Evidence that DIF-1 and hyper-osmotic stress activate a Dictyostelium STAT by inhibiting a specific protein tyrosine phosphatase.

becomes tyrosine phosphorylated and accumulates in the nucleus when Dictyostelium cells are exposed to the prestalk cell inducer Differentiation inducing factor 1 (DIF-1), or are subjected to hyper-osmotic stress. We show that the protein tyrosine phosphatase PTP3 interacts directly with duanyu1813c and that duanyu1813c is refractory to activation in PTP3 overexpressing cells. Conversely, overexpression of a dominant inhibitor of PTP3 leads to constitutive tyrosine phosphorylation and ectopic nuclear localisation of Treatment of cells with DIF-1 or exposure to hyper-osmotic stress induces a decrease in biochemically assayable PTP3 activity and both agents also induce serine-threonine phosphorylation of PTP3. These observations suggest a novel mode of activation, whereby serine-threonine phosphorylation of a cognate protein tyrosine phosphatase results in the inhibition of its activity, shifting the phosphorylation-dephosphorylation equilibrium in favour of phosphorylation.

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