Tyrosine phosphorylation-independent nuclear translocation of a Dictyostelium STAT in response to DIF signaling

We describe a Dictyostelium STAT, Dd-STATc, which regulates the speed of early development and the timing of terminal differentiation. Dd-STATc also functions as a repressor, which directs graded expression of the ecmA gene in different prestalk cell populations. Developing Dictyostelium cells produce a chlorinated hexaphenone, DIF, which directs prestalk cell differentiation. Dd-STATc is tyrosine phosphorylated, dimerizes, and translocates to the nucleus when cells are exposed to DIF. Surprisingly, however, SH2 domain-phosphotyrosine interaction is not necessary for the DIF-induced nuclear translocation of Dd-STATc. In this respect, Dd-STATc activation resembles several recently described, noncanonical mammalian STAT signaling processes. We show instead that DIF mediates nuclear translocation via sequences located in the divergent, N-terminal half of the Dd-STATc molecule.