From the Department of Molecular Physiology and Biophysics, and Howard Hughes Medical Institute, Vanderbilt University School of Medicine, Nashville, Tennessee 37232
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ABSTRACT |
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We have recently cloned a cDNA encoding a
phospholipase D (PLD) from rat brain and named it rPLD1. It shows 90%
amino acid identity with the human PLD isoform hPLD1b. We have
expressed rPLD1 as a histidine-tagged fusion protein in insect
(Sf9) cells using the expression vector pBlueBacHis and purified
the recombinant protein to homogeneity by
Ni2+-agarose affinity chromatography.
Phosphatidylinositol 4,5-P2 and phosphatidylinositol
3,4,5-P3 activated the PLD equipotently, but other acidic
phospholipids were ineffective. The activity of rPLD1 was dependent on
both Mg2+ and Ca2+. It was specific for
phosphatidylcholine and showed a broad dependence on pH with optimum
activity at pH 6.5-7.5. The enzyme was inhibited by oleate and
activated by the small G proteins ARF3 and RhoA in the presence of
guanosine 5'-3-O-(thio)triphosphate. Protein kinase C
(PKC)- and -
II, but not PKC-
, -
, -
, or -
, activated rPLD1 in a manner that was stimulated by phorbol ester but did not
require ATP. Neither synergistic interactions between ARF3 and RhoA nor
between these G proteins and PKC-
or -
II were observed. Recombinant PKC-
and -
II phosphorylated purified rPLD1 to high stoichiometry in vitro, and the phosphorylated PLD
exhibited a mobility shift upon electrophoresis. Phosphorylation of the
PLD by PKC was correlated with inhibition of its catalytic activity. rPLD1 bound to concanavalin A-Sepharose beads, and its electrophoretic mobility was altered by treatment with endoglycosidase F. The amount of
PLD bound to the beads was decreased in a
concentration-dependent manner when tunicamycin was added
to the Sf9 expression system. Tunicamycin also decreased
membrane localization of rPLD1. These results suggest that rPLD1 is a
glycosylated protein and that it is negatively regulated by
phosphorylation by PKC in vitro.
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INTRODUCTION |
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Phospholipase D type 1 (PLD1)1 plays an important role in signal transduction in a variety of cells. PLD catalyzes the hydrolysis of phosphatidylcholine (PC), the major phospholipid of membranes, to phosphatidic acid and choline in response to a variety of hormones, neurotransmitters, growth factors, and cytokines (1). Phosphatidic acid (PA), the direct product of PLD action, has been implicated in increased DNA synthesis, activation of protein kinases and a protein-tyrosine phosphatase, stimulation of the respiratory burst in neutrophils, stimulation of c-fos and c-myc transcription, activation of certain enzymes of inositol phospholipid metabolism, and effects on actin polymerization and the GTPase-activating proteins of small G proteins (1-4). PA can be further metabolized by PA phosphohydrolase to yield diacylglycerol and by phospholipase A2 to form the intercellular messenger lysophosphatidic acid. Diacylglycerol derived from PC via PA can result in prolonged activation of certain PKC isozymes (1, 5). Thus, agonist-induced stimulation of PLD through its activation of PKC could play a role in long term cellular responses such as proliferation and differentiation.
Regulation of PLD activity is not fully understood. PLD is known to be activated via multiple pathways involving PKC, heterotrimeric and small G proteins, protein-tyrosine kinases, Ca2+, and unsaturated fatty acids (1, 5). However, there is little information on the biochemical and molecular properties of mammalian PLDs because homogenous preparations of the enzyme have not been available until very recently.
Much attention has focused on the role of PKC in the regulation of PLD.
Studies using down-regulation of PKC and PKC inhibitors suggest the
involvement of PKC-dependent mechanisms in the activation of PLD by many agonists (1, 5). Surprisingly, the stimulation of PLD by
PKC in membranes from Chinese hamster lung (CCL39) fibroblasts (6) and
in a partially purified preparation of PLD from brain (7) has been
shown to occur in an ATP-independent manner, suggesting that PKC may
interact directly with PLD and activate it by a nonphosphorylation mechanism. PKC- and PKC-
isozymes have also been found to
activate HL-60 cell membrane-bound PLD in an ATP-independent manner
(8). In contrast, Lopez et al. (9) reported that
conventional isoform(s) of PKC activated neutrophil PLD by
phosphorylating a target protein located in the plasma membrane.
Although there are many reports indicating PKC involvement in PLD
activation, there is evidence that PKC is not required by some
agonists, suggesting an alternate pathway(s) (1, 5).
Early reports showed that GTPS-activated PLD in membranes or
permeabilized cells, suggesting the involvement of G proteins (10-12).
