Nuclear localization signal binding protein from Arabidopsis mediates nuclear import of Agrobacterium VirD2 protein

T-DNA nuclear import is a central event in genetic transformation of plant cells by Agrobacterium, Presumably, the T-DNA transport intermediate is a single-stranded DNA molecule associated with two bacterial proteins, VirD2 and VirE2, which most likely mediate the transport process, While VirE2 cooperatively coats the transported single-stranded DNA, VirD2 is covalently attached to its 5' end, To better understand the mechanism of VirD2 action, a cellular receptor for VirD2 was identified and its encoding gene cloned from Arabidopsis. The identified protein, designated AtKAP alpha, specifically bound VirD2 in vivo and in vitro. VirD2-AtKAP alpha interaction was absolutely dependent on the carboxyl-terminal bipartite nuclear localization signal sequence of VirD2. The deduced amino acid sequence of At-KAP alpha was homologous to yeast and animal nuclear localization signal-binding proteins belonging to the karyopherin alpha family, Indeed, AtKAP alpha efficiently rescued a yeast mutant defective for nuclear import, Furthermore, AtKAP alpha specifically mediated transport of VirD2 into the nuclei of permeabilized yeast cells.