Two acid phosphatases were partially purified from beet root plasma membranes. One phosphatase had high activity with p-nitrophenyl phosphate (pNPP) as a substrate and contained an 82 kDa protein. This phosphatase was inhibited by metal ions and molybdate or vanadate and had an optimal pH of 5.4. The other phosphatase had lower activity with pNPP but could dephosphorylate myelin basic protein. This phosphatase contained 36 kDa and 65 kDa polypeptides and was highly inhibited by okadaic acid, suggesting it is a PP2A protein phosphatase. About 45% of the acid and protein phosphatase activities in the plasma membranes could be due to

1. PHYSIOLOGIA PLANTARUM 121: 223–230. 2004 DOI: 10.1111/j.1399-3054.2004.00331.x
Printed in Denmark – all rights reserved Copyright # Physiologia Plantarum 2004
Acid phosphatases from beet root (Beta vulgaris) plasma membranes
´
Eduardo Armienta-Aldana and Luis E. Gonzalez de la Vara*
Departamento de Biotecnologı´a y Bioquı´mica, Unidad Irapuato, Centro de Investigacio´n y de Estudios Avanzados del IPN. Apartado
Postal 629, 36500 Irapuato Gto, Me´xico
*Corresponding author, e-mail: lgonzale@ira.cinvestav.mx
Received 25 November 2003; revised 28 January 2004
Several acid phosphatases (EC 3.1.3.2) were found in beet root basic protein (phospho-MBP). This phosphatase presented two
(Beta vulgaris L.) plasma membranes. Two of them were par- polypeptides with molecular masses of 36 and 65 kDa and was
tially purified by an extraction of plasma membranes with 83% inhibited by 2 nM okadaic acid, which suggests it is a
octylglucoside and successive gel-filtration and anion-exchange PP2A protein phosphatase. As the phosphatase activity was
chromatographies. With p-nitrophenyl-phosphate (pNPP) as high in soluble (non-membrane) fractions, the possibility that
substrate, most of the phosphatase activity was found in a frac- phosphatases in plasma membranes were soluble contaminants
tion containing an 82-kDa protein. This phosphatase showed an ´
was assessed. Following the method of Berczi and Møller (Plant
optimum pH of 5.4 and was inhibited by Cu21, Zn21, molybdate Physiol. 116:1029, 1998), it was found that about 45% of both
or vanadate. The other phosphatase had a lower specific activity acid and protein phosphatase activities could be due to soluble
with pNPP, but was able to dephosphorylate phospho-myelin enzymes trapped inside membrane vesicles.
Introduction
Acid phosphatases or orthophosphoric-monoester phos- the plant. Among acid phosphatases, purple acid phos-
phohydrolases (EC 3.1.3.2) are abundant in plant cells. phatases (PAPs) are probably the most thoroughly stud-
These enzymes appear to be important in the production, ied. PAPs are metallo-phosphatases that catalyse the
transport, and recycling of Pi (reviewed in Duff et al. hydrolysis of several phosphate esters and anhydrides.
1994). Acid phosphatases are widely distributed in In the Arabidopsis genome, 29 genes for PAPs have been
plants, and have been found in seeds (Park and Van predicted. The expression of some of them increases in
Etten 1986, Biswas and Cundiff 1991, Kawarasaki et al. response to phosphate deficiency (Li et al. 2002). Some
1996, Granjeiro et al. 1999), roots (Panara et al. 1990, acid phosphatases could be involved in protein dephos-
Penheiter et al. 1997), leaves (Staswick et al. 1994) and phorylations and therefore in signalling pathways (Duff
fruits (Turner and Plaxton 2001). However, no acid et al. 1994).
phosphatases have been found in plasma membranes On the other hand, protein phosphatases are grouped
from plant cells, although their presence in them has in three distinct families. The PPP and PPM families
been suggested by the discovery of a glycosylphosphati- include enzymes that remove phosphate groups from
dylinositol-anchored acid phosphatase in Spirodela oli- Ser-P and Thr-P residues; whereas members of the pro-
gorrhiza (Nakazato et al. 1998). tein tyrosine phosphatase (PTP) family include Tyr-P-
Plant acid phosphatases are involved in responses to specific and dual-specificity phosphatases. The PPP
phosphate deficiency (Duff et al. 1991, Tadano et al. family includes protein phosphatases belonging to the
1993), salt stress (Pan 1987) and water deficit (Barrett- PP1 and PP2A classes [which can be distinguished by
Lennard et al. 1982). These enzymes break down organic its sensitivity to the inhibitor okadaic acid (OA)], as well
molecules to liberate phosphate, making it available to as the PP2B group. The PPM family includes protein
Abbreviations – Brij 58 P, polyethylene glycol hexadecyl ether; CHAPS, 3-[ (3-cholamidopropyl) dimethylammonio]-1-propanesulphonate; EDTA,
ethylendinitrile tetraacetic acid; EGTA, ethylenglycol-bis (ether b-aminoethyl) N,N,N0 ,N0 -tetraacetic acid; HEDTA, N-hydroxyethylethylenediamine-
triacetic acid; MBP, myelin basic protein; pNPP, p-nitrophenyl phosphate; PAP, purple acid phosphatase.