Members of the ADP-ribosylation factor (ARF) family of small G proteins
were first recognized as activators of PLD (13, 14), and the presence
of phosphatidylinositol 4,5-bisphosphate (PIP2) in
substrate vesicles was shown to be essential for the ARF-regulated PLD
activity (13, 15). ARF is also required for coatomer assembly and
vesicle trafficking in Golgi (16). ARF-activated PLD is enriched in
Golgi membranes, and PLD may play a role in ARF-dependent
coatomer assembly (17). Two PLD activities have been separated from rat
brain. One is activated by ARF and PIP2 but inhibited by
oleate, whereas the other is completely dependent on oleate for
activity but insensitive to ARF and PIP2 (18).
Members of Rho family of small G proteins, including RhoA, Cdc42, and Rac have also been found to activate PLD in isolated membranes (8, 19-21). It has also been reported that treatment of Rat 1 fibroblasts with Clostridium botulinum C3 transferase, which ADP-ribosylates and inactivates Rho, results in decreased activation of PLD in response to stimuli (22). Rho proteins are involved in structural rearrangements of the actin cytoskeleton (23, 24) and in the regulation of phosphatidylinositol 3-kinase (25) and phosphatidylinositol-4-phosphate 5-kinase (26). Membrane-associated PLDs from HL-60 cells and porcine brain were shown to be synergistically activated by ARF and RhoA (20, 27), whereas rat liver membrane PLD was activated by RhoA but minimally by ARF (19).
Evidence for the existence of multiple isoforms of agonist-activated PLDs in mammalian cells is substantial (1, 18). It has been obtained in a variety of biochemical studies, which indicate differences in the subcellular localization of PLD activity and in its responses to Ca2+, small G proteins, fatty acids, detergents, and pH, and differences in substrate specificities and chromatographic properties of partially purified PLD enzymes.
Recently, Hammond et al. (29, 30) cloned alternatively
spliced forms of human PLD (hPLD1a and hPLD1b) and expressed them in
insect cells using baculovirus vectors. The enzymes were purified to
homogeneity using immunoaffinity chromatography and shown to be
activated by ARF, Rho family proteins, PIP2,
phosphatidylinositol 3,4,5-P3 (PIP3), and
PKC- (30). We have also cloned a PLD isoform from a rat brain
cDNA library (31) and have studied the properties of the enzyme
expressed in COS-7 cells. The present studies were undertaken to
investigate the properties and regulation of this rat brain PLD (rPLD1)
using the enzyme expressed as a histidine-tagged fusion protein in
baculovirus-infected insect cells and purified to homogeneity. The
results show that the enzyme has some properties that are very similar
and some that are different from those of hPLD1. In addition, the
enzyme is shown to be a glycoprotein and to be phosphorylated and
negatively regulated by PKC.
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MATERIALS AND METHODS |
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All reagents were analytical grade unless otherwise indicated.
Dioleoylphosphatidylethanolamine, dipalmitoyl-PC, PIP2, and other unlabeled phospholipids were obtained from Sigma.
Dipalmitoyl-C16-PIP3 and
dipalmitoyl-C16-phosphatidylinositol 3,4-bisphosphate were kind gifts from Dr. Ching-Shih Chen (University of Kentucky). -Octylglucopyranoside, GTP
S, and tunicamycin were purchased from
Boehringer Mannheim, and concanavalin A-Sepharose beads were from
Amersham Pharmacia Biotech. All radioactive reagents were from NEN Life
Science Products except for
1-palmitoyl-2-[1-14C]linoleoyl PC and
1-palmitoyl-2-[14C]linoleoyl PE, which were from Amersham
Pharmacia Biotech. LK60 silica gel thin layer plates were purchased
from Whatman. Nickel-agarose resin was from Qiagen, and anti-X press
antibody was from Invitrogen. Affinity-purified anti-C-terminal
dodecapeptide of hPLD antibody was a kind gift of Dr. S. H. Ryu
(POSTECH, Pohang, Korea). Purified recombinant PKC isoforms expressed
in Sf9 cells were from Panvera Corp. The cDNAs for RhoA,
ARF3, and myristoyl transferase were kind gifts of Drs. R. A. Cerione (Cornell University), J. Moss (National Institutes of
Health), and J. I. Gordon (Washington University),
respectively.
Construction of Recombinant Baculovirus-- The rPLD1 cDNA was inserted into the baculovirus transfer vector pBlueBacHis2B (Invitrogen) such that the PLD coding sequence was in frame with the sequences encoding the hexahistidine tag. This transfer plasmid and linearized AcMNPV viral DNA (Invitrogen) were cotransfected into Spodoptera frugiperda (Sf9) cells by lipofection (Invitrogen) according to the manufacturer's instructions.
