Epstein-Barr virus (EBV) expresses an immediate-early protein, Rta, to activate the viral lytic cycle. This study identifies PIASxα and PIASxβ as binding partners of Rta in a yeast two-hybrid screen and demonstrates the binding of Rta to PIASxα and PIASxβ in vitro by GST pull-down analysis. Coimmunoprecipitation experiments and indirect immunofluorescence analysis show that Rta interacts and colocalizes with PIASxα and PIASxβ in the nucleus. These interactions seem to enhance Rta sumoylation as transfecting plasmids expressing PIASxα, PIASxβ, Ubc9, or SUMO-1 increase the capacity of Rta to transactivate a promoter that contains an Rta-response element and the promoters of p21 and BNLF1 in transient transfection assay. This study also finds that Rta sumoylation is preferentially enhanced by PIASxβ, which could be attributed to the fact that PIASxβ, compared to PIASxα, has a strong affinity to Rta, suggesting that affinity of a SUMO E3 ligase to its target protein influences the function of protein sumoylation.
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