Physiol. Plant. 121, 2004 223

2. phosphatases 2C (PP2C) (Luan 2000). In the Arabidopsis 19 400 Â g for 30 min at 4 C. The supernatant (2 ml) was
thaliana genome, 112 protein phosphatase genes have loaded onto a Sephacryl S-300-HR gel-filtration column
been predicted (Kerk et al. 2002). Of these, 69 belong (50 Â 1 cm), equilibrated with 30 mM HEPES, 1 mM
to the PP2C group, 23 are classified as PPP (including 8 DTT, 1 mM EDTA and 100 mM NaCl (pH 7.0) and
PP1 and 5 PP2A phosphatases) and 18 as dual-specificity eluted with the same buffer. The fractions with highest
PTPs. phosphatase activity from the Sephacryl column were
There are several enzymes whose activities are con- pooled and loaded onto a DEAE-Sepharose column
trolled by phosphorylation in plasma membranes. For (6 Â 0.7 cm), equilibrated with 30 mM HEPES (pH 7.0),
instance, the H1-transporting ATPase is an enzyme 1 mM DTT and 1 mM EDTA (column buffer). Proteins
phosphorylated in vivo (Desbrosses et al. 1998). Its were eluted first with 1 ml column buffer with 0.1 M
phosphorylation status, which reflects the activity of NaCl, then with 4 ml of a 0.1–0.8 M NaCl gradient in
endogenous kinases and phosphatases, regulates its column buffer. Phosphatase activity (using pNPP as sub-
association with 14-3-3 proteins and, thereby, its activity strate) and protein content were measured in all (0.5 ml)
(Morsomme and Boutry 2000). In plasma membranes fractions collected.
from tomato leaves, the H1-ATPase activity increased To analyse the phosphatase activity not extracted with
when they were exposed to a fungal elicitor. This stimu- n-octylglucoside, the pellet obtained in this extraction
lation was correlated with the dephosphorylation of the was treated with 3% Triton X-114 (v/v) in column buffer
H1-ATPase by a membrane-bound phosphatase (Vera- (2 ml) for 30 min at 4 C with moderate shaking. To allow
Estrella et al. 1994). More recently, Camoni et al. (2000) a phase separation, this mixture was centrifuged at
showed that the activity of a PP2A phosphatase in maize 19 400 Â g for 3 min at room temperature. The upper
roots reduced drastically the association of 14-3-3 pro- (least hydrophobic) phase was collected, and the lower
teins with the plasma membrane H1-ATPase. phase was extracted with 1 ml of upper phase (prepared
Herein we report the purification and some properties without membranes). Upper phases were pooled
of two plasma membrane-associated acid phosphatases. (2 ml) and loaded onto a DEAE-Sepharose column
One of them appears to be a typical acid phosphatase, (4.5 Â 1.2 cm), equilibrated with column buffer. Phos-
whereas the other is probably a PP2A protein phospha- phatases were eluted with 5 ml of column buffer with
tase. To assess the possibility that these phosphatases 0.1 M NaCl. The fraction (1 ml) with highest phospha-
were soluble contaminants, plasma membrane phospha- tase activity was loaded onto a CM-Sepharose column
´
tases were sequentially extracted as proposed by Berczi (6 Â 0.7 cm), equilibrated with column buffer. Proteins
and Møller (1998). We found that an important fraction were then eluted with a 4-ml 0–0.6 M NaCl gradient in
of the phosphatase activity could be due to soluble the same buffer.
enzymes trapped inside membrane vesicles.