Putative recombinant viral plaques were identified by color screening. Virus from several putative recombinant plaques was tested for the ability to induce expression of (His6)-rPLD1 in infected Sf9 cells using Western blots of cell lysates with affinity-purified antibodies to the C terminus of hPLD1 (data not shown). The positive plaques were purified, and the pure viruses were amplified by infection of spinner cultures of Sf9 cells, which were stirred at 70 rpm and incubated at 27 °C.Insect Cell Culture and Baculovirus Expression-- Growth of Sf9 (High Five) cells (Invitrogen) was maintained at 27 °C in SF 900 II SFM media (Life Technologies, Inc.) supplemented with 5% fetal bovine serum containing antibiotic and antimycotic agents. Production of high titer viral stocks was performed in spinner flasks with 2 × 106 Sf9 cells/ml. The high titer virus stocks were stored at 4 °C and protected from light to ensure maintenance of titer. Monolayers of Sf9 cells (5 × 106 cells in a 100-mm dish) were infected with recombinant baculovirus encoding (His6)-rPLD at a multiplicity of 10 and cultured at 27 °C for various times. The cells were washed in ice-cold phosphate-buffered saline, scraped into lysis buffer (200 mM HEPES, pH 8.0, 50 mM NaCl, 10 µg/ml leupeptin, 10 µg/ml aprotinin, 1 mM phenylmethylsulfonyl fluoride, 5 mM MgCl2, and 10% glycerol), and lysed by five passages through a 25-gauge needle. Trypan blue staining of the lysate indicated >95% disruption of the cells. The lysate was centrifuged at 10,000 × g for 1 h to prepare cytosolic and particulate (membrane) fractions. Membranes were washed once by suspension in buffer and repeated centrifugation. PLD activity was higher in the soluble fraction than in the particulate fraction, and activity was detectable at 12 h, reached a maximum after 40 h of infection, and then declined slightly (data not shown). Western blotting using affinity-purified antibodies to the C-terminal dodecapeptide of hPLD also showed higher expression of rPLD1 in the soluble fraction, and the time course of enzyme expression measured by immunoblotting was similar to that measured by enzyme activity (data not shown).
Purification of (His6)-rPLD1 by
Ni2+-NTA-Agarose Affinity Chromatography--
Monolayers
of Sf9 cells (3 × 107 cells/150-mm dish; 10 dishes of cells) were used for each purification. They were infected at
a multiplicity of 10 for 48 h with recombinant baculovirus encoding (His6)-rPLD1. The medium was removed, and the
cells were lysed by incubation with 10 ml of buffer containing 20 mM HEPES, pH 8.0, 1% -octylglucopyranoside, 0.6 M NaCl, 5 mM MgCl2, 10 µg/ml
leupeptin, 10 µg/ml aprotinin, 1 mM phenylmethylsulfonyl fluoride, and 15% glycerol for 30 min at 4 °C.
Purification of Small G Proteins--
RhoA cDNA was
subcloned into the baculovirus transfer vector, pBlueBacHis
(Invitrogen) encoding a hexahistidine tag. The protein was expressed in
Sf9 cells by cotransfection of transfer plasmid and linearized
AcMNPV viral DNA, and purified by affinity chromatography using
Ni2+-NTA-agarose as described. Human ARF3 was coexpressed
with myristoyltransferase in Escherichia coli and purified
using DEAE-Sephacel and Superdex 75 column chromatography as described
by Weiss et al. (32). Both preparations of small G proteins
were shown by SDS-PAGE and silver staining to be near homogeneous and
were stored in aliquots at 70 °C.
Measurement of PLD Activity-- PLD activity was measured by the formation of phosphatidylbutanol in the presence of 1% butanol using phospholipid vesicles comprised PE, PIP2, and PC in a molar ratio of 16:1.4:1, as described by Brown et al. (13). In some assays, the lipid composition of the vesicles was altered as indicated. The lipid reconstitution buffer was 50 mM HEPES (pH 7.5), 3 mM EGTA, 2 mM CaCl2, 3 mM MgCl2, 1 mM dithiothreitol, and 80 mM KC1. The reaction mixture was incubated at 37 °C and terminated by the addition of 1 ml of chloroform/methanol/HCl (50:50:0.3) and 0.35 ml of 1 N HCl in 5 mM EGTA. The incubation time was 30 min unless otherwise noted. The mixture was centrifuged at 2000 × g for 5 min, and the lower phase was dried under N2, resuspended in 300 µl of chloroform/methanol (2:1), and spotted onto silica gel 60 A thin layer chromatography plates (Whatman). The plates were developed in the upper phase of a solvent system of ethyl acetate/isooctane/H2O/acetic acid (55:25:50:10) and then stained with iodine. A phosphatidylbutanol standard (Avanti) was used to locate the bands, which were scraped into scintillation vials containing 500 µl of methanol and 7.5 ml of Ready Organic scintillation mixture (Beckman).
