytic cycle, using multiple mechanisms, including inhibition of the host interferon response. As a tegument protein, ORF45 is necessary for the proper targeting of the viral capsid for envelopment and release, affecting the late stage of the viral life cycle. A growing list of ORF45 interaction partners have been identified, with one of the most well-characterized being the association of ORF45 with the host extracellular-regulated kinase (ERK) p90 ribosomal s6 kinase (RSK) signaling cascade. In this review, we describe ORF45 expression kinetics, as well as the host and viral interaction partners of ORF45 and the significance of these interactions in KSHV biology. Finally, we discuss the role of ORF45 homologs in gammaherpesvirus infections.