Phosphatase activity measurements
Materials and methods Phosphatase activity was routinely measured observing
the conversion of p-nitrophenyl phosphate (pNPP) into
Chemicals and plant material
p-nitrophenol in a reaction mixture (0.5 ml) containing
Beet roots (Beta vulgaris L.) were obtained at a local 15 mM BTP/succinic-acid buffer (at the indicated pH),
market and stored at 4 C, until used, usually 1 or 2 days 150 mM KCl, and 5 mM pNPP, unless otherwise indi-
later. Unless indicated otherwise, chemicals were obtained cated. Reactions were started by adding the enzyme-
from Sigma Chemical Co (St Louis, MO, USA). containing sample and reading the absorbance at 405
and 466 nm (control wavelength) at 0, 15, 30 and
60 min. To obtain the p-nitrophenol concentration pro-
Plasma membrane preparation
duced, a differential millimolar extinction coefficient
Plasma membranes from beet roots were prepared by (e(405À466)) of 8.317 (in 0.1 N NaOH) was used. This
two-phase aqueous partitioning as described (Lino et al. coefficient was obtained hydrolysing a solution of
1998) and kept at À70 C in 25 mM Tris/MES (pH 7.5), pNPP with alkaline phosphatase and determining the
2 mM EDTA, 1 mM DTT (United States Biochemical phosphate concentration and the A405–A466 value in it.
Corp. (USB), Cleveland, OH) and 45% (v/v) glycerol Extinction coefficients at different pH values were
until used. The supernatant obtained with the total mem- calculated using a pKa for p-nitrophenol of 7.15.
brane (microsomal) pellet was used as a source of soluble Alternatively, phosphatase activities were determined
proteins. by measuring the inorganic phosphate released according
to Ames (1966). Reaction mixtures (120 ml) contained
30 mM BTP/succinic-acid buffer (at the indicated pH),
Purification of acid phosphatases
5 mM pNPP (or other substrate at the indicated concen-
Plasma membranes (with 8 mg of protein) were extracted tration) and 150 mM KCl. Reactions were started by
with n-octylglucoside (1 g per g of protein) in 30 mM adding 8–10 ml of the sample with phosphatase activity.
HEPES, 1 mM DTT, 1 mM EDTA for 1 h at 4 C with After 60 min at 30 C, the reaction was stopped with
moderate shaking. The homogenate was centrifuged at 1.88 ml of the molybdate reagent (0.02 g ascorbic acid,
224 Physiol. Plant. 121, 2004

3. 0.42% ammonium heptamolybdate in 1 N H2SO4) used Electrophoresis
to measure the released phosphate. The amount of phos-
The electrophoretic separation of proteins was performed
phate released was measured by reading the A820 30 min
in 7.5% (w/v) polyacrylamide gels (SDS-PAGE) following
later. This method was used to determine phosphatase
the method described by Schagger and von Jagow (1987).
¨
activities at pH values lower than 6.0 (where the p-nitro-
Electrophoreses were run at room temperature. After
phenol absorbance is too low), or with substrates other
electrophoresis, gels were fixed in 50% methanol and
than pNPP.
10% acetic acid for 30 min, and stained with Coomassie
blue (Serva blue G, SERVA Electrophoresis GmbH,
Heidelberg, Germany) (Schagger and von Jagow 1987).
¨
Phosphoprotein phosphatase activity measurements
Protein phosphatase activity was determined using
32 Kinetic studies
P-labelled myelin basic protein (32P-MBP) as substrate.
32
P-MBP was prepared by phosphorylating MBP [Gibco Activity versus substrate concentration curves were
(Invitrogen, Carlsbad, CA) or Sigma] with a 63-kDa Ca21- obtained for pNPP (0.1–10 mM) and for 32P-MBP (0.05–
dependent protein kinase purified from beet root plasma 2 mg mlÀ1). When pNPP was used as substrate, the reac-
´
membranes (Carrillo, Lino and Gonzalez de la Vara; tions were run in a 30-mM MES/Tris (pH 6.2) buffer. 32P-
unpublished results). The phosphorylation mixture (75– MBP was dephosphorylated in 30 mM BTP/succinic acid
90 ml) contained 30 mM HEPES/Tris (pH 7.0), 1.5 mM (pH 6.6), 150 mM KCl, 1 mM EDTA and 5 mM MgSO4.
EGTA, 1.5 mM HEDTA, 2.4 mM CaCl2, 3.3 mM MgSO4 Activity versus pH curves were obtained using 30 mM
(calculated to obtain free ion concentrations of 10 mM and BTP/succinic acid (at the indicated pH, from 4.2 to 7.5).
