VZV ORF47 serine protein kinase and its viral substrates

Mechanism of herpesvirus protein kinase UL13 in immune escape and viral replication

Upon infection, the herpes viruses create a cellular environment suitable for survival, but innate immunity plays a vital role in cellular resistance to viral infection. The UL13 protein of herpesviruses is conserved among all herpesviruses and is a serine/threonine protein kinase, which plays a vital role in escaping innate immunity and promoting viral replication. On the one hand, it can target various immune signaling pathways in vivo, such as the cGAS-STING pathway and the NF-κB pathway. On the other hand, it phosphorylates regulatory many cellular and viral proteins for promoting the lytic cycle. This paper reviews the research progress of the conserved herpesvirus protein kinase UL13 in immune escape and viral replication to provide a basis for elucidating the pathogenic mechanism of herpesviruses, as well as providing insights into the potential means of immune escape and viral replication of other herpesviruses that have not yet resolved the function of it.

Varicella-Zoster Virus Activates CREB, and Inhibition of the pCREB-p300/CBP Interaction Inhibits Viral Replication In Vitro and Skin Pathogenesis In Vivo

Varicella-zoster virus (VZV) is an alphaherpesvirus that causes varicella upon primary infection and zoster upon reactivation from latency in sensory ganglion neurons. The replication of herpesviruses requires manipulation of cell signaling pathways. Notably, CREB, a factor involved in the regulation of several cellular processes, is activated upon infection of T cells with VZV. Here, we report that VZV infection also induced CREB phosphorylation in fibroblasts and that XX-650-23, a newly identified inhibitor of the phosphorylated-CREB (pCREB) interaction with p300/CBP, restricted cell-cell spread of VZV in vitro CREB phosphorylation did not require the viral open reading frame 47 (ORF47) and ORF66 kinases encoded by VZV. Evaluating the biological relevance of these observations during VZV infection of human skin xenografts in the SCID mouse model of VZV pathogenesis showed both that pCREB was upregulated in infected skin and that treatment with XX-650-23 reduced infectious-virus production and limited lesion formation compared to treatment with a vehicle control. Thus, processes of CREB activation and p300/CBP binding are important for VZV skin infection and may be targeted for antiviral drug development.

IMPORTANCE

Varicella-zoster virus (VZV) is a common pathogen that causes chicken pox and shingles. As with all herpesviruses, the infection is acquired for life, and the virus can periodically reactivate from latency. Although VZV infection is usually benign with few or no deleterious consequences, infection can be life threatening in immunocompromised patients. Otherwise healthy elderly individuals who develop zoster as a consequence of viral reactivation are at risk for postherpetic neuralgia (PHN), a painful and long-lasting complication. Current vaccines use a live attenuated virus that is usually safe but cannot be given to many immunodeficient patients and retains the capacity to establish latency and reactivate, causing zoster. Antiviral drugs are effective against severe VZV infections but have little impact on PHN. A better understanding of virus-host cell interactions is relevant for developing improved therapies to safely interfere with cellular processes that are crucial for VZV pathogenesis.

ORF9p phosphorylation by ORF47p is crucial for the formation and egress of varicella-zoster virus viral particles

The role of the tegument during the herpesvirus lytic cycle is still not clearly established, particularly at the late phase of infection, when the newly produced viral particles need to be fully assembled before being released from the infected cell. The varicella-zoster virus (VZV) protein coded by open reading frame (ORF) 9 (ORF9p) is an essential tegument protein, and, even though its mRNA is the most expressed during the productive infection, little is known about its functions. Using a GalK positive/negative selection technique, we modified a bacterial artificial chromosome (BAC) containing the complete VZV genome to create viruses expressing mutant versions of ORF9p. We showed that ORF9p is hyperphosphorylated during the infection, especially through its interaction with the viral Ser/Thr kinase ORF47p; we identified a consensus site within ORF9p recognized by ORF47p and demonstrated its importance for ORF9p phosphorylation. Strikingly, an ultrastructural analysis revealed that the mutation of this consensus site (glutamate 85 to arginine) strongly affects viral assembly and release, reproducing the ORF47 kinase-dead VZV phenotype. It also slightly diminishes the infectivity toward immature dendritic cells. Taken together, our results identify ORF9p as a new viral substrate of ORF47p and suggest a determinant role of this phosphorylation for viral infectivity, especially during the process of viral particle formation and egress.