In Vitro Phosphorylation Studies--
Phosphorylation reactions
were carried out in a total volume of 25 µl containing kinase buffer
(25 mM Tris-HCl, pH 7.5, 1.32 mM
CaCl2, 5 mM MgCl2, 1 mM
EDTA, 1.25 mM EGTA, 1 mM dithiothreitol) supplemented with 4-phorbol 12-myristate 13-acetate (PMA) (100 nM), phosphatidylserine (100 µg/ml), 50 µM
ATP, 10 µCi of [
-32P]ATP (NEN Life Science
Products), 50 ng of purified PKC-
or -
, in the presence or
absence of purified PLD for 20 min at 30 °C. Incubations were
stopped in Laemmli sample buffer and analyzed by SDS-PAGE on 4-12%
gradient gels followed by autoradiography. For the time course studies,
the incubations were performed with or without 10 µM
Ro-31-8220 for the indicated times. An assay of PLD activity was
performed as described above. For the phosphorylation reaction,
incubations were stopped in Laemmli sample buffer, PLD was separated
from PKC-
by SDS-PAGE on 4-12% gradient gels, and the gel was
dried. To quantitate the amount of 32P incorporated into
PLD, the dried gel was exposed to film, and the autoradiograph was used
as a template for excising radioactive PLD bands, which were added into
scintillation vials containing 10 ml of Ready Organic scintillation
mixture. The stoichiometry of phosphate incorporation was determined
from the specific radioactivity of the [
-32P]ATP
(counted under the same conditions) and the amount of PLD protein
applied to the gels (determined by Bio-Rad protein microassay).
Binding of PLD to Concanavalin A-Sepharose Beads-- Sf9 cells were infected at a multiplicity of 10 for 60 h with recombinant baculovirus encoding rPLD1 in the presence of various concentrations of tunicamycin (Boehringer Mannheim Corp.) and lysed by incubation with a lysis buffer containing 20 mM Hepes, pH 7.2, 1% Triton X-100, 10% glycerol, 50 mM NaF, 1 mM Na3VO4, 10 µg/ml leupeptin, 10 µg/ml aprotinin, 1 mM phenylmethylsulfonyl fluoride. The lysates were centrifuged at 15,000 × g for 10 min, and the resulting supernatant was incubated for 2 h at 4 °C with Sepharose beads coupled to Concanavalin A (ConA) (Amersham Pharmacia Biotech). The beads were washed five times with a washing buffer, followed by the addition of SDS-sample buffer, and boiling. The recovered protein was subjected to SDS-PAGE and transferred to polyvinylidene difluoride membranes, which were probed with anti-PLD antibody. Immunoreactive bands were visualized by chemiluminescence using horseradish peroxidase-conjugated anti-rabbit IgG and ECL reagent (Amersham Pharmacia Biotech).
Endoglycosidase F Digestion--
Purified rPLD1 (200 ng) was
denatured in 0.5% SDS, 1% -mercaptoethanol for 10 min at 100 °C
and then incubated for 5 h at 37 °C with 1000 units of
endoglycosidase F (New England Biolabs) in 50 mM sodium
phosphate (pH 7.5), 1% Nonidet P-40. The reaction was stopped by
adding SDS sample buffer and heating for 4 min at 100 °C, followed
by Western blotting with anti-PLD antibody.