2.5 mM for Ca21 and Mg21, respectively), 0.1 mM For the 82- and 95-kDa acid phosphatases, the phos-
[g-32P]ATP (37 TBq molÀ1) and 5 mg mlÀ1 MBP. The phate released was measured as described by Ames
kinase (with an activity of about 20 pmol minÀ1, using (1966). For the 36-kDa acid phosphatase we measured
MBP as substrate) was added to start the phosphorylation the dephosphorylation of 32P-MBP with the above buffer
reaction. After an overnight incubation at room tempera- at pH values in the same range. To calculate best-fitting
ture, low-molecular mass compounds were removed with a parameters, all curves were analysed with the program
1-ml spin gel-filtration (Sephadex G25) column, equili- ORIGIN version 6.1 (OriginLab, Northampton, MA).
brated with 5 mM HEPES/Tris (pH 7.0) and 100 mM KCl.
The phosphatase assay mixture (90 ml total volume)
contained 30 mM BTP/succinic acid (pH 6.6), 150 mM Sequential extraction of plasma membrane phosphatases
KCl, 1 mM EDTA and 0.5 mg mlÀ1 32P-MBP. The
To estimate if the phosphatases were bound to plasma mem-
dephosphorylation reaction was started by adding 50 ml
branes or were simply trapped inside the vesicles formed by
of the phosphatase preparation, and a 20-ml aliquot was
them (which would suggest that these phosphatases could be
immediately withdrawn from this mixture and spotted
soluble contaminants), proteins were sequentially extracted
onto a 2 Â 2 cm Whatman P81 phosphocellulose paper
´
from plasma membranes following the method of Berczi and
piece. After a 30-min incubation at room temperature,
Møller (1998) with some modifications. Frozen plasma
three 20-ml aliquots were withdrawn and spotted onto
membranes (with 5 mg of protein) were thawed and diluted
P81 paper pieces. These pieces were washed with 75 mM
to 5 ml in 0.2 M sucrose, 20 mM Tris/MES (pH 7.0) and
H3PO4 (3 Â 10 min), rinsed in ethanol (1 Â 5 min), air-
2 mM DTT, and centrifuged at 200 000 Â g at 4 C for
dried, placed in vials with scintillation liquid and counted
30 min in a Beckman NVT90 rotor (Beckman Coulter, Full-
for radioactivity. To calculate the phosphate released
erton, CA). This first supernatant (SN1) contains trapped or
from 32P-MBP, the radioactivity in the aliquots taken
weakly bound proteins in plasma membrane vesicles that are
at 30-min time was subtracted from that in the aliquot
released by thawing. The pellet (P1) was re-suspended and
taken at zero time.
diluted in 5 ml of 20 mM Tris/MES (pH 7.0), 2 mM DTT and
0.3 M KI. After a 5-min incubation, this suspension was
centrifuged as above. The supernatant (SN2, which contains
Estimation of native molecular masses
proteins bound to the outside of membrane vesicles by weak
Native molecular masses of acid phosphatases were esti- ionic or hydrophobic forces) was collected. The pellet (P2)
mated by gel filtration in the same Sephacryl S-300-HR was re-suspended in 5 ml of 0.2 M sucrose, 20 mM Tris/MES
column used in the purification process. One-millilitre (pH 7.0), 2 mM DTT, and 0.05% (w/v) polyethylene glycol
fractions were eluted with 30 mM HEPES, 1 mM DTT, hexadecyl ether (Brij 58 P). After a 15-min incubation, the
1 mM EDTA and 100 mM NaCl (pH 7.0) at a flow rate homogenate was centrifuged as above. The detergent Brij 58
of 0.25 ml minÀ1. Native relative molecular masses (Mr) P opens membrane vesicles and changes their sidedness
were calculated from a plot of Kd (partition coefficient) (Johansson et al. 1995), so that this third supernatant
against log (Mr) using the following protein standards: (SN3) contains mainly proteins trapped inside them. The
urease hexamer (545 kDa), urease trimer (272 kDa), pellet (P3) was re-suspended and diluted (to 5 ml) in 20 mM
b-amylase (200 kDa), alcohol dehydrogenase (150 kDa) Tris/MES (pH 7.0), 2 mM DTT and 0.3 M KI, to release the
and carbonic anhydrase (29 kDa). proteins weakly bound to the inner surface of the membrane
Physiol. Plant. 121, 2004 225

4. vesicles. After a 5-min incubation, this suspension was cen- in a single peak in the DEAE-Sepharose ion-exchange
trifuged as above. The supernatant (SN4) was collected, and column. Fractions in this peak contained few polypep-
the pellet (P4) was re-suspended in 5 ml of 0.2 M sucrose, tides, the most conspicuous among them was an 82-kDa
20 mM Tris/MES (pH 7.0), 2 mM DTT and 15 mM 3-[(3- one (Fig. 2). Because this polypeptide appeared in all
cholamidopropyl)dimethylammonio]-1-propanesulphonate preparations with acid phosphatase activity, we attribu-
(CHAPS). This suspension was incubated for 20 min and ted the phosphatase activity to it, even though we have
centrifuged as indicated. The supernatant (SN5), which no further evidence to support this.
