Arabidopsis transportin1 is the nuclear import receptor for the circadian clock-regulated RNA-binding protein AtGRP7

Ziemienowicz A, Haasen D, Staiger D, Merkle T. Arabidopsis transportin1 is the nuclear import receptor for the circadian clock-regulated RNA-binding protein AtGRP7. Plant Molecular Biology. 2003;53(1/2):201-212.We characterized the Arabidopsis orthologue of the human nuclear import receptor transportin1 (TRN1). Like the human receptor, Arabidopsis TRN1 recognizes nuclear import signals on proteins that are different from the classical basic nuclear localization signals. The M9 domain of human heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) is the prototype of such signals. We show that At TRN1 binds to similar domains in hnRNP-like proteins from plants. At TRN1 also interacts with human hnRNP A1 and with yeast Nab2p, two classical import cargo proteins of transportin in these organisms. Like all nuclear transport receptors of the importin-Ã family, At TRN1 binds to the regulatory GTPase Ran from Arabidopsis . We demonstrated that the amino terminus of At TRN1 is necessary for this interaction. Recombinant At TRN1 conferred nuclear import of fluorescently labelled BSA-M9 peptide conjugates in permeabilized HeLa cells, functionally replacing human TRN1 in these in vitro nuclear import assays. We identified three plant substrate proteins that interact with At TRN1 and contain M9-like domains: a novel Arabidopsis hnRNP that shows high similarity to human hnRNP A1 and two small RNA-binding proteins from Arabidopsis , At GRP7 and At GRP8. Nuclear import activity of the M9-like domains of these plant proteins was demonstrated in vivo by their ability to confer partial nuclear re-localisation of a GFP fusion protein containing a nuclear export signal. In addition, fluorescently labelled At GRP7 was specifically imported into nuclei of permeabilized HeLa cells by Arabidopsis At TRN1 and human TRN1. These results suggest that the transportin-mediated nuclear import pathway is highly conserved between man, yeast and plants