Molecular characterization of three Arabidopsis soluble ABC proteins which expression is induced by sugars

Among the complete set of Arabidopsis ABC proteins, 15 correspond to the soluble or NBD-type (nucleotide binding domains). We have cloned cDNAs coding for three closely homologous NBD-like proteins, AtNAP2, AtNAP4 and AtNAP9 that display a characteristic ABC signature motif. In transitory expression assays, the three AtNAP proteins fused to the green-fluorescent protein (GFP) were localized in the cytosol of tobacco and Arabidopsis cell cultures. The expression of these genes is enhanced by 1% sucrose in the culture medium but not by non-assimilable sugars. This result was confirmed with an AtNAP2::GUS translational fusion derived from a promoter trap line in which the T-DNA is inserted near the 3′ end of the AtNAP2 coding sequence. According to the GUS staining, AtNAP2 is expressed in the elongation zone and meristem of roots of white light-grown seedlings and in the hypocotyl of dark-grown seedlings. We suggest that AtNAP2 might be involved in cell elongation processes.