contains solubilized membrane proteins, was collected. The 82-kDa phosphatase was purified 129 times
The last pellet (P5) was re-suspended in 0.5 ml of 0.2 M from plasma membranes, to a specific activity of
sucrose, 20 mM Tris/MES (pH 7.0) and 2 mM DTT. Acid 3726 nmol mg À1minÀ1 (Table 1). Its optimum pH value
phosphatase activity in supernatants and P5 was measured was 5.6 Æ 0.2 (with pK1 and pK2 values of 4.5 and 6.6,
with pNPP as substrate. Protein phosphatase activity was respectively). With pNPP, the 82-kDa acid phosphatase
measured with 32P-MBP as described. presented Michaelis–Menten kinetics; the Km calculated
for this substrate was 7.7 Æ 2.0 mM.
The activity of the purified 82-kDa phosphatase was
Protein determination assayed using a variety of phosphorylated substrates at a
Protein concentration was determined by a modification 5-mM concentration. This phosphatase was able to dephos-
of Stoscheck (1990) method, using bovine serum albumin phorylate phosphoaminoacids (Tyr-P, Ser-P and Thr-P),
(BSA) as standard. ATP, GTP, PPi, a- and b-naphtyl phosphates, in addition
to pNPP. The 82-kDa phosphatase was unspecific: its high-
est (observed with sodium pyrophosphate) and lowest
(with Ser-P) activities were only 113 and 92%, respectively,
Results
of those obtained with pNPP. This phosphatase was
Partial purification of plasma membrane phosphatases unable to dephosphorylate 32P-MBP (data not shown).
As with most acid phosphatases, the 82-kDa one was
Acid phosphatase activity is conspicuous in beet root
inhibited by 1 mM ammonium heptamolybdate (63%),
plasma membranes. To partially purify some of these
1 mM sodium orthovanadate (55%) or 5 mM NaF
phosphatases, the procedure described in Materials and
(36%). This phosphatase was also strongly inhibited by
methods was followed. As shown in Table 1, treating
1 mM CuSO4 and by 5 mM ZnSO4: 72 and 58%, res-
plasma membranes with n-octylglucoside extracted about
pectively. The only significant activation (82%) was
60% of the acid phosphatase activity in them. Extracted
observed with 150 mM KCl. Divalent cations such as
proteins were separated in a Sephacryl S-300-HR gel-
Mg21, Mn21 (at 5 mM), Ca21 (1 mM), or chelators like
filtration column, from which most of the acid phospha-
EDTA, EGTA or HEDTA (at 5 mM) did not signifi-
tase activity was recovered in a peak corresponding to
cantly affect the activity of this acid phosphatase.
proteins with native Mr about 85 kDa (Fig. 1A). This
peak contained a low amount of protein (Fig. 1A) and
showed few protein bands in SDS-PAGE (Fig. 2). In the The 36-kDa protein phosphatase
following purification step (a DEAE-Sepharose column),
The phosphatase peak eluting with about 0.5 M NaCl
two phosphatase activity peaks were obtained (Fig. 1B).
from the DEAE-sepharose column (Fig. 1B), unlike the
The fraction with highest activity (using pNPP as sub-
main acid phosphatase peak, presented phosphoprotein
strate) showed an 82-kDa polypeptide (among others) in
phosphatase activity. Fractions with this activity showed
SDS-PAGE gels (Fig. 2). The minor activity peak (eluted
two conspicuous polypeptides with Mr values of 65 and
with about 0.5 M NaCl. Fig. 1B) presented two main poly-
36 kDa in SDS-PAGE (Fig. 2, lane 5). Using 32P-MBP as
peptides with molecular masses of 36 and 65 kDa (Fig. 2).
substrate, the highest specific activity obtained was
6845 pmol mg À1 minÀ1, 73.9 times the activity shown by
plasma membranes (Table 2). This protein phosphatase
The 82-kDa acid phosphatase
presented maximal activity at pH 6.6 Æ 0.7, with pK1 and
As seen in Fig. 1B, most of the phosphatase activity, pK2 values of 6.2 and 7.1, respectively. Activity versus
32
measured at pH 6.3 with pNPP as substrate, appeared P-MBP concentration curves showed a linear trend up
Table 1. Purification of an 82-kDa acid phosphatase from beet root plasma membranes. Phosphatase activity was measured at pH 6.3 with
5 mM p-nitrophenyl phosphate as substrate.
