Structure of the VP16 Transactivator Target in ARC/Mediator

The human Activator-Recruited Cofactor (ARC)/Mediator co-activator complex interacts with many transcriptional activators and facilitates recruitment of RNA polymerase II to promote target gene transcription. The MED25 (ARC92) subunit is a critical target of the potent Herpes simplex 1 viral transcriptional activator VP16. Here, we determine the solution structure of the MED25 VP16-binding domain (VBD), and define its binding site for the N-terminal portion of the VP16 transactivation domain (TADn). A hydrophobic furrow, formed by a β-barrel and two α-helices in MED25 VBD, interacts tightly with VP16 TADn. Mutations in this furrow prevent binding of VP16 TAD to MED25 VBD and interfere with the ability of over-expressed MED25 VBD to inhibit VP16-dependent transcriptional activation in vivo. This detailed molecular understanding of transactivation by the benchmark activator VP16 could provide important insights into viral and cellular gene activation mechanisms.