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Dynamic redistribution of STAT1 protein in IFN signaling visualized by GFP fusion proteins
1Department of Gene Regulation and Differentiation, GBF-National Research Institute for Biotechnology, Braunschweig, Germany.
Abstract:
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.
Insights
Signal transducers and activators of transcription (STAT) proteins are key in gene expression. Visualizing STAT1-GFP trafficking reveals cell-specific responses to interferon stimulation, with only a subpopulation fully engaging the response.
Area of Science:
- Cellular Biology
- Molecular Biology
- Immunology
Background:
- Signal transducers and activators of transcription (STAT) proteins are transcription factors mediating cytokine and growth factor signaling.
- STAT proteins are activated by tyrosine phosphorylation, leading to dimerization, nuclear translocation, and gene activation.
Purpose of the Study:
- To visualize and analyze the subcellular localization and trafficking of STAT1, STAT2, and p48 using green fluorescent protein (GFP) fusion proteins during interferon (IFN) stimulation.
- To determine the time kinetics of STAT1-GFP trafficking in living cells and assess its functional activity.
Main Methods:
- Utilized green fluorescent protein (GFP) fusion proteins to create STAT1-GFP, STAT2-GFP, and p48-GFP.
- Observed and analyzed the subcellular localization and trafficking dynamics of these fusion proteins in living cells upon IFN stimulation.
- Quantified the time kinetics of nuclear translocation and cytoplasmic retreat of STAT1-GFP.
Main Results:
- STAT1-GFP fusion protein functions identically to wild-type STAT1, exhibiting transcriptional activity and similar trafficking kinetics.
- Prolonged IFN exposure leads to STAT1-GFP relocation to the cytoplasm; restimulation with the same IFN type does not induce re-translocation unless the initial stimulus is brief.
- Different IFN types can elicit sequential STAT1-GFP activation, indicating STAT1 is not inhibited but can be reused.
- Significant heterogeneity in STAT1-GFP trafficking was observed within a cell population, suggesting varying cellular competence for IFN response.
Conclusions:
- GFP fusion proteins are effective tools for studying STAT protein trafficking and signaling dynamics in real-time.
- Cellular response to IFN is heterogeneous, with only a subpopulation of cells exhibiting a full STAT1 activation and translocation response.
- The findings highlight the complexity of STAT protein regulation and cellular responsiveness to cytokine signaling.