Molecular Cloning of a Novel Importin α Homologue from Rice, by Which Constitutive Photomorphogenic 1 (COP1) Nuclear Localization Signal (NLS)-Protein Is Preferentially Nuclear Imported

Nuclear import of proteins that contain classical nuclear localization signals (NLS) is initiated by importin α, a protein that recognizes and binds to the NLS in the cytoplasm. In this paper, we have cloned a cDNA for a novel importin α homologue from rice which is in addition to our previously isolated rice importin α1a and α2, and we have named it rice importin α1b. In vitro binding and nuclear import assays using recombinant importin α1b protein demonstrate that rice importin α1b functions as a component of the NLS-receptor in plant cells. Analysis of the transcript levels for all three rice importin α genes revealed that the genes were not only differentially expressed but that they also responded to dark-adaptation in green leaves. Furthermore, we also show that the COP1 protein bears a bipartite-type NLS and its nuclear import is mediated preferentially by the rice importin α1b. These data suggest that each of the different rice importin α proteins carry distinct groups of nuclear proteins, such that multiple isoforms of importin α contribute to the regulation of plant nuclear protein transport.