Regulation of PKR By Viral RNAS

Abstract

PKR is an interferon-induced kinase that plays a key role in the innate immunity response to viral infection. Upon binding dsRNA, PKR undergoes autophosphorylation reactions that activate the kinase. We have investigated the mechanism of PKR activation by two viral RNAs that regulate PKR. HIV-I TAR is a 23 bp RNA hairpin with three bulges that is known to dimerize. A single PKR binds with moderate affinity to TAR monomer whereas dimers bind two PKRs. TAR dimers activate PKR whereas monomers do not. The secondary structure defects in the TAR RNA stem function as antideterminants to PKR binding and activation. Our results support a model where dimerization of the TAR RNA hairpin facilitates sequential binding of two PKR monomers, leading to protein dimerization and subsequent activation. Adenovirus VAI is a 160 nt highly structured RNA that inhibits activation of PKR by dsRNA. The stoichiometry and affinity of PKR binding to VAI are regulated by Mg2+. In the presence of 5 mM Mg2+, PKR binds similarly to VAI and to a truncation mutant lacking the terminal stem, indicating that this region of VAI is dispensable for regulation of PKR activation.View Large Image | View Hi-Res Image | Download PowerPoint Slide