Kaposi's Sarcoma-Associated Herpesvirus ORF57 Protein Enhances mRNA Accumulation Independently of Effects on Nuclear RNA Export

Abstract

The ORF57 protein expressed by Kaposi's sarcoma-associated herpesvirus (KSHV) during lytic replication is essential for KSHV virion production. ORF57 enhanced the nuclear levels of mRNA and PAN, a nuclear KSHV RNA, and the activity of various ORF57 mutants on the levels of mRNA paralleled their ability to enhance nuclear PAN accumulation, suggesting that ORF57 may also act on messenger RNAs by export-independent effects on RNA stability. Finally, an ORF57 mutant lacking a region homologous to a nucleolar localization signal in herpesvirus saimiri was constructed. This mutant retained function, demonstrating that, unlike the ORF57 homolog in herpesvirus saimiri, nucleolar trafficking is not required for ORF57 function in enhancing mRNA accumulation.

Binding of Cellular Export Factor REF/Aly by Kaposi's Sarcoma-Associated Herpesvirus (KSHV) ORF57 Protein Is Not Required for Efficient KSHV Lytic Replication

“…In contrast to these findings, REF/Aly knockdown was reported to have a negligible effect on ORF57 enhancement of gene expression in KSHV-infected cells (31). Furthermore, in a separate report, ORF57 mutants unable to bind REF/Aly nevertheless retained partial ability to enhance KSHV target mRNA accumulation (36).…”

“…According to this model, KSHV lytic mRNAs, the majority of which are intronless, are inefficiently exported due to an inability to assemble ribonucleoprotein transcription/export (TREX) complexes whose formation is associated with the process of premRNA splicing (4). ORF57 binds the cellular RNA-binding protein, REF/Aly, which is a component of TREX, via an RNA-independent protein interaction (33,36). A site-specific mutant of ORF57 carrying two amino acid changes in the REF/Aly binding domain (PP208,211AA) was shown to be incapable of binding REF/Aly (4).…”

Rhesus Monkey Rhadinovirus ORF57 Induces gH and gL Glycoprotein Expression through Posttranscriptional Accumulation of Target mRNAs

“…Examples include translation and mRNA stability, the nuclear egress of mRNA and mRNA stability, and the transcription and nuclear egress of mRNA. In looking at the literature on ORF57 action, it is often difficult to distinguish what is multifunctional activity from what is a single activity manifest in different, linked, functional processes (6,17,20,26,32). Similar difficulties can be seen in the data reported here.…”

“…Still, the fact that nut-1/PAN RNA, which always localizes to the nucleus and is not exported to the cytoplasm (9,39), is enhanced by ORF57 (15,20,25) supports the idea that enhanced RNA accumulation in this case is independent of nuclear export (35). Furthermore, the nuclear and cytoplasmic fractionation of KSHV ORF59 RNA showed no sign of enhancement of nuclear RNA export (32).…”

“…The timing and intensity of viral gene expression is orchestrated by comprehensive, interrelated regulatory mechanisms. ORF57 is expressed in the immediate-early phase of lytic replication (20,24,28) and is a key orchestrator of the viral lytic cycle by inducing delayed early and late gene expression (15,20,25,26,32). The extremely low levels of expression of envelope glycoproteins until the final stage of lytic replication may serve in part to limit any cytotoxicity of virus-encoded glycoproteins and may also aid virus replication by limiting the exposure of these antigens to the host immune response.…”