Transport of UDP-galactose in plants. Identification and functional characterization of AtUTr1, an Arabidopsis thaliana UDP-galactose/UDP-glucose transporter

The synthesis of non-cellulosic polysaccharides and glycoproteins in the plant cell Golgi apparatus requires UDP-galactose as substrate. The topology of these reactions is not known, although the orientation of a plant galactosyltransferase involved in the biosynthesis of galactomannans in fenugreek is consistent with a requirement for UDPgalactose in the lumen of the Golgi cisternae. Here we provide evidence that sealed, rightside-out Golgi vesicles isolated from pea stems transport UDP-galactose into their lumen and transfer galactose, likely to polysaccharides and other acceptors. In addition, we identified and cloned AtUTr1, a gene from Arabidopsis thaliana that encodes a multitransmembrane hydrophobic protein similar to nucleotide sugar transporters. Northern analysis showed that AtUTr1 is indeed expressed in Arabidopsis. AtUTr1 is able to complement the phenotype of MDCK ricin resistant cells, a mammalian cell line deficient in transport of UDP-galactose into the Golgi. In vitro assays using a Golgi enriched vesicle fraction obtained from Saccharomyces cerevisiae expressing AtUTr1-mycHis is able to transport UDP-galactose but also UDP-glucose. AtUTr1-mycHis does not transport GDPmannose, GDP-fucose, CMP-sialic acid, UDP-glucuronic acid or UDP-xylose when expressed in S. cerevisiae. AtUTr1 is the first transporter described, able to transport UDPgalactose and UDP-glucose. Thus AtUTr1 may play an important role in the synthesis of glycoconjugates that contain galactose and glucose in Arabidopsis