Structure of the glycosylphosphatidylinositol anchor of an arabinogalactanprotein from Pyrus communis suspension-cultured cells

Arabinogalactan proteins (AGPs) are proteoglycans of: higher plants, which are implicated in growth and development. We recently have shown that two A GPs, NaAGP1 (from Nicotiana alata styles) and PcAGP1 (from Pyrus communis c ell suspension culture), are modified by the addition of a glycosylphosphat idylinositol (GPI) anchor. However, paradoxically, both AGPs were buffer so luble rather than membrane associated, We now show that pear suspension cul tured cells also contain membrane-bound GPI-anchored AGPs. This GPI anchor has the minimal core oligosaccharide structure, D-Man alpha(1-2)-D-Man alph a(1-6)-D-Man alpha(1-4)-D-GlcN-inositol, which is consistent with those fou nd in animals, protozoa, and yeast, but with a partial beta(1-4)-galactosyl substitution of the 6-linked Man residue, and has a phosphoceramide lipid composed primarily of phytosphingosine and tetracosanoic acid. The secreted form of PcAGP1 contains a truncated GPI lacking the phosphoceramide moiety , suggesting that it is released from the membrane by the action of a phosp holipase D. The implications of these findings are discussed in relation to the potential mechanisms by which GPI-anchored AGPs may be involved in sig nal transduction pathways.