Identification of a Novel Conserved Motif in the STAT Family That Is Required for Tyrosine Phosphorylation

The rapid transcriptional activation of cellular genes by either type 1 interferons (IFNα/β) or type 2 interferon (IFNγ) is responsible for many of the pleiotropic effects of these cytokines, including their antiviral, antigrowth, and immunomodulatory activities. Interferon-stimulated gene expression is mediated by transcription factors termed Stats, which upon being tyrosine-phosphorylated, translocate to the nucleus and bind enhancers of interferon-activated genes. We have recently characterized a new Jurkat cell variant, named H123, where IFNα stimulates programmed cell death. H123 clones that are resistant to the apoptotic actions of IFNα have been selected. One of these clones (Clone 8) is defective in its responses to IFNα with regard to activation of genes that require tyrosine phosphorylation of Stat2. Stimulation of Clone 8 cells with IFNα induces normal tyrosine phosphorylation of Stat1 and Stat3. Sequencing of Stat2 RNA reveals a substitution of proline 630 located within the Src homology 2 domain of Stat2 to leucine (P630L). Pro-630 and its adjacent amino acids are conserved in all Stat family members but are absent in other proteins that contain Src homology 2 domains. Expression of Stat2 P630L in cells inhibits IFNα-stimulated gene expression. These results not only define a critical motif in Stat2 required for its transcriptional activity, but they also provide evidence that resistance to type one IFNs can be mediated by mutations in Stat2 as well as those previously described for Stat1.