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RESULTS |
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Expression and Affinity Purification of
(His6)-rPLD1--
To produce amounts of enzymatically
active rPLD1 sufficient for in vitro characterization, we
expressed the enzyme using the baculovirus expression system and the
transfer vector pBlueBacHis, which contains an epitope for the
Anti-Xpress antibody to allow affinity purification of the recombinant
enzyme. We compared the (His6)-rPLD1 expression level in
two different insect cell lines, the S. frugiperda cell line
Sf9 and the Trichoplusiani cell line High Five,
respectively. Although it is reported that High Five cells offer up to
25-fold higher expression of secreted proteins than Sf9 cells,
the Sf9 line was used, since infection of these cells yielded
approximately 10-fold more fusion protein than High Five cells (data
not shown). Four different detergents (-octyl glucoside, Triton
X-100, Nonidet P-40, sodium cholate) were tested for solubilization of
the expressed PLD, and octyl glucoside was found to be most suitable
(data not shown). Membrane-bound PLD could also be extracted with 0.6 M NaCl. The 10 × 150-mm dishes of Sf9 cells
each containing 2 × 107 cells were infected at a
multiplicity of infection of 10 with recombinant baculovirus for
expression of the PLD. At 48 h postinfection, the cells were
washed with phosphate-buffered saline and extracted with buffer
containing 1%
-octyl glucoside and 0.6 M NaCl. After centrifugation, supernatants were adsorbed to 0.4 ml of
Ni2+-NTA resin overnight at 4 °C. The resin was then
washed with buffer containing 50 mM imidazole and 800 mM NaCl. (His6)-rPLD1 was eluted in 80 mM imidazole-containing fractions and analyzed by SDS-PAGE and Western blotting. The molecular mass of rPLD1 was estimated to be
120 kDa by SDS-PAGE (Fig. 1). The enzyme
was purified to apparent homogeneity as shown by staining with
Coomassie Brilliant Blue (Fig. 1A), and approximately 50 µg of pure rPLD1 protein was obtained. The purified protein was
recognized by an antibody to the C-terminal dodecapeptide of hPLD1
(Fig. 1B) as well as the Anti-Xpress antibody (Fig.
1C).
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Regulation by Polyphosphoinositides--
We studied the effects of
acidic phospholipids on the activity of purified rPLD1 using substrate
vesicles containing 5 µM test phospholipid in the
presence of PE and PC. The assays also contained 50 µM
GTPS and the small G protein ARF, since this greatly enhances the
activity of PLD (13, 14). PIP2 and PIP3 stimulated the activity of rPLD1 significantly, and
phosphatidylinositol 4-phosphate and phosphatidylinositol
3,4-bisphosphate were largely ineffective (Fig.
2A). The activation of PLD by
PIP2 and PIP3 was dependent on the molar
fraction of polyphosphoinositide in the vesicles (Fig. 2B).
Significant activation was observed with 1 µM and maximal
activation occurred at 5 µM.
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Effect of pH and Oleate on PLD Activity--
The effect of pH on
rPLD1 activity was investigated using pH values ranging from 4.0 to 9.0 in the presence of 1 mM free Mg2+. The activity
was assayed in the presence of GTPS and ARF. PLD activity showed a
broad dependence on pH, with optimum activity at pH 6.5-7.5 (data not
shown).
Substrate Specificity-- We examined the substrate selectivity of purified rPLD1 toward different phospholipids. The enzyme was incubated with 1-palmitoyl-2-[1-14C]linoleoyl PC, 1-palmitoyl-2-[1-14C]linoleoyl PE and 1-stearoyl-2-[1-14C]arachidonoyl phosphatidylinositol under standard assay conditions. [14C]phosphatidylbutanol was produced only from PC (data not shown), thus demonstrating that rPLD1 is PC-specific.
Dependence on Ca2+ and
Mg2+--
Ca2+ and Mg2+ have been
implicated in the activation of PLD (28). To determine the influence of
these divalent cations on PLD activity assayed in the presence of
GTPS and ARF, free Ca2+ and Mg2+ in the
assay medium were controlled using
Ca2+/Mg2+-EGTA buffers at pH 7.5. ARF-dependent rPLD1 activity was very low in the absence of
Ca2+ and Mg2+, and stimulation by both cations
was observed in the micromolar range and reached a peak at 1 mM (Fig. 3). The PLD activity
showed two phases of stimulation by the cations. The first phase
occurred at 1-100 µM Ca2+ and
Mg2+ concentrations, and the second one was a sharp
increase at 1 mM concentration of both cations. At higher
concentrations of Ca2+, there was a sharp decrease in
activity, but this was not observed with Mg2+.
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Activation by ARF and Rho--
The activation of rPLD1 by purified
ARF3 and RhoA in the presence of GTPS is shown in Fig.
4A. The activation of the
enzyme by these G proteins was dependent on GTP
S, since in the
absence of the nucleotide, no stimulation was seen (data not shown).
The PLD activity was increased by ARF in a dose-dependent
manner. Stimulation was detectable at 50 nM and was maximal
at 2 µM. RhoA was 2-fold less efficacious, but its
concentration-dependence was very similar to that of ARF.
