A plant in vitro system for the nuclear import of proteins

This paper reports the development of an in vitro system that allows the direct assay of protein import into plant nuclei. In this assay the import of fluorescently labelled karyophilic protein substrates into nuclei isolated from evacuolated tobacco BY-2 suspension cells is monitored. It is demonstrated that import of the fluorescently labelled peptide conjugates is rapid, saturable and nuclear localization signal (NLS)-dependent. Exclusion of high molecular weight (70 kDa) dextran and substrates carrying mutated NLS sequences further underline the specificity of this system. Nuclear translocation of karyophilic import substrates in tobacco, similar to mammalian systems, is inhibited by the non-hydrolysable GTP analogue GTP-gamma-S. In contrast, protein uptake is not blocked by wheat germ agglutinin, N-ethyl-maleinimide and iodoacetic acid. Furthermore, it is shown that nuclear import of proteins is only partially inhibited by low temperature (0-4 degrees C). The in vitro nuclear import assay does not depend on exogenously added ATP or cytosolic factors. However, a block of nuclear import with GTP-gamma-S could be overcome by the addition of cytosolic extract, suggesting the dependence on cytosolic factors or proteins. These data indicate that the characteristics of nuclear protein import in plant and mammalian cells are similar, but may be, at least in some respects, also different from each other.