Plant importin alpha binds nuclear localization sequences with high affinity and can mediate nuclear import...

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