Role of GPI anchor in plant cell wall construction

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The Spirodela oligorrhiza purple acid phosphatase (PAP) inducibly synthesized under phosphate-deficient conditions was at the first time proved to be a glycosylphosphatidylinositol (GPI)-anchored protein found in higher plants. To know the primary structure of the GPI-anchored PAP its cDNA clone has been obtained by a combination of cDNA library screening and 5' rapid amplification of cDNA from plants (-P plants) grown under phosphate-deficient conditions. The open reading frame of the S.oligorrhiza PAP cDNA consists of 1365 bp encoding a 455-amino acid protein. Its deduced amino acid sequence shows 82% and 80% similarity with that of PAP from Arabidopsis thaliana and Phaseolus vulgaris, respectively. The amino acid residue Ala-439 followed by two more small amino acid residues, Asp and Ser, is predicted to be the GPI-anchoring ω-site. The absence of a dibasic motif upstream of the putativeω-site suggests that the PAP is a cell wall protein. This presumption is supported by the finding that PAP was released by digestion of the cell wall fraction with cellulase. The GPI anchor is speculated to be a signal for transporting PAP to the cell wall. Immuno-histochemical results using-P plant roots demonstrate that PAP is preferentially distributed in the outer-most cortical cells of roots but not in the epidermis, suggesting its role for acquiring inorganic phosphate under phosphate-deficient conditions.Screening of new GPI-anchored proteins has beencarried out using cultured cells of the moss Marchantia polymorpha. Radio-labeled ethanolamine and mystic acid were both incorporated significantly into a 47 kDa protein which was the sole major secretory protein of the moss. The N-terminal amino acid sequencing of the protein indicates thatthe 47 kDa protein is a protein similar to the desiccation-related protein from Craterostigma plantagineum. The moss protein would be a new GPI-anchored protein.