Nuclear factor-90 of activated T-cells: A double-stranded RNA-binding protein and substrate for the double-stranded RNA-dependent protein kinase, PKR

NFAT transcription factors play a central role in initiating T-cell activat ion through the induction of immediate-early T-cell specific genes includin g interleukin-2 (IL-2). NFAT transcription factors bind to a sequence in th e IL-2 enhancer known as the antigen receptor response element 2 (ARRE-2). Multiple proteins exhibiting ARRE-2 binding activity have been isolated, in cluding a heterodimer from stimulated T-cell nuclear extracts consisting of M-r = 90 000 (NF90) and M-r = 45 000 (NF45) subunits. The subunits of this heterodimer have been cloned, and NF90 was found to encode a protein conta ining two domains that are predicted to form motifs capable of binding to d ouble-stranded RNA. Using in vitro translated polypeptides, we have demonst rated that NF90 specifically binds to double-stranded RNA. Furthermore, NF9 0 was phosphorylated in a double-stranded RNA-dependent manner likely by th e interferon-induced, double-stranded RNA-dependent protein kinase, PKR. Th e NF90 protein was found to be expressed not only in T-cells, but also in n onimmune HeLa cells. In HeLa cells, the protein was almost exclusively loca lized to the ribosome salt wash fraction of cell lysates.