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Functional subdomains of STAT2 required for preassociation with the alpha interferon receptor and for signaling
1Department of Molecular Biology, Research Institute, Cleveland Clinic Foundation, Ohio 44195, USA.
Abstract:
Two members of the STAT signal transducer and activator of transcription family, STAT1 and STAT2, are rapidly phosphorylated on tyrosine in response to alpha interferon (IFN-alpha). Previous work showed that in the mutant human cell line U6A, which lacks STAT2 and is completely defective in IFN-alpha signaling, the phosphorylation of STAT1 is very weak, revealing that activation of STAT1 depends on STAT2. We now find that STAT2 binds to the cytoplasmic domain of the IFNAR2c (also known as IFNAR2-2) subunit of the IFN-alpha receptor in extracts of untreated cells. STAT1 also binds but only when STAT2 is present. The activities of chimeric STAT2-STAT1 proteins were assayed in U6A cells to define regions required for IFN-alpha signaling. Previous work showed that a point mutation in the Src homology 2 (SH2) domain prevents STAT2 from binding to phosphotyrosine 466 of the IFNAR1 subunit of the activated receptor. However, we now find that the entire SH2 domain of STAT2 can be replaced by that of STAT1 without loss of function, revealing that other regions of STAT2 are required for its specific interaction with the receptor. A chimeric protein, in which the N-terminal third of STAT2 has replaced the corresponding region of STAT1, did preassociate with the IFNAR2c subunit of the receptor, became phosphorylated when IFN-alpha was added, and supported the phosphorylation of endogenous STAT1. These results are consistent with a model in which STAT2 and STAT1 are prebound to the IFNAR2c subunit of the resting receptor. Upon activation, the IFNAR1 subunit is phosphorylated on Tyr-466, allowing the SH2 domain of STAT2 to bind to it; this is followed by the sequential phosphorylation of STAT2 and STAT1.
Insights
STAT2 binding to the interferon-alpha receptor (IFNAR2c) is crucial for STAT1 activation and signal transduction. This interaction facilitates STAT1 phosphorylation, enabling interferon-alpha signaling pathways.
Area of Science:
- Cellular signaling
- Molecular biology
- Immunology
Background:
- Interferon-alpha (IFN-alpha) signaling is mediated by STAT proteins, including STAT1 and STAT2.
- STAT2 is essential for IFN-alpha-induced STAT1 phosphorylation and signaling.
- The IFN-alpha receptor (IFNAR) complex, particularly the IFNAR2c subunit, plays a key role in initiating this signaling cascade.
Purpose of the Study:
- To elucidate the molecular mechanisms underlying STAT1 and STAT2 interactions with the IFN-alpha receptor.
- To identify the specific regions of STAT2 responsible for receptor binding and signal transduction.
- To define the sequential events in STAT protein activation following IFN-alpha stimulation.
Main Methods:
- Co-immunoprecipitation assays to study protein-protein interactions between STAT proteins and IFNAR subunits.
- Use of mutant cell lines (U6A) lacking STAT2 to assess signaling defects.
- Construction and functional analysis of chimeric STAT2-STAT1 proteins in U6A cells.
- Phosphorylation assays to measure STAT1 and STAT2 activation.
Main Results:
- STAT2 directly binds to the cytoplasmic domain of IFNAR2c in resting cells; STAT1 binding is dependent on STAT2.
- The Src homology 2 (SH2) domain of STAT2 is not essential for IFNAR1 binding, contrary to previous assumptions.
- A chimeric protein with the N-terminal region of STAT2 conferred IFNAR2c association, phosphorylation, and supported STAT1 phosphorylation.
- A model is proposed where STAT2 and STAT1 pre-associate with IFNAR2c, with STAT2's SH2 domain mediating subsequent binding to phosphorylated IFNAR1.
Conclusions:
- STAT2 acts as a crucial adaptor, pre-associating with IFNAR2c and facilitating STAT1 recruitment and activation.
- The N-terminal region of STAT2, not solely the SH2 domain, is critical for specific receptor interaction and signal initiation.
- This study refines the model of IFN-alpha signal transduction, highlighting the coordinated roles of STAT2 and STAT1 in receptor complex assembly and activation.