Structure of the VP16 transactivator target in the Mediator

Abstract

Herpesvirus VP16 is a classic example of a transcriptional activator and interacts with the Mediator complex via MED25 (ARC92). The structure of the VP16 interaction domain from MED25 is now solved and the binding site for the VP16 transactivation domain defined, giving insight into how a transcription activator contacts a key component involved in transcription initiation. The human Mediator coactivator complex interacts with many transcriptional activators and facilitates recruitment of RNA polymerase II to promote target gene transcription. The MED25 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 overexpressed 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.