A TRANSCRIPTION FACTOR WITH SH2 AND SH3 DOMAINS IS DIRECTLY ACTIVATED BY AN INTERFERON-ALPHA-INDUCED CYTOPLASMIC PROTEIN TYROSINE KINASE(S)

Interferon-stimulated gene factor 3 (ISGF3), the primary transcription factor induced by interferon-alpha, is a complex of four (113, 91, 84, and 48 kd) proteins. This paper reports that the 113, 91, and 84 kd (ISGF3-alpha) proteins of ISGF3 contain conserved SH2 and SH3 domains. A specific interferon alpha-induced cytoplasmic protein tyrosine kinase(s) can form a transient complex with ISGF3-alpha proteins. These ISGF3-alpha proteins can be immunoprecipitated by anti-phosphotyrosine antibodies only after interferon-alpha treatment. Phosphoamino acid analyses of P-32-labeled ISGF3-alpha proteins confirm that ISGF3-alpha proteins are directly tyrosine phosphorylated both in vitro and in vivo in response to interferon-alpha, and this tyrosine phosphorylation can be inhibited by staurosporine and genistein. Phosphatase treatment of these ISGF3-alpha proteins results in inhibition of ISGF3 complex formation in vitro. These observations indicate that interferon alpha-induced direct tyrosine phosphorylation of ISGF3-alpha proteins is necessary for activation of the transcription factor ISGF3.