Regulation of Stat3 nuclear import by importin α5 and importin α7 via two different functional sequence elements

Regulated import of STAT proteins into the nucleus through the nuclear pores is a vital event. We previously identified Arg214/215 in the coiled-coil domain and Arg414/417 in the DNA binding domain involved in the ligand-induced nuclear translocation of Stat3. In this study, we investigated the mechanism for Stat3 nuclear transport. We report here that among five ubiquitously expressed human importin αs, importin α5 and α7, but not importin α1, α3, and α4, bind to Stat3 upon cytokine stimulation. Similar results were observed for Stat1, but not for Stat5a and 5b, which were unable to interact with any of the importin αs. The C-terminus of importin α5 is necessary but not sufficient for Stat3 binding. Truncation mutant of Stat3 (aa1-320) that contains Arg214/215 exhibits specific binding to importin α5, and an exclusive nuclear localization. Point mutations of Arg214/215 in this mutant destroy importin α5 binding and its nuclear localization. In contrast, the truncation mutant (aa320-770) including Arg414/417 fails to interact with importin α5 and is localized in the cytoplasm. However, both sequence elements are necessary for the full-length Stat3's interaction with importin α5. These results suggest that Arg214/215 is likely the binding site for importin α5, whereas Arg414/417 may not be involved in the direct binding, but necessary for maintaining the proper conformation of Stat3 dimer for importin binding. A model for Stat3 nuclear translocation is proposed based on these data.