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Activation of rPLD1 by the - and
II-Isozymes of PKC--
It
has been reported that classical isozymes of PKC stimulate PLD in
several in vitro systems (6-9). To investigate the effects of PKC, rPLD1 was incubated with increasing concentrations of purified
recombinant PKC isozymes. Among the isozymes tested, PKC-
and -
II
(Fig. 4B), but not PKC-
, -
, -
, or -
(data not shown), stimulated the enzyme in a concentration-dependent
manner with half-maximal effects at approximately 10 nM and
with similar maximal effects. The addition of 100 nM PMA
increased the stimulation by PKC-
and PKC-
II 1.5- and 1.3-fold,
respectively. The effects of the two isozymes were observed in the
absence of ATP, and, in fact, the addition of 100 µM ATP
inhibited the effects of the isozymes by 53 and 77% in the presence
and absence of PMA, respectively. This suggested, contrary to
expectation, that phosphorylation of the enzyme might be
inhibitory.
Lack of Synergism among Arf, Rho, and PKC on rPLD1
Activity--
It has been reported that ARF and RhoA synergistically
activate PLD in some systems (27, 33) and that PKC- has a
synergistic effect with the two G proteins (7). This has also been
shown for the 1a and 1b isozymes of hPLD1 (30). To investigate possible synergism among ARF, Rho, and PKC on PLD activity, rPLD1 was incubated with GTP
S and with ARF3, RhoA, PKC-
, and PKC-
II and their
combinations. However, the combination of maximally effective
concentrations of ARF3 and RhoA produced no greater stimulation than
seen with ARF3 alone (Fig. 5) Likewise,
the combination of either ARF3 or RhoA with PKC-
or PKC-
II
resulted in no increase in activity above that observed with either G
protein alone. Finally, the combination of ARF3 plus RhoA with either
PKC isozyme produced no increase above that seen with ARF3 alone (Fig.
5). These results with pure rPLD1 provide no indication of synergism
among ARF, Rho, and PKC in the regulation of this enzyme, in agreement
with results obtained using intact COS-7 cells expressing rPLD1 and using membranes from these cells (31).
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PKC- and -
II Phosphorylate rPLD1 in Vitro--
Although the
results of Fig. 4B indicated that the stimulatory effects of
PKC-
and -
II did not require ATP and previous work has shown that
the regulatory domain alone of PKC may be sufficient to provide
stimulation of PLD by PKC (7), we investigated whether PKC isozymes
could phosphorylate purified rPLD in vitro, particularly
since a search of the rPLD1 sequence revealed 14 putative PKC
phosphorylation sites. Purified recombinant PKC-
and -
II were
added to an in vitro phosphorylation assay using [
-32P]ATP and purified rPLD1. Interestingly, PKC-
and PKC-
II phosphorylated the PLD (Fig.
6A) in a
time-dependent manner (Fig.
7). The phosphorylation occurred at high
stoichiometry (approximately 4 mol of phosphate/mol of enzyme at 60 min, determined as described under "Materials and Methods") and was
accompanied by a mobility shift on SDS-PAGE (Fig. 6B). Under
the assay conditions containing PMA, Ca2+, and
phosphatidylserine, PKC-
and PKC-
II were autophosphorylated (Fig.
6A).
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Phosphorylation of rPLD1 by PKC Inhibits Its Enzymatic
Activity--
It was observed that when ATP was included in activity
assays, stimulation of PLD by PKC- or -
II was reduced. This
observation was also reported by Hammond et al. (30). To
investigate the effect of PKC phosphorylation on rPLD1 activity,
in vitro phosphorylation and PLD activity assays were
performed in the presence or absence of the PKC inhibitor, Ro-31-8220
(Fig. 7). Phosphorylation of rPLD1 by PKC-
was markedly inhibited by
the addition of Ro-31-8220, as expected. In the presence of ATP and
PKC-
, there was an initial increase in PLD activity, but thereafter
the activity declined. The addition of the PKC inhibitor increased PLD
activity at all times (Fig. 7). These data indicate that
phosphorylation of rPLD1 by PKC negatively regulates its activity.
rPLD1 Is a Glycosylated Protein-- We investigated whether rPLD1 is glycosylated in cells, since the rPLD1 sequence revealed 5 putative N-glycosylation sites. After the addition of 0-20 µg/ml of tunicamycin, an inhibitor of N-linked glycosylation, to the culture medium of Sf9 cells infected with recombinant baculovirus, lysates were incubated with Sepharose beads coupled to ConA. Binding of rPLD1 to the beads was observed, and the amount of PLD bound was decreased in a concentration-dependent manner by tunicamycin (Fig. 8A). Further evidence of glycosylation was obtained when it was found that treatment of the enzyme with endoglycosidase F caused a shift in its mobility on electrophoresis (Fig. 8B).
