Regulation of Protein Kinase R by Epstein-Barr Virus EBER1 RNA.

Protein kinase R (PKR) is a key antiviral component of the innate immune pathway and is activated by viral double-stranded RNAs (dsRNAs). Adenovirus-associated RNA 1 (VAI) is an abundant, noncoding viral RNA that functions as a decoy by binding PKR but not inducing activation, thereby inhibiting the antiviral response. In VAI, coaxial stacking produces an extended helix that mediates high-affinity PKR binding but is too short to result in activation. Like adenovirus, Epstein-Barr virus produces high concentrations of a noncoding RNA, EBER1. Here, we compare interactions of PKR with VAI and EBER1 and present a structural model of EBER1. Both RNAs function as inhibitors of dsRNA-mediated PKR activation. However, EBER1 weakly activates PKR whereas VAI does not. PKR binds EBER1 more weakly than VAI. Assays at physiological ion concentrations indicate that both RNAs can accommodate two PKR monomers and induce PKR dimerization. A structural model of EBER1 was obtained using constraints derived from chemical structure probing and small-angle X-ray scattering experiments. The central stem of EBER1 coaxially stacks with stem loop 4 and stem loop 1 to form an extended RNA duplex of ∼32 bp that binds PKR and promotes activation. Our observations that EBER1 binds PKR much more weakly than VAI and exhibits weak PKR activation suggest that EBER1 is less well suited to function as an RNA decoy.