Protein Total activity Specific activity Purification Yield
Step (mg) (nmol minÀ1) (nmol mgÀ1 minÀ1) (fold) (%)
Plasma membranes 5.51 159.5 28.9 1.0 100
Octylglucoside extract 1.96 96.7 49.4 1.7 60.7
Gel filtration 0.0499 23.5 470.2 16.2 14.7
Ion-exchange 0.0018 6.71 3726 129 4.2
226 Physiol. Plant. 121, 2004

5. Fig. 1. Purification of two acid phosphatases from beet root plasma
membranes. The acid phosphatase activity (*) and the protein
concentration (*) of the eluted fractions are shown. Solid arrows Fig. 2. Protein patterns of fractions obtained in the purification of
point to peaks with highest acid phosphatase activity. (A) Elution acid phosphatases. Proteins were separated by SDS-PAGE, and the
profile of a Sephacryl S-300 HR chromatography column loaded with gels were stained with Coomassie blue. The electrophoresis gel was
an n-octylglucoside extract of beet root plasma membranes. One- loaded with plasma membrane proteins (10 mg of protein mixed with
millilitre fractions were collected. Broken-line arrows on the top point n-octylglucoside; lane 1), proteins extracted with octylglucoside
to the fractions in which urease hexamer (545 kDa), urease trimer (5 mg; lane 2), the fraction with highest acid phosphatase activity
(272 kDa), b-amylase (200 kDa), alcohol dehydrogenase (150 kDa) from a Sephacryl S-300 column (2 mg; lane 3), a fraction from a
and carbonic anhydrase (29 kDa) were eluted. (B) DEAE-Sepharose DEAE-Sepharose column with the 82-kDa acid phosphatase (1 mg;
chromatography of the fraction with highest acid phosphatase activity lane 4), and a fraction with the 36-kDa phosphatase from the same
from the Sephacryl column. Fraction volume was 0.5 ml. Proteins were column (1 mg; lane 5). Positions of molecular mass markers (values
eluted with 0.1 M NaCl (1 ml), a linear 0.1–0.8 M NaCl gradient (4 ml) in kDa) are shown at the left margin. The solid arrow point to the
and 1 M NaCl (1 ml), as indicated by the broken line without symbols. 36-kDa polypeptide (probable catalytic subunit of a PP2A
phosphatase), the broken-line arrow to the 82-kDa acid
phosphatase, and the dotted-line one to the 65-kDa polypeptide
(probably, a regulatory subunit of a PP2A phosphatase).
to 2 mg mlÀ1, which did not allow us to calculate any Km
value for this substrate (data not shown).
The effects of various possible inhibitors or activators The polypeptide composition of this phosphatase (Fig. 2,
on this phosphatase are shown in Table 3. The best inhi- lane 5) also suggests it could be a PP2A: the 36-kDa
bitor tested was okadaic acid, which caused an inhibition polypeptide could be the catalytic subunit and the 65-
greater than 80% at a concentration of 2 nM (100% at kDa one, a ‘type-A’ regulatory subunit (Luan 2000).
2 mM). This phosphatase was also inhibited by 5 mM
fluoride. As expected, it was not inhibited by vanadate
Sequential extraction of plasma membrane phosphatases
(1 mM), a known inhibitor of phospho-tyrosine phos-
phatases. The high sensitivity to okadaic acid shown by During the preparation of plasma membranes, a high
this phosphatase suggests it belongs to the PP2A class. acid-phosphatase activity (measured with pNPP) was
Table 2. Purification of a phosphoprotein phosphatase from beet root plasma membranes. Phosphatase activity was measured at pH 6.6 with
0.5 mg mlÀ1 32P-MBP as substrate.
Protein Total activity Specific activity Purification Yield
Step (mg) (pmol minÀ1) (pmol mgÀ1 minÀ1) (fold) (%)
Plasma membranes 7.58 701.7 92.6 1 100
Octylglucoside extract 1.98 232.7 117.3 1.27 33.2
Gel filtration 0.0544 83.01 1526 16.5 11.8
Ion-exchange 0.0013 9.15 6845 73.9 1.3
Physiol. Plant. 121, 2004 227

6. Table 3. Effects of various compounds on the activity of the
purified 36-kDa acid phosphatase. Phosphatase activity was
measured with 0.5 mM 32P-MBP as substrate, in media containing
150 mM KCl (with the exception of the NaCl-containing reaction
medium). Activities are means of two measurements, and are
expressed as a percentage of the activity with KCl only
(2863 pmol mg À1 minÀ1).
