Characterization of an Arabidopsis thaliana Homologue of the Nuclear Export Receptor CAS by its Interaction with Importin α[*]

Abstract

We have previously described four genes encoding different Importin α-like proteins from Arabidopsis thaliana. Here we describe the putative nuclear export receptor for Importin α. Using protein interaction assays in the yeast two-hybrid system, we characterized an Arabidopsis protein showing high similarity to human CAS, the nuclear export receptor for Importin α. Arabidopsis CAS specifically bound to four different plant Importin α proteins but not to proteins containing leucine-rich nuclear export signals (NESs) that are recognized by Exportin 1 (XPO1/CRM1). Like all members of the Importin β family, Arabidopsis CAS also interacted with the regulatory GTPase Ran. Deletion of 15 amino acid residues from the amino terminus of CAS abolished binding of Importin α, but did not influence the interaction with the GTPase Ran. We found two regions of Importin α1 that profoundly influence the binding to CAS: the amino terminal Importin beta-binding (IBB) domain and the carboxy terminus. Whereas the IBB domain did not directly bind to CAS, but might rather affect the interaction through conformational changes within the Importin α protein, the carboxy terminal domain strongly interacted with CAS.

Abbreviations

CAS: cellular apoptosis susceptibility protein

CHS: chalcone synthase

GFP: green fluorescent protein

IBB: Importin β-binding

NES: nuclear export signal

NLS: nuclear localization signal

NPC: nuclear pore complex

Arabidopsis thaliana - CAS - IBB domain - Importin α - nuclear export

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1 The nucleotide sequence of the AtCAS cDNA has been submitted to EMBL Nucleotide sequence Database under the accession number AJ297282.

Section Editor: W. B. Frommer