STAT1 SERINE PHOSPHORYLATION OCCURS INDEPENDENTLY OF TYROSINE PHOSPHORYLATION AND REQUIRES AN ACTIVATED JAK2 KINASE

Gamma interferon (IFN-gamma) induces both tyrosine and serine phosphor ylation of Stat1, Stat1 serine phosphorylation is required for maximal transcriptional activity of Stat1. In this report, we present evidenc e that Stat1 tyrosine phosphorylation is not a prerequisite for Stat1 serine phosphorylation, although an active Jak2 kinase is required for both phosphorylation events, Stat1 serine phosphorylation occurs with a more delayed time course than tyrosine phosphorylation. The occurre nce of serine phosphorylation without tyrosine phosphorylation suggest s that serine phosphorylation takes place ire the cytoplasm. Experimen ts performed,vith cells expressing either dominant-negative or constit utively active Ras protein indicated that the Ras-mitogen-activated pr otein kinase pathway is probably not involved in IFN-gamma-induced Sta t1 serine phosphorylation. Finally, a kinase capable of correct Stat1 serine phosphorylation was detected in partially purified cytoplasmic extracts from both IFN-gamma-treated and untreated cells.