Activity
Compound Concentration (%)
Okadaic acid 2 nM 16.6
Okadaic acid 2 mM 0
Ammonium heptamolybdate 1 mM 55.1
Sodium orthovanadate 1 mM 99.3
NaF 5 mM 10.7
MgSO4 5 mM 31.2
MnSO4 5 mM 51.7
FeSO4 5 mM 80
KCl 150 mM 100
NaCl 150 mM 105
found in the supernatant containing soluble proteins.
When this supernatant was processed in the same way as
the octylglucoside extract from plasma membranes, the
phosphatase activity was found in the same chromatogra-
phy fractions (the fractions with native Mr about 85 kDa
in the gel filtration column and the fractions eluting with
0.1 M NaCl in the ion-exchange one) where the plasma- Fig. 3. Sequential extraction of acid phosphatase activity in plasma
membrane vesicles using pNPP (A) or 32P-MBP (B) as substrate.
membrane phosphatases were found. Phosphoprotein Plasma membranes kept frozen were thawed and centrifuged as
phosphatase activity was also found in the ion-exchange indicated in Materials and Methods to get the first supernatant
column fractions eluting with about 0.5 M NaCl (data not (SN1). The resulting pellet was extracted with 0.3 M KI to get the
proteins weakly bound to the outer surface of plasma membranes in
shown). These observations suggested that the purified the second supernatant (SN2). The pellet was treated with 0.05%
phosphatases from plasma membranes could be contam- (w/v) Brij 58 P to remove soluble proteins trapped inside vesicles
´
inating soluble proteins (Berczi and Asard 2003). (SN3). Membranes were then extracted with 0.3 M KI to get the
To evaluate this possibility, phosphatases were proteins weakly bound to the inner surface in SN4. Finally, the
resulting pellet was extracted with 15 mM CHAPS to get the
extracted sequentially from plasma membranes as sug- solubilized membrane proteins in SN5, and the proteins resisting
´
gested by Berczi and Møller (1998). With this method, this extraction procedure in the last pellet (P5). Acid (A) and protein
proteins that are released just by freezing and thawing (B) phosphatase activities were measured as described in Materials
and Methods.
the membrane vesicles are found in the first supernatant
(SN1). Later on, proteins weakly bound to the outside of
the vesicles (SN2), trapped inside them (SN3), weakly
phosphatase activity, some (27%) protein phosphatase
bound to the inside of the vesicles (SN4) and strongly
activity was extracted with KI, which showed the pres-
bound to membranes (SN5) are extracted in sequence. In
ence of protein phosphatases that were weakly bound to
Fig. 3A, it can be seen that a great proportion of the acid
plasma membranes.
phosphatase activity (measured with pNPP) was released
by only opening the membrane vesicles (in SN1 and
SN3), but a strong detergent was needed to extract a
Phosphatases not extracted with octylglucoside
significant percentage of it (in SN5). Less than 15% of
the total phosphatase activity was released with the KI As shown in Table 1, about 40% of the acid phosphatase
treatments (SN2 and SN4), which suggests that very few activity was not extracted from the plasma membranes
acid phosphatases were weakly bound to plasma mem- with octylglucoside. Proteins in these membranes were
brane surfaces. All these results showed that more than extracted with Triton X-114, and the phosphatases in
45% of the acid phosphatase activity found in plasma this new extract were purified partially by two ion-
membranes could be due to soluble phosphatases exchange chromatography steps as described in Mater-
trapped inside the membrane vesicles. ials and Methods. Several peaks with acid phosphatase
The phosphoprotein phosphatase activity in the frac- activity were observed, and one acid phosphatase (opti-
tions of this sequential extraction is shown in Fig. 3B. mum pH: 6.0) was purified to near homogeneity. Frac-
About 45% of this activity was extracted from plasma tions with this phosphatase presented a protein band
membranes with treatments that only open membrane with a Mr of 95 kDa in SDS-PAGE. This phosphatase
vesicles, and a further 24% needed a strong detergent hydrolysed pNPP, but not 32P-MBP (data not shown).
treatment to be extracted. However, unlike the acid We are actually characterizing it.
