The Golgi localized bifunctional UDP-rhamnose/UDP-galactose transporter family of Arabidopsis.

Plant cells are surrounded by a cell wall that plays a key role in plant growth, structural integrity, and defense. The cell wall is a complex and diverse structure that is mainly composed of polysaccharides. The majority of noncellulosic cell wall polysaccharides are produced in the Golgi apparatus from nucleotide sugars that are predominantly synthesized in the cytosol. The transport of these nucleotide sugars from the cytosol into the Golgi lumen is a critical process for cell wall biosynthesis and is mediated by a family of nucleotide sugar transporters (NSTs). Numerous studies have sought to characterize substrate-specific transport by NSTs; however, the availability of certain substrates and a lack of robust methods have proven problematic. Consequently, we have developed a novel approach that combines reconstitution of NSTs into liposomes and the subsequent assessment of nucleotide sugar uptake by mass spectrometry. To address the limitation of substrate availability, we also developed a two-step reaction for the enzymatic synthesis of UDP-l-rhamnose (Rha) by expressing the two active domains of the Arabidopsis UDP-l-Rha synthase. The liposome approach and the newly synthesized substrates were used to analyze a clade of Arabidopsis NSTs, resulting in the identification and characterization of six bifunctional UDP-l-Rha/UDP-d-galactose (Gal) transporters (URGTs). Further analysis of loss-of-function and overexpression plants for two of these URGTs supported their roles in the transport of UDP-l-Rha and UDP-d-Gal for matrix polysaccharide biosynthesis.