Role of STAT2 in the alpha interferon signaling pathway.

We have isolated U6A, a mutant cell line which lacks the STAT2 subunit of the transcription factor interferon (IFN)-stimulated gene factor 3 (ISGF3). The response of U6A cells to IFN-alpha is almost completely defective, but the response to IFN-gamma is normal. Complementation of U6A cells with a cDNA encoding STAT2 restores the IFN-alpha response, proving that STAT2 is required in this pathway. Binding of IFNs to their receptors triggers tyrosine phosphorylation and activation of the receptors, JAK family kinases, STAT1, and STAT2. In IFN-alpha-treated U6A cells, phosphorylation of the essential tyrosine kinases TYK2 and JAK1 is normal, but the phosphorylation of STAT1 is weak. A mutant STAT2 protein in which the phosphorylated tyrosine at position 690 is changed to phenylalanine does not restore normal phosphorylation of STAT1 in response to IFN-alpha. The dependence of STAT1 phosphorylation on the presence of STAT2 but not vice versa (T. Improta, C. Schindler, C. M. Horvath, I. M. Kerr, G. R. Stark, and J. E. Darnell, Jr., Proc. Natl. Acad. Sci. USA 91:4776-4780, 1994) indicates that in the formation of ISGF3, these two proteins may be phosphorylated sequentially in response to IFN-alpha and that phosphorylated STAT2 may be required to allow unphosphorylated STAT1 to bind to the activated IFN-alpha receptor.