228 Physiol. Plant. 121, 2004

7. Discussion the maize and beet phosphatases, respectively) are of the
same order of magnitude, and reflect their degrees of
In this article, we show the presence of three acid phospha- purification. On the other hand, the protein yield of the
tases in beet root plasma membranes. With one extraction 36-kDa phosphatase from beet root plasma membranes
and two chromatography steps, two of them were purified. (1.3 mg) was much lower than that of the maize root
After the first chromatographic step: a gel filtration, most of enzyme (25 mg). This low protein yield could make uncer-
the activity appeared in a peak with proteins having mol- tain the estimation of specific activities.
ecular masses about 85 kDa. In the second chromatographic ´
Berczi and Asard (2003) have pointed out that the
step (ion-exchange), the fraction with highest specific activ- possibility of contamination of membrane preparations
ity was obtained eluting the DEAE-Sepharose column with with soluble proteins is often overlooked. To find out
a low salt concentration. This fraction contained a conspic- if the phosphatases in plasma membranes were only
uous 82-kDa protein, a molecular mass similar to that soluble enzymes trapped inside membrane vesicles, the
estimated in a gel-filtration column; which suggests that ´
sequential extraction procedure proposed by Berczi and
this phosphatase is a monomer. This phosphatase presented Møller (1998) was followed. With this procedure, we
an acid pH optimum: 5.4, and was able to hydrolyse many found that important fractions of the acid and protein
substrates. Its Km value for pNPP (7.7 mM) is higher than phosphatase activities appear to be soluble contami-
most of the values reported for acid phosphatases (Duff et al. nants. Particularly, it is possible that the 82-kDa phos-
1994). This phosphatase was inhibited most strongly by phatase were only a trapped soluble enzyme, since a
Cu21 and Zn21 ions, and by phosphate analogues such as phosphatase with its chromatographic properties is
molybdate and vanadate. The inhibitions by Cu21 and abundant in the soluble fraction obtained during the
vanadate suggest the participation of SH groups in the preparation of plasma membranes, and almost no acid
catalytic mechanism (Granjeiro et al. 1999). phosphatase activity weakly bound to these membranes
The 82-kDa phosphatase is, most probably, different was found. The phosphatase activity strongly bound to
from purple acid phosphatases (PAPs). PAPs from Ara- plasma membranes (solubilized with CHAPS) could be
bidopsis thaliana (Coello 2002, Li et al. 2002) or from due to phosphatases not extracted by octylglucoside,
Lupinus albus (Wasaki et al. 1999, 2000) have monomer such as the 95-kDa phosphatase.
molecular masses lower than 82 kDa. Taking into In contrast, a significant fraction of the protein phos-
account its molecular mass and its sensitivity to Cu21 phatase activity was found to be weakly bound to plasma
and Zn21 ions, this phosphatase could be similar to those membranes. This suggests that soluble phosphatases
isolated from white clover root cell walls, one of which could bind to, and dephosphorylate, plasma membrane
presents a Mr of 113 kDa (Zhang and McManus 2000). phosphoproteins. In fact, the PP2A phosphatase from
Although phosphoprotein phosphatase activities have maize, able to dephosphorylate the plasma membrane
been described for some plant acid phosphatases (Duff H1-ATPase, was purified from a soluble (cytosolic) frac-
et al. 1994, Gellatly et al. 1994), and the 82-kDa acid tion (Camoni et al. 2000).
phosphatase purified in this work was able to dephos- In conclusion, three acid phosphatases were purified
phorylate Ser-P, Thr-P and Tyr-P; it is, most probably, from beet root plasma membranes. The 82-kDa one
not a protein phosphatase involved in signal transduc- shows biochemical properties similar to those observed
tion, as it was unable to dephosphorylate MBP phos- with other plant acid phosphatases, and could be a solu-
phorylated on Ser or Thr residues. ble contaminating enzyme trapped inside plasma mem-
In contrast to the 82-kDa phosphatase, the 36-kDa brane vesicles. The 36-kDa phosphatase is probably a
APase showed a very low specific activity with pNPP PP2A-type protein phosphatase. It will be interesting to
as substrate. However, it was able to dephosphorylate study if it is able to dephosphorylate plasma membrane
phospho-MBP. The fractions with highest specific activity phosphoproteins. Finally, the 95-kDa phosphatase,
presented two protein bands in SDS-PAGE with mol- which was not extracted with octylglucoside, is worthy
ecular masses of 65 and 36 kDa. This protein pattern, the of further characterization.
sensitivity of this enzyme to nanomolar concentrations of
okadaic acid, its insensitivity to vanadate and its lack of ´
Acknowledgements – We thank Barbara Lino for providing us with
activation with divalent cations (PP2B and PP2C phospha- the kinase needed to phosphorylate MBP. This work was supported
tases are activated by these cations), suggest that it could by a grant from Conacyt, Mexico. Conacyt also granted a doctoral
scholarship to E.A.A.
be a Ser/Thr phosphatase of the PP2A type. However, it is
necessary to confirm the identity of this phosphatase by
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230 Physiol. Plant. 121, 2004