Recognition of viral RNA stem-loops by the tandem double-stranded RNA binding domains of PKR.

In humans, the double-stranded RNA (dsRNA)-activated protein kinase (PKR) is expressed in late stages of the innate immune response to viral infection by the interferon pathway. PKR consists of tandem dsRNA binding motifs (dsRBMs) connected via a flexible linker to a Ser/Thr kinase domain. Upon interaction with viral dsRNA, PKR is converted into a catalytically active enzyme capable of phosphorylating a number of target proteins that often results in host cell translational repression. A number of high-resolution structural studies involving individual dsRBMs from proteins other than PKR have highlighted the key features required for interaction with perfectly duplexed RNA substrates. However, viral dsRNA molecules are highly structured and often contain deviations from perfect A-form RNA helices. By use of small-angle X-ray scattering (SAXS), we present solution conformations of the tandem dsRBMs of PKR in complex with two imperfectly base-paired viral dsRNA stem-loops; HIV-1 TAR and adenovirus VA(I)-AS. Both individual components and complexes were purified by size exclusion chromatography and characterized by dynamic light scattering at multiple concentrations to ensure monodispersity. SAXS ab initio solution conformations of the individual components and RNA-protein complexes were determined and highlight the potential of PKR to interact with both stem and loop regions of the RNA. Excellent agreement between experimental and model-based hydrodynamic parameter determination heightens our confidence in the obtained models. Taken together, these data support and provide a framework for the existing biochemical data regarding the tolerance of imperfectly base-paired viral dsRNA by PKR.