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Dynamic redistribution of STAT1 protein in IFN signaling visualized by GFP fusion proteins.

STAT proteins (signal transducers and activators of transcription) are a family of transcription factors which are used by many cytokines and cell growth factors for initiating gene expression. They are activated by tyrosine phosphorylation through the cytoplasmic domain of stimulated receptors. Upon phosphorylation STAT proteins dimerize, translocate to the nucleus and activate transcription by binding to specific recognition sites. Different cytokines activate different subsets of STATs and other signaling proteins. We have made use of green fluoresencent protein (GFP) fusion proteins to visualize the subcellular localization and trafficking of STAT1, STAT2 and p48 during interferon (IFN) stimulation and have analysed in detail STAT1-GFP trafficking in living cells. Analysis of GFP fusion proteins allowed the determination of time kinetics of subcellular trafficking in individual living cells. STAT1-GFP is indistinguishable from its wild-type protein displaying strong activity as transcriptional activator as well as the same time kinetics of transport to the nucleus and retreat to the cytoplasm. After prolonged exposure to IFN, STAT1-GFP is no longer retained in the nucleus and relocation to the cytoplasm is observed. Restimulation with the same type of IFN does not lead to repeated nuclear translocation of STAT1-GFP. STAT1 is not subject of inhibition, as restimulation with another type of IFN allows immediate reuse of previously activated STAT1-GFP. However, restimulation with the same type of IFN can be achieved when the primary stimulus is removed after a short induction period. This method of visualizing signal transduction reveals a considerable inhomogeneity with respect to the extent of STAT1-GFP shuttling within a clonal cell population, indicating that competence for full-blasted IFN response is restricted to a cellular subpopulation whereas other cells respond incompletely, retarded or not at all.