Arabidopsis thaliana Atrab28:: a nuclear targeted protein related to germination and toxic cation tolerance

The Arabidopsis gene Atrab28 has been shown to be expressed during late embryogenesis. The pattern of expression of Atrab28 mRNA and protein during embryo development is largely restricted to provascular tissues of mature embryos, and in contrast to the maize Rab28 homologue it cannot be induced by ABA and dehydration in vegetative tissues. Here, we have studied the subcellular location of Atrab28 protein and the effect of its over-expression in transgenic Arabidopsis plants. The Atrab28 protein was mainly detected in the nucleus and nucleolus of cells from mature embryos. In frame fusion of Atrab28 to the reporter green fluorescent protein (GFP) directed the GFP to the nucleus in transgenic Arabidopsis and in transiently transformed onion cells. Analysis of chimeric constructs identified an N-terminal region of 60 amino acids containing a five amino acid motif QPKRP that was necessary for targeting GFP to the nucleus. These results indicate that Atrab28 protein is targeted to the nuclear compartments by a new nuclear localization signal (NLS). Transgenic Arabidopsis plants, with gain of Atrab28 function, showed faster germination rates under either standard or salt and osmotic stress conditions. Moreover, improved cation toxicity tolerance was also observed not only during germination but also in seedlings. These results suggest a role of Atrab28 in the ion cell balance during late embryogenesis and germination.