The function and specificity of Golgi sugar nucleotide transporters in cell wall synthesis

Plant cells are surrounded by rigid cell walls. Long chains made of a range of different types of sugars, known as polysaccharides, are major components of plant cell walls. Different plant cells and tissues vary in their properties largely due to differences in the wall polysaccharides. For example, as fruits ripen and soften, the polysaccharide pectin which functions as a glue holding cells together begins to be broken down. Cell walls polysaccharides are very important in our diet, as dietary fibre, and protect us against cancer and other diseases. By forming part of the barrier to attacking organisms, polysaccharides are also involved in plant resistance to diseases. However, we do not know the importance to the plant or to our diet of each particular polysaccharide component. If we could breed plants with different polysaccharide structures, we could investigate their importance to us and the plant. The Golgi apparatus is a structure that makes the polysaccharides within each cell. Individual sugar units are delivered to the enzymes making the long chain polysaccharides through the membrane of the Golgi apparatus. To do this, there are a large number of transporter proteins that take the sugars across the membrane. We have recently discovered four genes that direct the synthesis of four of these transporters, which we call GONSTs. We would like to study mutant plants missing each of these genes, to understand what the importance of each transporter is to the plant. We expect that these mutant plants will not be able to make all the normal polysaccharides, but we do not know precisely which polysaccharides will be affected. We also do not know what the consequences for the plant will be of having changed cell walls. Data we already have on one mutant tells us to expect that pectin synthesis will be affected. This will mean that the plant does not grow normally. When we have carried out this study, we will know how the structure of some of the wall polysaccharides can be changed in the laboratory. We should eventually be able to breed crop plants with changed polysaccharides in their wall, bringing dietary, industrial and agricultural benefits.

Cell wall polysaccharides are major components of plant cell walls, and are involved in processes determining plant growth, shape and resistance to pathogens. Remarkably little is known about the enzymes that synthesise pectins and hemicelluloses. In the last few years it has been shown that substrate-specific nucleotide sugar transporters are present in the plant Golgi apparatus membrane. These are thought to provide substrates to luminal glycosyltransferases. However, their importance and function has not been demonstrated in plants. We have discovered a family of four Golgi-localised sugar nucleotide transporters, GONSTs (Baldwin et al. 2001, Plant Cell 13, 2283; Handford et al. 2004, MGG 272, 397). Based on sequence similarity and preliminary studies of substrate transport activity, we hypothesise that these are likely responsible for the transport of GDP-linked sugars in plants. The likely important substrates are GDP-Fucose and GDP-L-Galactose. We now have an opportunity now to study this complete family of four transporters, using both mutant analysis and in vitro assays, to address the hypotheses: Are these transporters responsible for the transport of GDP-Fucose and GDP-L-Galactose in plants? Are they responsible for individually for synthesis of specific polysaccharide, perhaps by substrate channeling? Alternatively, do they generally supply substrate to several different polysaccharide synthesizing enzymes? What is the consequence of Fucose and L-Galactose deficiency for plant growth? Preliminary results suggest an important role for GONST1 in substrate supply for pectin Rhamnogalacturonan II synthesis. These studies will show for the first time how substrate supply across membranes can alter the glycosylation of cell wall polysaccharides. By studying this limited family of four GDP-sugar transporters, we will provide the basis for future studies on the role of the more numerous and diverse UDP-sugar transporters.