STAT pathways in Dictyostelium. (a) cAMP induces activation of a tyrosine kinase which activates STATa, whilst protein tyrosine phosphatase PTP1 acts antagonistically in this process. GSK3, which can be activated by cAMP-ZAK1/2 signaling mentioned earlier, contributes to the nuclear export of STATa. (b) Unlike other STAT proteins, STATb proteins are constitutively localized in the nucleus in a tyrosine phosphorylation independent manner. Elucidation of the mechanism of this unusual STAT protein regulation will be of interest. (c) One of the major prestalk cell morphogens, DIF-1, triggers STATc tyrosine phosphorylation, nuclear localization, and the stimulation of STATc-mediated gene expression. STATc tyrosine phosphorylation is antagonized by protein tyrosine phosphatase PTP3. DPYK3 is known to function positively at the upstream of PTP3 and thus negatively regulate STATc activity [14].