Structure of an Arabinogalactan-Protein Glycosylphosphatidylinositol Anchor
The observation that two AGPs (Nal from Nicotiana alata and Pc1 from pear) were present in extracellular secretions, when the respective cDNAs indicated that they both contained a C-terminal transmembrane domain, prompted us to examine these AGPs for evidence of C-terminal processing. The AGPs Nal and Pcl were purified and deglycosylated with anhydrous hydrogen fluoride. Their protein backbones and respective peptides produced by chemical and enzymic cleavage were analyzed by electrospray-ionization mass-spectrometry, protein sequencing and amino acid analysis. The results showed that the entire C-terminal hydrophobic domain was absent from both proteins and that they each contained an ethanolamine residue on the C-terminal amino acid (Youl et al 1998). This finding is a very good indication that both AGPs contained glycosylphosphatidylinositol (GPI) membrane anchors, although, interestingly, neither contained lipid. The GPI-glycan moiety was obtained, however, after dephosphorylation of AGP Pcl, suggesting that the lipid had been removed by a phospholipase. A lipid-anchored form of AGP with a glycan moiety identical to that found in the secreted Pcl was purified from detergent extracts of suspension-cultured pear cells. The complete structure of the GPI anchor has been determined (Oxley and Bacic 1999). We are currently investigating the mechanism by which GPI-AGPs are released from the plasma membrane.