Characterization of Proteins Homologous to Nucleotide Sugar Transporters in Arabidopsis thaliana

The nucleotide sugar transporters (NST) and the plastidic phosphate transporters (pPT) proteins share structural and functional similarities. They mediate the transport of phosphorylated and nucleotide-coupled metabolites through the membranes of plastids, Golgi apparatus and endoplasmic reticulum (ER). Recently, a great number of proteins homologous to the NST/pPT proteins have been identified in Arabidopsis and only a few of them have been characterized. The majority of these homologous proteins cluster in three subfamilies, namely the KV/A/G, KT and KD subfamilies. Two members of the KV/A/G subfamily, KVAG1 and KVAG2, have a putative plastidic targeting signal. This made them interesting candidates as putative providers of nucleotide sugars for the synthesis of specific plastid lipids. In this study, the intracellular localization and in planta expression of these proteins was analyzed. cDNA fusions to GFP revealed the localization of both proteins in the Golgi apparatus and not in plastids. Transport experiments using liposomes and the proteins heterologously expressed in yeast were performed; however no significant transport of nucleotide sugars was detected. Homozygous knock-out T-DNA lines, kvag1-1 and kvag2-1, presented general wild type phenotype and development, except for a small reduction in the phospholipid content in leaves. The transport of CDP-choline and CDP-ethanolamine, precursors of phospholipid biosynthesis, by KVAG2 was also evaluated; however the transport rates were not significantly higher than the controls. Additional evidence is presented for the Golgi localization of other members of the KV/A/G subfamily. T-DNA knock-out mutants for these genes were identified and they all showed a similar wild type phenotype