FUNCTION OF STAT2 PROTEIN IN TRANSCRIPTIONAL ACTIVATION BY ALPHA-INTERFERON

Alpha interferon (IFN-alpha)-induced transcriptional activation requir es the induction of a complex of DNA-binding proteins, including tyros ine-phosphorylated Stat1 and Stat2, and of p48, a protein which is not phosphorylated on tyrosine and which comes from a separate family of DNA-binding proteins. The isolation and characterization of U6A cells, which lack Stat2, have allowed the introduction of normal and mutant forms of Stat2 so that various functions of the Stat2 protein can be e xamined, As reported earlier, Stat1, which is the second target of tyr osine phosphorylation in IFN-alpha-treated cells, is not phosphorylate d in the absence of Stat2. We show that all mutations that block Stat2 phosphorylation also block Stat1 phosphorylation. These include not o nly the mutations of Y-690 and SH2 domain residues that are involved i n tyrosine phosphorylation but also short deletions at the amino termi nus of the protein, Two mutants of Stat2 that are not phosphorylated o n tyrosine can act as dominant negative proteins in suppressing wild-t ype Stat2 phosphorylation, most likely by competition at the receptor- kinase interaction site(s), We also show that the COOH-terminal 50 ami no acids are required for transcriptional activation in response to IF N-alpha. Mutants lacking these amino acids can be phosphorylated, form IFN-stimulated gene factor 3, and translocate to the nucleus but cann ot stimulate IFN-alpha-dependent transcription, Seven acidic residues are present in the deleted COOH-terminal residues, but 24 acidic resid ues still remain in the 100 carboxy-terminal amino acids after deletio n, Thus, transcriptional activation is unlikely to depend on acidic am ino acids alone.