AtSUC3, a gene encoding a new Arabidopsis sucrose transporter, is expressed in cells adjacent to the vascular tissue and in a carpel cell layer

The cDNA corresponding to the open reading frame T17M13.3 from Arabidopsis chromosome II was isolated and the encoded protein was characterized as a m ember of a subgroup of higher plant sucrose transporters. The AtSUC3 (Arabi dopsis thaliana sucrose transporter 3) open reading frame encodes a protein with 594 amino acid residues, being 81 and 82 residues longer than the pre viously described Arabidopsis sucrose carriers AtSUC1 and AtSUC2. About 50 of these additional amino acids are part of an extended cytoplasmic loop se parating the N-terminal from the C-terminal half of the protein. For functi onal characterization the AtSUC3 cDNA was expressed in baker's yeast. Subst rate specificities, energy dependence and K-m values of the recombinant pro tein were determined. Removal of the enlarged cytoplasmic loop and expressi on of the truncated cDNA caused no detectable change in the kinetic propert ies of the protein, suggesting a transport-independent function for this cy toplasmic domain. Immunolocalization with an AtSUC3-specific antiserum iden tified the protein in a cell layer separating the phloem from the mesophyll and in a single, subepidermal cell layer of the carpels that is important for pod dehiscence. These localizations suggest a possible role of AtSUC3 i n the funnelling of sucrose from the mesophyll towards the phloem, and poss ibly in pod shatter.