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-stran ded (ds) RNA-binding proteins thought to be involved in gene expression. Or iginally 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 d sRNA-activated protein kinase, PKR. In this report, we characterize the bio chemical interactions between these two dsRBM containing proteins. NF90 bin ds to PKR through two independent mechanisms: an RNA-independent interactio n occurs between the N terminus of NF90 and the C-terminal region of PKR, a nd an RNA-dependent interaction is mediated by the dsRBMs of the two protei ns. Co-immunoprecipitation analysis demonstrates that NF90, NF45, and PKR f orm a complex in both nuclear and cytosolic extracts, and both proteins ser ve 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-termi nal region. The C-terminal region also inhibits PKR function, probably thro ugh competitive binding to dsRNA. A model for NF90-PKR interactions is prop osed.