The evolutionarily conserved Kaposi's sarcoma-associated herpesvirus ORF57 protein interacts with REF protein and acts as an RNA export factor

ORF57 (MTA) one of the earliest Kaposi's sarcoma-associated herpesvirus (KSHV) regulatory proteins to be expressed is essential for virus lytic replication. A counterpart is present in every herpesvirus sequenced, indicating the importance of this signature viral protein and those examined act post-transcriptionally, affecting RNA splicing and transport. In KSHV-infected cells, ORF57 protein was present in a complex with REF (Aly) and TAP (NXF1), factors involved in cellular mRNA export. The ORF57 N-terminal region interacts with REF, whereas both N- and C-terminal domains of REF interact with ORF57. The ORF57-REF interaction was direct, whereas TAP appeared to be recruited via REF. In somatic cells, ectopically expressed ORF57 protein was shown to function as a CRM1-independent nuclear mRNA export factor, promoting export of mRNAs that are poor substrates for splicing. The γ-herpesvirus ORF57 protein, and its α-1 herpesvirus ICP27 counterpart both export RNA through pathways involving REF and TAP proteins, although divergence of these herpesvirus subfamilies occurred some 180-210 million years ago. The TAP-mediated cellular mRNA export pathway is CRM1-independent. However, human immunodeficiency virus type 1 Rev protein-mediated RNA export, which is CRM1-dependent, was considerably inhibited by ORF57, suggesting that Rev and ORF57 compete for a common export component. These data strengthen arguments that TAP and CRM1 pathways converge in accessing similar components of the nuclear pore complex. We propose that ORF57-mediated RNA export may use different export factors to accommodate the KSHV-infected host cell environments, for example, in B-cells or endothelial cells and during the different phases of lytic virus replication.