Glycosylphosphatidylinositol Ceramide Lipid Anchor on Rose Arabinogalactan-Proteins

Abstract

Arabinogalactan-proteins (AGPs) are cell-surface proteoglycans that are widely found throughout the plant kingdom and are present in leaves, roots, floral parts, and seeds. The functions of AGPs are unknown but appear to involve fundamental processes in plant development including cell proliferation, cell expansion, cell wall assembly, cell identity marking, and cell death. Characterization of AGPs from the plasma membrane, cell wall, and culture medium of suspension-cultured rose cells has revealed structural similarities between certain AGPs found in these cell surface domains. The present investigation focused on the question: What is the mechanism through which some AGPs associate with the plasma membrane while other AGPs with very similar aminoacyl and glycosyl compositions are soluble molecules in the aqueous phases of the cell wall and extracellular spaces? The results show that some plasma membrane AGPs, as well as a small proportion of culture medium AGPs, carry a ceramide-class glycosylphosphatidylinositol (GPI) lipid anchor (Svetek et al 1999). The evidence supporting this conclusion includes: (a) the release of AGPs from plasma membrane vesicles in vitro by treatment with exogenous phosphatidylinositol-specific phospholipase C (PI-PLC); (b) the shifting of elution characteristics from hydrophobic to hydrophilic when certain AGPs are treated with exogenous PI-PLC and analyzed by reverse-phase chromatography; (c) the presence of characteristic GPI-linker oligosaccharide components including inositol, glucosamine, and mannose; and (d) the presence of ceramide components, i.e. long-chain fatty acid and long-chain base, in hydrophobic AGPs that are sensitive to PI-PLC. The predominant lipid components are tetracosanoic acid and phytosphingosine. Plasma membrane vesicles readily shed AGPs by an inherent mechanism that appears to involve a phospholipase. The presence of GPI lipid anchors may be a common feature of rose AGPs. This finding has important implications towards understanding the biosynthesis, the subcellular localization, and the biological functions of AGPs, and towards stimulating searches for other GPI-linked proteins in higher plants.