Rapid and massive nuclear translocation of signaling proteins is an important step in the induction of transcription upon extracellular stimulation. Despite the importance of this process, the molecular mechanisms that govern this process have been elucidated only for few signaling components. In these cases, signaling proteins that utilize the classical nuclear localization signal (NLS) interact with Impα and Impβ, to facilitate their nuclear translocation. However, it is clear today that many other signaling proteins translocate to the nucleus upon stimulation using distinct, NLS-Impα/β- independent mechanisms.
In a search for NLS-independent shuttling proteins, we have resorted to the MAPK family members JNK and p38. Unlike ERK1/2, the subcellular localization of JNKs and p38s has not been properly established so far. In this study we show that in resting cells, JNK1/2 and p38α/β are localized mainly in the cytoplasm, and translocate to the nucleus upon stimuli, independent of their activation. We further found that despite the pronounced similarity between the MAPK family members, none of the JNK or p38 proteins contain the ERK1/2-NTS in their KID regions. Furthermore, mutations in the aligned residues of this region resulted in a marginal effect on the nuclear translocation of JNK1/2 and p38α/β, indicating that the mechanism of the translocation is not only NLS- but also NTS- independent. We hypothesized that the nuclear translocation is still dependent on other, ill-defined, β-like importins. Therefore, we used Co-IP and SiRNA experiments with all these importins, and found that the translocation of JNK1/2 and p38α/β is mediated through their interactions with either Imp7 or Imp9, which require further dimerization with Imp3. Thus, the stimuli-dependent nuclear translocation of these MAPKs is mediated by the dimerization of different β-like importins. As such, it consists an unexplored layer of transcriptional regulation.