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The rapid inactivation of nuclear tyrosine phosphorylated Stat1 depends upon a protein tyrosine phosphatase
R L Haspel1, M Salditt-Georgieff, J E Darnell
1Laboratory of Molecular Cell Biology, The Rockefeller University, New York, NY 10021-6399, USA.
Abstract:
After interferon-gamma (IFN-gamma) treatment of cells the appearance of tyrosine phosphorylated Stat1 in the nucleus was maximal within 20-30 min, remained for 2-2.5 h and activated molecules disappeared by 4 h. In the absence of continued signaling from the receptor (imposed by staurosporine treatment) previously activated Stat1 disappeared completely within 60 min, implying continuous generation and removal of active molecules during extended IFN-gamma treatment. Proteasome inhibitors prolonged the time of activation of Stat1 by prolonging signaling from the receptor but not by blocking removal of already activated Stat1 molecules. By analyzing with 35S labeling the distribution of total Stat1 and activated Stat1, we concluded that the Stat1 molecules promptly cycle into the nucleus as tyrosine phosphorylated molecules and later return quantitatively to the cytoplasm as non-phosphorylated molecules. Therefore, the removal of the activated Stat1 molecules from the nucleus appears not to be proteolytic but must depend on a protein tyrosine phosphatase(s).
Insights
Interferon-gamma (IFN-gamma) activates Stat1 in the nucleus, which is rapidly removed. This removal is mediated by protein tyrosine phosphatases, not proteasomes, indicating a continuous cycling of Stat1 molecules.
Area of Science:
- Cellular signaling
- Molecular biology
- Immunology
Background:
- Interferon-gamma (IFN-gamma) is a key cytokine in immune responses.
- Signal transducer and activator of transcription 1 (Stat1) is a crucial transcription factor activated by IFN-gamma.
- Understanding the dynamics of Stat1 activation and deactivation is essential for comprehending IFN-gamma signaling.
Purpose of the Study:
- To investigate the temporal dynamics of tyrosine phosphorylated Stat1 in the nucleus following IFN-gamma treatment.
- To determine the mechanisms responsible for the removal of activated Stat1 from the nucleus.
- To elucidate the role of proteasomes and protein tyrosine phosphatases in Stat1 regulation.
Main Methods:
- Cell treatment with IFN-gamma and staurosporine.
- Analysis of tyrosine phosphorylated Stat1 localization and timing.
- Use of proteasome inhibitors to assess their impact on Stat1 dynamics.
- 35S labeling to track total and activated Stat1 distribution.
Main Results:
- Nuclear Stat1 phosphorylation peaked within 20-30 minutes and persisted for 2-2.5 hours, disappearing by 4 hours.
- In the absence of receptor signaling, activated Stat1 was cleared within 60 minutes.
- Proteasome inhibitors extended Stat1 activation by prolonging receptor signaling, not by inhibiting removal of activated Stat1.
- Stat1 molecules were observed to shuttle into the nucleus as phosphorylated forms and return to the cytoplasm as non-phosphorylated forms.
Conclusions:
- Activated Stat1 undergoes rapid nuclear-cytoplasmic cycling.
- The removal of activated Stat1 from the nucleus is primarily mediated by protein tyrosine phosphatases, not proteasomal degradation.
- These findings highlight a phosphatase-dependent mechanism for regulating Stat1 activity during IFN-gamma signaling.