Residues R199H200 of prototype foamy virus transactivator Bel1 contribute to its binding with LTR and IP promoters but not its nuclear localization

Prototype foamy virus encodes a transactivator called Bel1 that enhances viral gene transcription and is essential for PFV replication. Nuclear localization of Bel1 has been reported to rely on two proximal basic motifs R199H200 and R221R222R223 that likely function together as a bipartite nuclear localization signal. In this study, we report that mutating R221R222R223, but not R199H200, relocates Bel1 from the nucleus to the cytoplasm, suggesting an essential role for R221R222R223 in the nuclear localization of Bel1. Although not affecting the nuclear localization of Bel1, mutating R199H200 disables Bel1 from transactivating PFV promoters. Results of EMSA reveal that the R199H200 residues are vital for the binding of Bel1 to viral promoter DNA. Moreover, mutating R199H200 in Bel1 impairs PFV replication to a much greater extent than mutating R221R222R223. Collectively, our findings suggest that R199H200 directly participate in Bel1 binding to viral promoter DNA and are indispensible for Bel1 transactivation activity.