Mutagenesis of varicella-zoster virus glycoprotein I (gI) identifies a cysteine residue critical for gE/gI heterodimer formation, gI structure, and virulence in skin cells

Varicella-zoster virus (VZV) is the alphaherpesvirus that causes chicken pox (varicella) and shingles (zoster). The two VZV glycoproteins gE and gI form a heterodimer that mediates efficient cell-to-cell spread. Deletion of gI yields a small-plaque-phenotype virus, ΔgI virus, which is avirulent in human skin using the xenograft model of VZV pathogenesis. In the present study, 10 mutant viruses were generated to determine which residues were required for the typical function of gI. Three phosphorylation sites in the cytoplasmic domain of gI were not required for VZV virulence in vivo. Two deletion mutants mapped a gE binding region in gI to residues 105 to 125. A glycosylation site, N116, in this region did not affect virulence. Substitution of four cysteine residues highly conserved in the Alphaherpesvirinae established that C95 is required for gE/gI heterodimer formation. The C95A and Δ105-125 (with residues 105 to 125 deleted) viruses had small-plaque phenotypes with reduced replication kinetics in vitro similar to those of the ΔgI virus. The Δ105-125 virus was avirulent for human skin in vivo. In contrast, the C95A mutant replicated in vivo but with significantly reduced kinetics compared to those of the wild-type virus. In addition to abolished gE/gI heterodimer formation, gI from the C95A or the Δ105-125 mutant was not recognized by monoclonal antibodies that detect the canonical conformation of gI, demonstrating structural disruption of gI in these viruses. This alteration prevented gI incorporation into virus particles. Thus, residues C95 and 105 to 125 are critical for gI structure required for gE/gI heterodimer formation, virion incorporation, and ultimately, effective viral spread in human skin.

Human cytomegalovirus UL97 kinase and nonkinase functions mediate viral cytoplasmic secondary envelopment

Previous studies have revealed critical roles for the human cytomegalovirus (HCMV) UL97 kinase in viral nuclear maturation events. We have shown recently that UL97 affects the morphology of the viral cytoplasmic assembly compartment (AC) (M. Azzeh, A. Honigman, A. Taraboulos, A. Rouvinski, and D. G. Wolf, Virology 354:69-79, 2006). Here, we employed a comprehensive ultrastructural analysis to dissect the impact of UL97 on cytoplasmic steps of HCMV assembly. Using UL97 deletion (ΔUL97) and kinase-null (K355M) mutants, as well as the UL97 kinase inhibitor NGIC-I, we demonstrated that the loss of UL97 kinase activity resulted in a unique combination of cytoplasmic features: (i) the formation of pp65-rich aberrant cytoplasmic tegument aggregates, (ii) distorted intracytoplasmic membranes, which replaced the normal architecture of the AC, and (iv) a paucity of cytoplasmic tegumented capsids and dense bodies (DBs). We further showed that these abnormal assembly intermediates did not result from impaired nuclear capsid maturation and egress per se by using 2-bromo-5,6-dichloro-1-(β-d-ribofuranosyl) benzimidizole (BDCRB) to induce the artificial inhibition of nuclear maturation and the nucleocytoplasmic translocation of capsids. The specific abrogation of UL97 kinase activity under low-multiplicity-of-infection conditions resulted in the improved release of extracellular virus compared to that of ΔUL97, despite similar rates of viral DNA accumulation and similar effects on nuclear capsid maturation and egress. The only ultrastructural correlate of the growth difference was a higher number of cytoplasmic DBs, tegumented capsids, and clustered viral particles observed upon the specific abrogation of UL97 kinase activity compared to that of ΔUL97. These combined findings reveal a novel role for UL97 in HCMV cytoplasmic secondary envelopment steps, with a further distinction of kinase-mediated function in the formation of the virus-induced AC and a nonkinase function enhancing the efficacy of viral tegumentation and release.

Viral serine/threonine protein kinases

Phosphorylation represents one the most abundant and important posttranslational modifications of proteins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is variable, they invariably play important roles in virus virulence. The current review provides an overview of the different viral serine/threonine protein kinases of several large DNA viruses and discusses their function, importance, and potential as antiviral drug targets.