Cloning and characterization of cDNA of the GPI-anchored purple acid phosphatase and its root tissue distribution in Spirodela oligorrhiza

A cDNA clone of the glycosylphosphatidylinositol (GPI)-anchored purple acid phosphatase (PAP) has been obtained by a combination of cDNA library scree ning and 5' rapid amplification of cDNA ends from Spirodela oligorrhiza pla nts grown under phosphate-deficient ( - P) conditions. The open reading fra me 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 t o Arabidopsis thaliana and Phaseolus vulgaris PAP, respectively. The amino acid residue, Ala439, followed by two more small amino acid residues, Asp a nd Ser, is predicted to be the GPI-anchoring (omega) site. The absence of a dibasic motif upstream of the putative omega 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 GP I anchor is speculated to be a signal for transporting PAP to the cell wall . Immunohistochemical results using - P plant roots demonstrate that PAP is preferentially distributed in the outermost cortical cells of roots but no t in the epidermis, suggesting its role in acquiring inorganic phosphate un der phosphate-deficient conditions. Northern blot analysis using the S. oli gorrhiza PAP cDNA as a probe demonstrates that expression of the PAP gene i ncreased during growth of - P plants and this time-dependent occurrence in mRNA levels of the PAP in - P plants was also observed in their protein and activity levels.