Rac and nuclear translocation of signal transducers and activators of transcription factors.

Abstract Signal transducers and activators of transcription (STATs) are tyrosine phosphorylated upon cytokine stimulations, and tyrosine‐phosphorylated STATs (p‐STATs) enter the nucleus to activate a variety of target genes. However, how activated STATs are transported to the nucleus has remained unclear. We have demonstrated that the small Rho GTPase Rac1 and the GTPase activating protein MgcRacGAP promote nuclear translocation of p‐STATs via the importin α/β pathway through mediating a complex formation of p‐STATs with importin α. This chapter discusses the use of an in vitro nuclear transport assay with Flag‐tagged purified proteins, providing detailed protocols regarding such experiments. Although this chapter focuses on STATs, Rac, and MgcRacGAP the methods described here can be adapted easily to other nuclear factors and their regulators.