The Nuclear Import of the Small GTPase Rac1 is Mediated by the Direct Interaction with Karyopherin α2

The small GTPase Rac1 is involved in multiple cytosolic functions but recent data point out that Rac1 also translocates to the nucleus to regulate signalling pathways that control gene expression and progression through the cell cycle. Here, we identify the nuclear import receptor karyopherin alpha 2 (KPNA2) as a direct interaction partner of Rac1. The C-terminal polybasic region of Rac1 contains a nuclear localization signal (NLS), whereas Rac2 and Rac3 lack a functional NLS and do not bind to KPNA2. The presence of the NLS in Rac1 determines the specificity of the interaction and is a prerequisite for the nuclear import. Although this interaction is independent of the Rac1 GDP/GTP loading, the induction of the translocation requires Rac1 activation. The activation of Rac1 via the cytotoxic necrotizing factor 1 and the concurrent inhibition of its proteasomal degradation are crucial for the nuclear accumulation of Rac1. Conversely, the reduction of KPNA2 expression inhibits the nuclear import of Rac1. For the first time, our results show a direct interaction between Rac1 and KPNA2 and argue for a KPNA2-dependent nuclear import of Rac1. Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis revealed that nuclear Rac1 coimmunoprecipitates with numerous proteins. In the nucleus, Rac1 may participate in a variety of so far uncharacterized processes.