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

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