Interferon alpha/beta (IFNalpha/beta) mediate their antiviral and antiproliferative effects through the induction of a set of Interferon Stimulated early response Genes (ISGs). The STAT transcription factors have been identified as an important part of the IFNalpha/beta induced signaling cascade. STAT1 and STAT2 become tyrosine phosphorylated in response to IFNalpha/beta, and subsequently translocate to the nucleus where they interact with several enhancer elements to induce transcription. Genetic evidence revealed that the tyrosine kinases Jak1 and Tyk2 are required for STAT activation by IFNalpha/beta, however, it is still unclear whether these kinases actually phosphorylate STAT1 or act upon an upstream target. Dr. David and his colleagues have established conditions that allow IFNalpha/beta mediated phosphorylation of STAT1 to occur in Jak1 or Tyk2 deficient cells. However, in contrast Tyk2-/- cells, where the tyrosine phosphorylated STAT1 can be detected in the nucleus, no such nuclear accumulation of tyrosine phosphorylated STAT1 takes place in Jak1-/- cells. These results not only indicate that a kinase distinct from Jak1 or Tyk2 might be phosphorylating STAT1 in response to IFNalpha/beta, but also show that tyrosine phosphorylation and dimerization of STAT1 is necessary but not sufficient for nuclear accumulation of STAT1. The two aims of Part I are focused on identifying the kinase that phosphorylates STAT1 in response to IFNalpha/beta. In Aim 1, the investigators will verify that the phosphorylation of STAT1 in Jak1 or Tyk2 deficient cells by IFNalpha occurs on Tyr701. In Aim 2, they propose experiments to isolate and identify the kinase that phosphorylates STAT1 in response to IFNalpha and to test whether this kinase belongs to the src family of tyrosine kinases. They will conduct experiments to elucidate the inhibitory role of csk in IFNalpha-mediated STAT activation. Part II of the proposal contains three aims that investigate the mechanism of the nuclear localization of STAT1 in response to IFNalpha. In Aim 3 the investigators will examine Jak1 domains, in particular its kinase domain, for their function in the regulation of STAT1 nuclear translocation. In Aim 4 they will demonstrate that the subcellular localization of STAT1 is, at least in part, controlled through the regulation of the nuclear export. They will determine the presence of a nuclear export signal in STAT1, and will investigate whether STAT1 interacts with the nuclear export protein Crm1. In Aim 5, they propose to investigate the role of a newly identified methyl-arginine transferase in the events that lead to the nuclear accumulation of STAT1. The investigators will test whether STAT1 undergoes methylation that is required for its nuclear accumulation. Results from these proposed studies will not only improve our understanding of the mechanism of STAT activation, but will also shed light on the processes involved in the subcellular distribution of these signaling molecules.