A nuclear protein tyrosine phosphatase is required for the inactivation ofStat1

The Stall activation-inactivation cycle involves phosphorylation of Stat1 i n the cytoplasm, translocation to the nucleus, and then a return of the pro tein to the cytoplasm in a dephosphorylated state. However, the intracellul ar site of Stat1 dephosphorylation has not been determined, As receptor sig naling declines, the flow of activated Stall molecules should be to the sit e of their dephosphorylation. We found that upon receptor-Janus kinase inac tivation, either gradual or abruptly induced by staurosporine treatment, th e flow of Stall was from cytoplasm to the nucleus and the nucleus was the f inal compartment in which phosphorylated Stat1 was detected. N-terminal mut ants of Stall, previously shown to remain phosphorylated for a longer time than wild-type Stat1, were able to enter the nucleus and were not inactivat ed in the presence of staurosporine, directly demonstrating that these muta tions affect phosphatase access and/or activity during the normal dephospho rylation of Stall. In the presence of sodium vanadate, a phosphatase inhibi tor, phosphorylated Stat1 accumulated in the nucleus as the total amount of Stall in the cytoplasm declined to low levels. We conclude that the nucleu s is the site of Stall inactivation and that dephosphorylation is required for the rapid nuclear export of Stat1.