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Plant Physiol. (1998) 118: 1015-1020 The Glycosylphosphatidylinositol-Anchored Phosphatase from Spirodela oligorrhiza Is a Purple Acid Phosphatase1
Laboratory of Environmental Molecular Biology, Graduate School of Environmental Earth Science, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan (H.N., T.O., M.N., K.W., H.O.); Hokkaido National Industrial Research Institute, AIST, Toyohira-Ku, Sapporo 062-8517, Japan (N.M., K.H.); and Department of Botany, University of Texas, Austin, Texas 78713 (G.A.T.)
We recently presented clear evidence that the major low-phosphate-inducible phosphatase of the duckweed Spirodela oligorrhiza is a glycosylphosphatidylinositol (GPI)-anchored protein, and, to our knowledge, is the first described from higher plants (N. Morita, H. Nakazato, H. Okuyama, Y. Kim, G.A. Thompson, Jr. [1996] Biochim Biophys Acta 1290: 53-62). In this report the purified 57-kD phosphatase is shown to be a purple metalloenzyme containing Fe and Mn atoms and having an absorption maximum at 556 nm. The phosphatase activity was only slightly inhibited by tartrate, as expected for a purple acid phosphatase (PAP). Furthermore, the protein cross-reacted with an anti-Arabidopsis PAP antibody on immunoblots. The N-terminal amino acid sequence of the phosphatase was very similar to those of Arabidopsis, red kidney bean (Phaseolus vulgaris), and soybean (Glycine max) PAP. Extracts of S. oligorrhiza plants incubated with the GPI-specific precursor [3H]ethanolamine were treated with antibodies raised against the purified S. oligorrhiza phosphatase. Radioactivity from the resulting immunoprecipitates was specifically associated with a 57-kD band on sodium dodecyl sulfate-polyacrylamide gels. These results, together with previous findings, strongly indicate that the GPI-anchored phosphatase of S. oligorrhiza is a PAP.
Animal and fungal cells contain a diverse assortment of membrane
proteins, which are anchored to the outside surface of the plasma
membrane solely by a covalently linked GPI moiety (Englund, 1993 In contrast to the more than 150 examples of GPI-anchored proteins now
known in animals and yeast, until recently, there have been no
indications that this type of protein anchorage occurs in algae or
higher plants. Reports of a GPI-anchored nitrate reductase in Chlorella saccharophila (Stöhr et al., 1995 The lipid moiety of the S. oligorrhiza phosphatase anchor
has been tentatively identified as a ceramide (Morita et al., 1996 Materials
Purification of the Phosphatase from S. oligorrhiza Purification of the S. oligorrhiza phosphatase was carried out as described previously (Nakazato et al., 1997aElectrophoresis Proteins were analyzed by SDS-PAGE according to the method of Laemmli (1970)
Amino Acid Sequencing Five milligrams of the purified phosphatase was electrophoresed as described above and transferred onto a PVDF membrane. Bands corresponding to the 57-kD protein were excised from the membrane. The protein was sequenced by the Hokkaido University Analytical Center (Sapporo, Japan) on an automatic gas-phase sequencer (model 477A/120A, Applied Biosystems).Metal Analysis To determine the content of metals, the purified phosphatase (0.37-1.37 mg in 10 mM Tris-HCl buffer, pH 6.8) was dialyzed overnight against distilled water. The concentration of the enzyme was adjusted to 0.15 mg/mL with 1 N HNO3. The contents of Fe, Mn, Zn, Cu, Tl, Pb, Ni, and Co were determined on triplicate samples by microwave-induced plasma/quadrupole MS (Douglas and French, 1981Immunological and Blotting Methods Immunoblotting was performed using anti-S. oligorrhiza phosphatase and anti-Arabidopsis PAP antibodies (a generous gift from Thomas D. McKnight, Texas A & M University, College Station). Two micrograms of the purified phosphatase was electrophoresed under denaturing conditions and transferred onto a PVDF membrane. Immunological detection by immunoblotting was carried out with the purified rabbit anti-PAP IgG, followed by goat anti-rabbit IgG conjugated to horseradish peroxidase (Bio-Rad). Biotinylated Mr standards (Bio-Rad) were visualized after they were coupled to avidin-linked horseradish peroxidase.Affinity Purification of the Anti-S. oligorrhiza Phosphatase Antibody Antiserum to the S. oligorrhiza-purified phosphatase was obtained as previously described (Nakazato et al., 1997aRadioisotope Labeling and Immunoprecipitation About 500 mg of P plants was floated on two 5-mL aliquots of P
medium containing 0.75 mCi of
[3H]ethanolamine. After the samples were
exposed to 18 h of continuous light, proteins were extracted from
labeled plants. Proteins were precipitated from the crude extract with
cold acetone for 3 h at 30°C and then redissolved and
reprecipitated with acetone to remove traces of
[3H]ethanolamine. The proteins were spun
down and resuspended in 1 mL of buffer (50 mM Tris, pH 7.4, 150 mM NaCl, 1% Triton X-100, 5 mM EDTA, 0.2%
SDS, and 1 mM PMSF). Proteins were precipitated with cold
acetone, redissolved in buffer, and again precipitated with cold
acetone. Five microliters of the anti-N-terminal peptide antibody was
added, and the mixture was incubated at 4°C for 16 h with
agitation. Immune complexes were precipitated by the addition of 50 µL of 10% protein A-Sepharose (Sigma) and incubated at 4°C for
3 h. The supernatant after centrifugation was subjected to a
second round of immunoprecipitation. The combined precipitates were
resuspended in 50 µL of SDS-sample buffer and boiled for 4 min to
disrupt the complexes.
