Presence of a glycosylphosphatidylinositol lipid anchor on rose arabinogalactan proteins

Arabinogalactan proteins constitute a class of plant cell surface proteogly cans with widespread occurrence and suggested functions in various aspects of plant growth and development, including cell proliferation, expansion, m arking, and death. Previous investigations of subcellular fractions from su spension-cultured cells of "Paul's Scarlet" rose (Rosa sp.) have revealed e xtensive structural similarity between some soluble arabinogalactan protein s from the cell wall space and some plasma membrane-associated arabinogalac tan proteins, thus inspiring the present investigation of the mechanism thr ough which these inherently water-soluble molecules are held on the plasma membrane. Several lines of evidence gained through a combination of methods including reversed-phase chromatography, treatment with phosphatidylinosit ol-specific phospholipase C, and chemical structural analysis now show that some rose arabinogalactan proteins carry a ceramide class glycosylphosphat idylinositol Lipid anchor. The predominant form of the ceramide is composed of tetracosanoic acid and 4-hydroxysphinganine. Plasma membrane vesicles r eadily shed arabinogalactan proteins by an inherent mechanism that appears to involve a phospholipase. This finding has significance toward understand ing the biosynthesis, localization, and function of arabinogalactan protein s and toward stimulating other studies that may expand the currently very s hort List of higher plant proteins found to carry such membrane lipid ancho rs.