Nuclear import of serum response factor in airway smooth muscle.

We have previously shown that the transcription-promoting activity of serum response factor (SRF) is partially regulated by its extranuclear redistribution. In this study, we examined the cellular mechanisms that facilitate SRF nuclear entry in canine tracheal smooth muscle cells. We used in vitro pull-down assays to determine which karyopherin proteins bound SRF and found that SRF binds KPNA1 and KPNB1 through its nuclear localization sequence. Immunoprecipitation studies also demonstrated direct SRF-KPNA1 interaction in HEK293 cells. Import assays demonstrated that KPNA1 and KPNB1 together were sufficient to mediate rapid nuclear import of SRF-GFP. Our studies also suggest that SRF is able to gain nuclear entry through an auxiliary, nuclear localization sequence-independent mechanism.