Plant importin α binds nuclear localization sequences with high affinity and can mediate nuclear import independent of importin β

Abstract

Nuclear import of conventional nuclear localization sequence (NLS)- containing proteins initially involves recognition by the importin (IMP) α/β heterodimer, where IMPα binds the NLS and IMPβ targets the IMPα/NLS- containing protein complex to the nuclear pore. Here we examine IMPα from the plant Arabidopsis thaliana (At-IMPα), which exhibits nuclear envelope localization typical of IMPβ rather than IMPα in other eukaryotic cell systems. We show that At-IMPα recognizes conventional NLSs of two different types with high affinity (K(d) of 5-10 nM), in contrast to mouse IMPα (mIMPα), which exhibits much lower affinity (K(d) of 50-70 nM) and only achieves high affinity in the presence of m-IMPβ. Unlike m-IMPα, At-IMPα is thus a high affinity NLS receptor in the absence of IMPβ. Interestingly, At-IMPα was also able to bind with high affinity to NLSs recognized specifically by m-IMPβ and not m-IMPα, including that of the maize transcription factor Opaque-2. Reconstitution of nuclear import in vitro indicated that in the absence of exogenous IMPβ subunit but dependent on RanGDP and NTF2, At-IMPα was able to mediate nuclear accumulation to levels comparable with those mediated by m-IMPα/β. Neither m-IMPα nor -β was able to mediate nuclear import in the absence of the other subunit. At- IMPα's novel NLS recognition and nuclear transport properties imply that plants may possess an IMPα-mediated nuclear import pathway independent of IMPβ in addition to that mediated by IMPα/β.