IN-VITRO CHARACTERIZATION OF RICE IMPORTIN BETA-1 - MOLECULAR INTERACTION WITH NUCLEAR TRANSPORT FACTORS AND MEDIATION OF NUCLEAR-PROTEIN IMPORT

We recently isolated two cDNAs encoding importin beta homologues (rice importin beta 1 and beta 2), the first such homologues identified in plants. To address the function of rice importin beta 1 in the process of nuclear import of proteins, we carried out in vitro binding and nu clear import assays, Recombinant protein of rice importin pi assembled a complex (PTAC) with rice importin alpha 1 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 alpha 1 and dissociated the he terodimer formed between rice importin al and rice importin beta 1. An in vitro nuclear import assay using digitonin-permeabilized HeLa cell s revealed that rice importin pi can mediate nuclear envelope docking of NLS proteins and their subsequent translocation into the nucleus. T hese data strongly suggest that rice importin beta 1 functions as a co mponent of the NLS receptor in plant cells. (C) 1998 Federation of Eur opean Biochemical Societies.