Simian varicella virus gene 61 encodes a viral transactivator but is non-essential for in vitro replication

Abstract

Simian varicella virus (SVV) is closely related to varicella-zoster virus (VZV), the causative agent of chickenpox and shingles. The SVV and VZV gene 61 polypeptides are homologs of the HSV-1 ICP0, a viral transactivator which appears to play a role in viral latency and reactivation. In this study, the molecular properties of the SVV 61 were characterized. The SVV open reading frame (ORF) 61 encodes a 54.1-kDa polypeptide with 37% amino acid identity to the VZV 61. Homology to the HSV-1 ICP-0 is limited to a conserved RING finger motif at the amino terminus of the protein. A nuclear localization sequence (nls) at the carboxy-terminus directs the SVV 61 to the cell nucleus, while a SVV 61nls(-) mutant is confined to the cell cytoplasm. The SVV 61 transactivates its own promoter as well as SVV immediate early (IE, ORF 62), early (ORFs 28 and 29), and late (ORF 68) gene promoters in transfected Vero cells. The RING finger and nls motifs are required for efficient SVV 61 transactivation. The SVV 61 has no effect on the ability of the major SVV transactivator (IE62) to induce SVV promoters. Generation and propagation of a SVV gene 61 deletion mutant demonstrated that the SVV 61 is non-essential for in vitro replication. SVV gene 61 is expressed in liver, lung, and neural ganglia of infected monkeys during acute simian varicella.

Simian Varicella Virus Expresses a Latency-Associated Transcript That Is Antisense to Open Reading Frame 61 (ICP0) mRNA in Neural Ganglia of Latently Infected Monkeys

“…Primer set 61-5 did not detect ORF 61 RNA, indicating that the left-most terminus of the ORF 61 transcript maps within a 132-bp region between SVV DNA nt 102,392 and 102,261. Neither SVV ORF 61 transcripts nor cellular transcripts homologous to ORF 61 were detected in neural ganglia derived from an uninfected monkey, as confirmed in a previous study (15).…”

“…SVV ORF 61, which is located at nucleotides (nt) 102,915 to 104,426 of the SVV genome ( Fig. 1), is a homolog of the HSV-1 ICP0 gene and VZV gene 61 and encodes a viral protein that transactivates viral IE, early, and late genes (15). This study demonstrates that SVV expresses a LAT that is antisense to the ORF 61 mRNA in the neural ganglia of latently infected vervet monkeys.…”

“…The finding that SVV gene 61 mRNA predominates in acutely infected ganglia while the ORF 61 antisense LAT predominates in latently infected ganglia suggests that the balance of these transcripts may play a role in the molecular basis of SVV latency and reactivation. Possibly, viral latency is maintained by LAT binding to the complementary gene 61 mRNA, thus inhibiting translation of the ORF 61 protein, a transactivator of SVV IE, early, and late gene expression (15). Conversely, viral reactivation could be induced by changes in the neuronal cell environment, resulting in a shift in the gene 61 mRNA/ LAT ratio such that the ORF 61 transactivator protein is expressed.…”

The ORF61 Protein Encoded by Simian Varicella Virus and Varicella-Zoster Virus Inhibits NF-κB Signaling by Interfering with IκBα Degradation

“…We further demonstrate that SVV ORF61, which shares 42.8% amino acid identity with VZV ORF61 (4) and regulates viral gene expression (32), also prevents IB␣ degradation. Interestingly, IB␣ phosphorylation was allowed but degradation was prevented in ORF61-expressing cells stimulated with TNF-␣, resulting in the accumulation of phosphorylated IB␣ (pIB␣).…”

“…The inhibition of NF-B activation observed in VZV-infected cells has been attributed to the ORF61 protein (21). VZV and SVV ORF61 proteins share 42.8% overall amino acid identity (4), and both proteins are implicated in the transactivation of expression of other viral genes (26,32). To assess whether the ORF61 protein was responsible for the inhibition of NF-B signaling in SVV-infected cells, we used recombinant adenovectors to ectopically express the protein.…”