Extracellular signal-dependent nuclear import of Stat1 is mediated by nuclear pore-targeting complex formation with NPI-1, but not Rch1

Abstract

In response to interferon-γ (IFN-γ), Stat1 is tyrosine phosphorylated and translocates to the nucleus where it activates transcription. In this study, we identified factors which mediate the nuclear import of Stat1. Tyrosine-phosphorylated Stat1 associated with the β subunit (a 97 kDa component) of the nuclear pore-targeting complex via the NPI-1 family, but not the Rch1 family, of α subunit (a 58 kDa component) as a result of IFN-γ stimulation. Antibodies against NPI-1 or β subunit consistently inhibited the IFN-γ-dependent nuclear import of Stat1 in living cells, although antibodies reactive to Rch1 had no effect. Solution binding assays with deletion mutants of NPI-1 showed that the Stat1-binding domain of NPI-1 was located in the carboxy-terminal region, which is clearly distinct from the SV40 large T antigen nuclear localization signal (NLS)-binding regio. These results indicate that the extracellular signal-dependent nuclear transport of Stat1 is mediated by NPI-1, but not Rch1, in conjunction with β subunit, and that these factors participate in, not only constitutive, but also the conditional nuclear import of proteins.