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DISCUSSION |
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PLD activity has been found in many mammalian tissues and cells (1), but its study has been hindered by difficulties in purifying the enzyme. A major breakthrough came when the gene for PLD in the plant Ricinus communis was cloned (34). With information from the plant and yeast PLD sequences, the cDNA for a mammalian PLD was cloned (29). Recently, Hammond et al. (30) have cloned and expressed alternatively spliced forms of human hPLD1 and also a second PLD isozyme (PLD2) (35), which is regulated very differently from PLD1. PLD2 has also been cloned from rat brain (36).
Many biochemical studies have indicated the existence of multiple forms of PLD (1). To identify other PLD isozymes, we have cloned several PLD cDNAs from mammalian tissues. The most fully characterized is rPLD1, which was cloned from a rat brain cDNA library and is regulated by ARF, Rho, and PKC (31). This enzyme is present in both soluble and membrane fractions when expressed in Sf9 cells (present study) and COS-7 cells (31). The present results provide an explanation for this finding, namely that the enzyme undergoes N-glycosylation, presumably in the Golgi, and that the glycosylated form is targeted to the membranes (Fig. 9).
Affinity purification of (His6)-rPLD1 from
baculovirus-infected Sf9 cells with -octyl glucoside using
Ni2+-NTA agarose resin provided a homogeneous active
enzyme. Although rPLD1 was extracted with the detergent, the elution of
enzyme from the resin was performed in a buffer without detergent. The purified enzyme was stable for 5 months when maintained in
detergent-free condition in the presence of 20% glycerol. The
molecular mass of purified enzyme is 120 kDa, and it is specific for
PC.
The original detection of ARF-stimulated PLD activity, assayed with
exogenous substrate, required the presence of PIP2 in the
substrate vesicles (13). PIP2 and PIP3
stimulated rPLD1 activity almost equipotently, but a variety of other
inositol phospholipids were ineffective (Fig. 2A).
Surprisingly, phosphatidylinositol 3,4-bisphosphate was ineffective,
suggesting that the stereochemical position of the inositol phosphate
groups is critical for the regulation of PLD. The regulation of PLD
activity by PIP3 and PIP2 may be of
physiological significance, since PIP3 is a recognized signaling molecule (37), and PIP3 and PIP2 bind
to the pleckstrin homology (PH) the domains of various proteins (38)
and modify their localization and function. The mechanism by which
PIP2 and PIP3 activate PLD is unclear, since
there is no evident PH domain or other binding site for
phosphoinositides in its sequence. It has been reported that
PIP2 stimulates the rate of GDP dissociation from ARF1 and
the rate of GTPS binding (39). It has also been shown that ARF1
directly binds PIP2 (40), and it has been suggested that
this binding is required for ARF1 to interact with target proteins such
as its GTPase-activating protein or PLD (13, 40-43). However, it is
clear that the action of PIP2 on PLD is not just related to
ARF, since the lipid is required for the stimulation of PLD by RhoA and
PKC in vitro (Fig. 4 and Refs. 7, 8, 27, and 30).
rPLD1 has a calculated PI value of 9.0, but the optimal pH for ARF-activated PLD in vitro was between 6.5 and 7.5. The enzyme was similarly dependent on Ca2+ and Mg2+ and was most active at a 1 mM concentration of these divalent cations. ARF-stimulated hPLD1 was similarly sensitive to changes in Mg2+ (30), but Ca2+ was not tested alone. The cations appeared to act on the same site, since the addition of Ca2+ produced no stimulation above that seen with 1 mM Mg2+ alone (30). Since the Mg2+ concentration of cytosol is in the millimolar range, at which hPLD1 and rPLD1 would be fully stimulated by this cation, these data indicate that these isozymes are unlikely to be regulated by changes in cytosolic Ca2+, which occur in the micromolar and submicromolar range.
As found for hPLD1, oleate strongly inhibited rPLD1 (Fig. 5). This is in contrast to some other forms of the enzyme that are stimulated by this fatty acid (28). The finding that PLD isozymes are regulated by unsaturated fatty acids raises the possibility that alterations in phospholipase A2 activity in vivo could secondarily alter PLD activity. However, to date, there is no experimental support for this possibility.
