In vitro characterization of rice importin β1: molecular interaction with nuclear transport factors and mediation of nuclear protein import

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Abstract
We recently isolated two cDNAs encoding importin β homologues (rice importin β1 and β2), the first such homologues identified in plants. To address the function of rice importin β1 in the process of nuclear import of proteins, we carried out in vitro binding and nuclear import assays. Recombinant protein of rice importin β1 assembled a complex (PTAC) with rice importin α1 and NLS protein, and also bound to the nuclear envelope of tobacco BY‐2 cells. Ran‐GTP, but not Ran‐GDP, interacted with rice importin β1 and dissociated the heterodimer formed between rice importin α1 and rice importin β1. An in vitro nuclear import assay using digitonin‐permeabilized HeLa cells revealed that rice importin β1 can mediate nuclear envelope docking of NLS proteins and their subsequent translocation into the nucleus. These data strongly suggest that rice importin β1 functions as a component of the NLS receptor in plant cells.