Crosstalk between STATs and small GTPases in the control of cell division and differentiation.

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In this research project, we have investigated cross-talk between activation of STAT3/STAT5 and Rho family small GTPases. In 2004-2005, using mouse embryonic fibroblasts where Racl can be conditionally deleted by Cre recombinase as well as a dominant negative Racl and siRNA for Racl, we demonstrated that Racl is essential for nuclear transport of STAT3 and STAT5. However, the underlying molecular mechanisms remained elusive. In 2006, we investigated how Racl controls nuclear transport of STAT3 and STAT5. To this end, we established a nuclear transport assay using permialized cells and recombinant proteins produced in F9 insect cells. The nuclear transport assay revealed that an active form of Racl, MgcRacGAP and importin a/b are required for the nuclear transport of phosphorylated STAT3 and STAT5. Interestingly, STAT/Racl/MgcRacGAP complex binds importin a only when Racl is in its GTP-bound form, indicating that MgcRacGAP plays an important role in the binding and dissociation of STAT and importins (Kawashima et al., J Cell Biol, 2006). We also found that MgcRacGAP harbors nuclear localization signal (NLS). Although STAT3 and 5 do not possess typical NLS, the present results show that MgcRacGAP and Racl works as a nuclear shaperon for the activated STAT3 and 5. In addition, we also found that MgcRacGAP also binds JAK2 that is known to phosphorylate STAT proteins. Knockdown of either MgcRacGAP or Racl not only inhibited nuclear transport of STAT3/5 but also attenuated their phosphorylation. Altogether, the present results suggested that MgcRacGAP and Racl plays critical roles in the nuclear transport of STAT3 and STAT5 as well as transport to the plasma membrane and activation of these transcription factors.