In agreement with Hammond et al. (30), who studied two alternatively spliced forms of hPLD1, rPLD1 was stimulated by ARF, RhoA, and PKC in vitro. The role of ARF in intracellular (Golgi) protein trafficking is well established (16), and it has been suggested that PLD plays a part in this process, perhaps by modulating membrane lipids to facilitate formation and fusion of transport vesicles (17, 44). Although ARF-responsive PLD activity has been found in other subcellular fractions besides Golgi (plasma membrane, nucleus, and cytosol) (1), its role at these sites is unknown. Some studies have shown agonist-induced membrane translocation of ARF (45, 46), but the nature of the membrane target is unclear. Rho regulates the formation of actin stress fibers and focal adhesions (23, 24) and has also been implicated in the regulation of phosphatidylinositol 3-kinase (25) and phosphatidylinositol 4-phosphate 5'-kinase (26), which synthesize PIP3 and PIP2, respectively. Although the present and previous (30) findings indicate that Rho can directly activate PLD, the possibility that the G protein exerts part of its stimulatory effects on PLD in vivo through alterations in PIP2 is suggested by the findings of Jakobs and co-workers (47). Irrespective of the mechanisms by which Rho regulates PLD, there is growing evidence that this protein plays a role in coupling cell surface receptors to PLD activation and other cellular responses (22, 48). The data of Hammond et al. (30) show little difference in the effects of RhoA, Rac1, and Cdc42 on the activity of hPLD1 or hPLD1b, suggesting that the enzyme does not play a role in mediating the very diverse actin cytoskeleton changes induced by these Rho proteins. However, the comparative effects of these Rho proteins on PLD activity have not been examined in intact cells.
PKC- or -
but not other PKC isozymes activated rPLD1 in a manner
that was stimulated by phorbol ester but did not require ATP. The
mechanism of the phosphorylation-independent PLD activation is unknown
at present, but it presumably involves a protein-protein interaction
that results in a conformational change that causes activation.
Interestingly, the addition of ATP inhibits the effects of PKC-
and
-
II on rPLD1 (Fig. 7 and Ref. 30). This can be attributed to
phosphorylation of the enzyme (Figs. 6 and 7), which results in
inhibition in vitro, although other effects of ATP are
possible. In some in vivo and in vitro studies,
inhibition of the kinase activity of PKC has been shown to inhibit the
effects of phorbol esters and agonists on PLD activity (9, 48). These results suggest that other PLD isozymes may be stimulated by PKC phosphorylation or that PKC may phosphorylate other proteins that have
stimulatory effects on PLD in vivo.
A surprising finding of the present study was the lack of synergism in the effects of ARF, Rho, and PKC on rPLD1 (Fig. 5). This is in contrast to the findings with the two forms of hPLD1 (30) but in agreement with our observations on rPLD1 expressed in COS-7 cells (31). These data suggest that the domains at which these regulatory proteins bind to rPLD1 are noninteracting. Preliminary data2 indicate that PKC interacts with an N-terminal sequence in the enzyme, whereas ARF and RhoA interact at other sites. It is possible that ARF and PKC activate rPLD1 by a shared mechanism, since their maximum effects were not additive (Fig. 5). The failure of these proteins to interact with Rho in the regulation of rPLD1 versus hPLD1 could be due to the substantial differences in the sequences of the two isozymes between residues 507 and 574, i.e. in the middle of the enzymes (31). However, these explanations are highly speculative and obviously need experimental support.
An interesting finding is that rPLD1 is a glycoprotein, as assessed by its binding to ConA and the effects of endoglycosidase F and tunicamycin (Figs. 8 and 9). The findings suggest that N-linked glycosylation is a factor in the membrane localization of the enzyme. Except when the enzyme is transiently overexpressed in cells, it is almost exclusively membrane-bound (1, 4). This suggests that membrane translocation of PLD is probably not a factor in its regulation. However, translocation of its regulatory proteins may be very important (1).
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ACKNOWLEDGEMENTS |
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We thank Judy Childs for assistance in the preparation of this manuscript, Dr. S. H. Ryu (POSTECH, Pohang, Korea) for supplying anti-PLD antibody, and Dr. C.-S. Chen (University of Kentucky) for providing PIP3 and phosphatidylinositol 3,4- bisphosphate.
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FOOTNOTES |
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* This work was supported in part by National Institutes of Health Grant DK47448.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Investigator of the Howard Hughes Medical Institute. To whom all
correspondence should be addressed. Tel.: 615-322-6494; Fax: 615-322-4381; E-mail: john.exton{at}mcmail.vanderbilt.edu.
1
The abbreviations used are: PLD, phospholipase
D; hPLD, human PLD; rPLD, rat PLD; PC, phosphatidylcholine; PE,
phosphatidylethanolamine; PA, phosphatidic acid; PKC, protein kinase C;
ARF, ADP-ribosylation factor; PIP2, phosphatidylinositol
4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-trisphosphate; GTPS, guanosine
5'-3-O-(thio)triphosphate; PAGE, polyacrylamide gel
electrophoresis; ConA, concanavalin A; PMA, 4
-phorbol 12-myristate
13-acetate; NTA, nitrilotriacetic acid.
2 S.-K. Park, D. S. Min, and J. H. Exton, unpublished observations.
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REFERENCES |
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