Measurement of Radioactivity The immunoprecipitated proteins were separated by SDS-PAGE on 5% polyacrylamide gels. For measurement of radioactivity, the gel was dried, and then the lane containing radioactive proteins was sliced into 1-mm sections with a cutting device. The gel slices were each put into a vial and dissolved with 2 mL of 30% H2O2. Radioactivity was measured in a scintillation counter.
Amino Acid Sequence Following purification of the phosphatase (Nakazato et al., 1997a
Immunoblotting Immunoblotting of the purified phosphatase was performed using an affinity-purified anti-S. oligorrhiza phosphatase antiserum and an anti-Arabidopsis PAP antiserum. As shown in Figure 2, the Arabidopsis PAP antiserum cross-reacted strongly with the S. oligorrhiza phosphatase. As controls, wheat germ and bovine alkaline phosphatases did not cross-react with either of the antisera.Metal Content When calculating the metal content, a phosphatase molecular mass of 57 kD was used. However, a significant but unquantified proportion of this mass is known to be carbohydrate. The metal content was determined on triplicate samples. As shown in Table I, Fe and Mn were detected at 0.3 and 0.25 mol mol 1 phosphatase
subunit, respectively, whereas Zn was
detected at 0.10 mol mol 1 phosphatase subunit. Levels of
Ni, Co, Cu, Tl, and Pb were negligible (data not shown).
Absorption Spectra As analyzed by Beck et al. (1986)
Insensitivity to Tartrate Inhibition Of all of the animal acid phosphatases, only the PAPs are resistant to inhibition by tartrate (Vincent and Averill, 1990
[3H]Ethanolamine Labeling About 500 mg of P plants was labeled with
[3H]ethanolamine, a universal precursor of the
GPI anchor, following the general procedure used previously (Morita et
al., 1996
One of the most widespread and extensively studied GPI-anchored
proteins of animals is alkaline phosphatase (Low, 1989
* Corresponding author; e-mail hoku{at}bio.sci.hokudai.ac.jp; fax 81-11-757-5994. Received March 13, 1998;
accepted August 14, 1998.
Abbreviations: GPI, glycosylphosphatidylinositol. PAP, purple acid phosphatase.
We are grateful to Dr. T. D. McKnight (Texas A & M University, College Station) for providing the antiserum against Arabidopsis PAP and the cDNA clone of Arabidopsis PAP. We also appreciate the synthesis and provision of the N-terminal oligopeptide of S. oligorrhiza PAP by Drs. C. Mazur and L. Wolfe (U.S. Environmental Protection Agency, Athens, GA).
Beck J, McConachie LA, Summors AC, Arnold WN, De Jersey J, Zerner B (1986) Properties of a purple phosphatase from red kidney bean: a zinc-iron metalloenzyme. Biochim Biophys Acta 869: 61-68 Cashikar AG, Kumaresan R, Rao NM (1997) Biochemical characterization and subcellular localization of the red kidney bean purple acid phosphatase. Plant Physiol 114: 907-915 [Abstract]
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Copyright Clearance Center: 0032-0889/98/118//06
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