Plasma membrane microdomains from hybrid aspen cells are involved in cell wall polysaccharide biosynthesis
Résumé
Detergent-resistant plasma membrane microdomains (DRMs) were isolated recently from several plant species. As for animal cells, a large range of cellular functions, such as signal transduction, endocytosis, protein trafficking, etc, have been attributed to plant lipid rafts and DRMs. The data available are essentially based on proteomics and more approaches need to be undertaken to elucidate the precise function of individual populations of DRMs in plants. We report here the first isolation of DRMs from purified plasma membranes of a tree species, the hybrid aspen Populus tremula x tremuloides, and their biochemical characterization. Plasma membranes were solubilised with Triton X-100 and the resulting DRMs were isolated by flotation in sucrose gradients. The DRMs were enriched in sterols, sphingolipids and glycosylphosphatidylinositol-anchored proteins and thus exhibited similar properties as DRMs from other species. However, they contained key carbohydrate synthases involved in cell wall polysaccharide biosynthesis, namely callose ((1→3)-β-D-glucan) and cellulose synthases. The association of these enzymes with DRMs was demonstrated using specific glucan synthase assays and antibodies, as well as biochemical and chemical approaches for the characterization of the polysaccharides synthesized in vitro by the isolated DRMs. More than 70% of the total glucan synthase activities present in the original plasma membranes was associated to the DRM fraction. In addition to shedding light on the lipid environment of callose and cellulose synthases, our data demonstrate the involvement of DRMs in the biosynthesis of important cell wall polysaccharides. This novel concept suggests a function of plant membrane microdomains in cell growth and morphogenesis.
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