THE GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PHOSPHATASE FROM SPIRODELA-OLIGORRHIZA IS A PURPLE ACID-PHOSPHATASE

We recently presented clear evidence that the major low-phosphate-indu cible phosphatase of the duckweed Spirodela oligorrhiza is a glycosylp hosphatidylinositol (GPI)-anchored protein, and, to our knowledge, is the first described from higher plants (N. Morita, H. Nakazato, H. Oku yama, Y. Kim, C.A. Thompson, Jr. [1996] Biochim Biophys Acta 1290: 53- 62). In this report the purified 57-kD phosphatase is shown to be a pu rple metalloenzyme containing Fe and Mn atoms and having an absorption maximum at 556 nm. The phosphatase activity was only slightly inhibit ed by tartrate, as expected for a purple acid phosphatase (PAP). Furth ermore, the protein cross-reacted with an anti-Arabidopsis PAP antibod y on immunoblots. The N-terminal amino acid sequence of the phosphatas e was very similar to those of Arabidopsis, red kidney bean (Phaseolus vulgaris), and soybean (Glycine max) PAP. Extracts of 5. oligorrhiza plants incubated with the GPI-specific precursor [H-3]ethanolamine wer e treated with antibodies raised against the purified S. oligorrhiza p hosphatase. Radioactivity from the resulting immunoprecipitates was sp ecifically associated with a 57-kD band on sodium dodecyl sulfate-poly acrylamide gels. These results, together with previous findings, stron gly indicate that the GPI-anchored phosphatase of S. oligorrhiza is a PAP.