Dynamic turnover of arabinogalactan proteins in cultured Arabidopsis cells

Arabinogalactan proteins (AGPs) are very large proteoglycans thought to have more of a signaling than a structural role when secreted into the plant cell wall. AGPs are also the first known family of abundant plant proteins synthesized with glycosylphosphatidylinositol(GPI) anchors. Nascent cellular Arabidopsis AGPs, still bearing an intact GPI anchor, and AGPs copiously discharged into the culture medium after phospholipase-cleavage of their anchor were each represented by more than 15 seemingly homologous molecular species of increasing size. In washed cells 3H-ethanolamine was slowly incorporated into each AGP’s GPI anchor via phosphatidylethanolamine. Pulse labeling of AGPs by 3H-acetate and by 3H-galactose was much more rapid, allowing labeled AGP detection in the growth medium within 1 h. HPLC analysis of the radiolabel distribution in AGPs secreted within 1–8 h revealed a sharp preference for the larger molecular species. After several hours a population of smaller radioactive AGP species began to appear in the medium. Following certain manipulations of the cells newly secreted AGP species measured by HPLC on a relative mass basis formed a pattern surprisingly different from the radioactivity pattern, although larger species still dominated. Thus Arabidopsis cells appear capable of releasing higher mass AGP species apparently stored in cell wall sites along with a unique mixture of freshly synthesized AGPs in combinations potentially active in signaling.