Nuclear Factor 90 Is a Substrate and Regulator of the Eukaryotic Initiation Factor 2 Kinase Double-stranded RNA-activated Protein Kinase

Nuclear factor 90 (NF90) is a member of an expanding family of double-stranded (ds) RNA-binding proteins thought to be involved in gene expression. Originally identified in complex with nuclear factor 45 (NF45) as a sequence-specific DNA-binding protein, NF90 contains two double stranded RNA-binding motifs (dsRBMs) and interacts with highly structured RNAs as well as the dsRNA-activated protein kinase, PKR. In this report, we characterize the biochemical interactions between these two dsRBM containing proteins. NF90 binds to PKR through two independent mechanisms: an RNA-independent interaction occurs between the N terminus of NF90 and the C-terminal region of PKR, and an RNA-dependent interaction is mediated by the dsRBMs of the two proteins. Co-immunoprecipitation analysis demonstrates that NF90, NF45, and PKR form a complex in both nuclear and cytosolic extracts, and both proteins serve as substrates for PKR in vitro. NF90 is phosphorylated by PKR in its RNA-binding domain, and this reaction is partially blocked by the NF90 N-terminal region. The C-terminal region also inhibits PKR function, probably through competitive binding to dsRNA. A model for NF90-PKR interactions is proposed.