With the exception of cellulose and callose, the complex polysaccharide sugars in plant cell walls are synthesized in the Golgi apparatus by enzymes called glycosyltransferases. These polysaccharides are assembled from substrates of simple nucleotide sugars which are transported into the Golgi apparatus from the cytosol, the gel-like liquid that fills a plant cell's cytoplasm. Despite their importance, few plant nucleotide sugar transporters have been functionally characterized at the molecular level. A big part of the holdup has been a lack of substrates that are necessary to carry out such characterizations.
"Substrates of mammalian nucleotide sugar transporters are commercially available because of the medical interest but have not been available for plants, which made it difficult to study both nucleotide sugar transporters and glycosyltransferases," Scheller says.
For their assay, Scheller, Rautengarten, Ebert and their collaborators, created several artificial substrates for nucleotide sugar transporters, then reconstituted the transporters into liposomes for analysis with mass spectrometry. The researchers used this technique to characterize the functions of the six new nucleotide sugar transporters they identified in Arabidopsis, a relative of mustard that serves as a model plant for research in advanced biofuels.
"We found that these six new nucleotide sugar transporters are bispecific, which is a surprise since the two substrates are not very similar from a physical standpoint to the human eye," Scheller says. "We also found that limiting substrate availability has different effects on different polysaccharide products, which suggests that cell wall polysaccharide biosynthesis in the Golgi apparatus of plants is also regulated by substrate transport mechanisms."
In addition to these six nucleotide sugar trans
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DOE/Lawrence Berkeley National Laboratory