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Mol. Cell. Biol., Apr 1997, 2048-2056, Vol 17, No. 4
X Li, S Leung, IM Kerr and GR Stark
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.
Copyright © 1997, American Society for Microbiology
Functional subdomains of STAT2 required for preassociation with the alpha interferon receptor and for signaling
Department of Molecular Biology, Research Institute, Cleveland Clinic Foundation, Ohio 44195, USA.
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