The KSHV RNA regulator ORF57: target specificity and its role in the viral life cycle

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes ORF57, which enhances the expression of intron-less KSHV genes on multiple post-transcriptional levels mainly affecting RNA stability and export to the cytoplasm. Yet, it remains elusive how ORF57 recognizes viral RNAs and discriminates them from cellular messenger RNAs (mRNAs). Although one common binding motif on three separate KSHV RNAs has been described, most other lytic genes lack this sequence element. In this article we will review the sequence requirements for ORF57 to enhance RNA expression and discuss a model how ORF57 achieves specificity for viral RNAs. Finally, the role of ORF57 is integrated into the viral life cycle as a complex interplay with other viral and host factors and with implications for cellular gene expression.

Quantitative Proteomics Analysis of Lytic KSHV Infection in Human Endothelial Cells Reveals Targets of Viral Immune Modulation

“…We observe three groups of viral proteins in which (1) expression of KSHV ORFs is strongly inhibited by ORF57 knockdown, (2) expression is moderately inhibited by ORF57 knockdown, and (3) expression is independent of or enhanced by ORF57 knockdown. Thus, our data on KSHV gene products that require ORF57 for efficient expression confirm the results of previous studies ( Majerciak et al., 2007 ; Verma et al., 2015 ; Vogt and Bohne, 2016 ) and extend the list of ORF57-dependent viral gene products.…”

“…Together with 200 cellular proteins, these 4 viral proteins make up 20% of total protein abundance in the cell. The functions of ORF25, ORF57, and ORF59 are well defined: ORF25 is a major capsid protein that antagonizes p53-mediated apoptosis ( Chudasama et al., 2015 ), ORF57 is a potent posttranscriptional regulator of lytic KSHV gene expression ( Majerciak and Zheng, 2015 ; Vogt and Bohne, 2016 ), and ORF59 is a viral DNA polymerase processivity factor that transports viral DNA polymerase into the nucleus for efficient KSHV DNA synthesis ( Chen et al., 2005 ). The role of ORF6 is less clear.…”

“…Both families represent therapeutic targets for cancer treatment ( Bassiouni et al., 2016 ; Boudesco et al., 2018 ; Carr et al., 2017 ). Poly(A) RNA binding was the most abundant category (74 proteins) of upregulated proteins ( Figure 5 C), and many members of this group are reported to associate with KSHV ORF57, the key player in viral mRNA processing ( Majerciak and Zheng, 2015 ; Vogt and Bohne, 2016 ) ( Table 1 ). The GO terms in the proteins downregulated by lytic KSHV were particularly enriched in surface receptors, protein kinases, and related biological processes, such as cell adhesion and signal transduction ( Figures 5 D and S5 C–S5E).…”

“…ORF57 is a multifunctional KSHV protein that is known to stabilize viral transcripts, promote nuclear export, and enhance translation of several lytic viral gene products ( Majerciak and Zheng, 2015 ). Although several studies previously focused on identification of KSHV transcripts and proteins whose expression depends on ORF57 ( Majerciak et al., 2007 ; Verma et al., 2015 ; Vogt and Bohne, 2016 ), the complete set of KSHV proteins that require ORF57 for efficient expression remains unknown. To generate a KSHV CRISPR ORF57 mutant, we transduced HuAR2T.rKSHV.219 Cas9 cells with control or ORF57-specific sgRNAs and analyzed the ORF57 knockdown upon KSHV reactivation by immunoblot.…”

Structural identification of conserved RNA binding sites in herpesvirus ORF57 homologs: implications for PAN RNA recognition

“…Several PAN RNA interaction sites have been identified in the KSHV episome and the cellular genome, indicating that this viral long non-coding RNA modulates viral and cellular gene expression (20). ksORF57 is also a potent multifunctional regulator of gene expression that functions at a post-transcriptional level (21,22), and is necessary for productive viral replication (23,24). ksORF57 binds selectively to transcripts, especially intronless viral RNAs, stabilizing them, which results in their nuclear accumulation (25–29).…”