Characterization and regulation of yeast Ca2+-dependentphosphatidylethanolamine-phospholipase D activity Xiaoqing Tang, Michal Waksman, Yona Ely and Mordechai Liscovitch Department of Bi
Trang 1Characterization and regulation of yeast Ca2+-dependent
phosphatidylethanolamine-phospholipase D activity
Xiaoqing Tang, Michal Waksman, Yona Ely and Mordechai Liscovitch
Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel
An unconventional phospholipase D (PLD) activity was
identified recently in Saccharomyces cerevisiae which is
Ca2+-dependent, preferentially hydrolyses
phosphatidyl-ethanolamine (PtdEtn) and phosphatidylserine and does not
catalyse a transphosphatidylation with primary short-chain
alcohols We have characterized the cytosolic and
mem-brane-bound forms of the yeast PtdEtn-PLD and examined
the regulation of its activity under certain growth, nutritional
and stress conditions Both forms of PtdEtn-PLD activity
were similarly activated by Ca2+ions in a biphasic manner
Likewise, other divalent cations affected both cytosolic and
membrane-bound forms to the same extent The yeast
PtdEtn-PLD activity was found to interact with immobilized
PtdEtn in a Ca2+-dependent manner The partially purified
cytosolic form and the salt-extracted membrane-bound form
of yeast PtdEtn-PLD exhibited a similar elution pattern on
size-exclusion chromatography, coeluting as low apparent
molecular weight peaks PtdEtn-PLD activity was
stimu-lated, along with Spo14p/Pld1p activity, upon dilution of
stationary phase cultures in glucose, acetate and galactose
media, but PtdEtn-PLD activation was less pronounced Interestingly, PtdEtn-PLD activity was found to be elevated
by 40% in sec14tsmutants at the restrictive temperature, whereas in other sec mutants it remained unaffected The activity of PtdEtn-PLD was reduced by 30–40% upon addition to the medium of inositol (75 lM) in either wild-type yeast or spo14D mutants and this effect was seen regardless
of the presence of choline, suggesting that transcription of the PtdEtn-PLD gene is down-regulated by inositol Finally, exposure of yeast cells to H2O2 resulted in a transient increase in PtdEtn-PLD activity followed by a profound, nearly 90% decrease in activity In conclusion, our results indicate that yeast PtdEtn-PLD activity is highly regulated: the enzyme is acutely activated upon entry into the cell cycle and following inactivation of sec14ts, and is inhibited under oxidative stress conditions The implications of these find-ings are discussed
Keywords: oxidative stress; phosphatidylethanolamine; phospholipase D; phospholipid metabolism; yeast
The ability of cells to respond to changes in their
environ-ment depends on multiple adaptive mechanisms Many such
mechanisms require the formation, inside the cells, of
specific molecules that act as messengers, informing various
cell systems of the need to change their activity or modify
their function Phospholipase D (PLD) is an enzyme that
generates such a messenger, phosphatidic acid (PtdA), in
response to environmental signals and thus plays an
important role in regulating cell function [1–3] A number
of eukaryotic PLD genes have been molecularly cloned in
recent years These PLD genes all belong to an extended
gene family, termed the HKD family, that also includes
certain bacterial PLDs, as well as non-PLD
phosphati-dyltransferases [2,4–6] Although the activation of PLD
enzymes has been implicated in signal transduction and membrane traffic events, their precise cellular localization and function are still poorly defined [7,8] Furthermore, forms of PLD that do not belong to the HKD family may also exist A yeast PLD gene, SPO14/PLD1, encodes a
Ca2+-independent PLD that hydrolyses phosphatidylcho-line (PtdCho) and is stimulated by phosphatidylinositol 4,5-bisphosphate (PtdInsP2) [9–11] Spo14p function is essential for sporulation [9] Upon induction of sporulation the enzyme is relocalized from the cytosol onto the spindle pole bodies and then encircles the mature spores membranes [12] Spo14p is also essential for SEC14-independent secretion, i.e in sec14ts-bypass mutants [13,14] A second PLD activity present in the yeast Saccharomyces cerevisiae was recently identified [15,16] The second yeast PLD enzyme, provi-sionally designated ScPLD2, has distinct catalytic proper-ties Its activity is Ca2+-dependent; it preferentially hydrolyses phosphatidylethanolamine (PtdEtn) and phos-phatidylserine (PtdSer); and its activity is not stimulated by PtdInsP2 In addition, unlike Spo14p/Pld1p and most other eukaryotic PLDs (but similar to certain bacterial PLDs [17]), the yeast Ca2+-dependent PLD is incapable of catalysing the characteristic transphosphatidylation reac-tion with primary short-chain alcoholic acceptors [15,16] This PLD activity was assayed with PtdEtn as substrate and
is therefore abbreviated herein as PtdEtn-PLD
Important-ly, SPO14/PLD1 is the sole PLD representative of the HKD gene family that is present in the yeast genome [18] The yeast Ca2+-dependent PtdEtn-PLD activity must
Correspondence to M Liscovitch, Department of Biological
Regulation, Weizmann Institute of Science, PO Box 26,
Rehovot 76100, Israel.
Fax: + 972 8934 4116, Tel.: + 972 8934 2773,
E-mail: moti.liscovitch@weizmann.ac.il
Abbreviations: PLD, phospholipase D;
PtdA, phosphatidic acid; PtdCho, phosphatidylcholine; PtdEtn,
phosphatidylethanolamine; PtdSer, phosphatidylserine; PtdInsP 2 ,
phosphatidylinositol 4,5-bisphosphate; C 6 -NBD,
[6-N-(7-nitrobenzo-2-O-1,3-diazol-4-yl)-amino]-caproyl; PtdIns, phosphatidylinositol;
YNB, yeast nitrogen base; SC, synthetic complete minimal medium.
(Received 26 November 2001, revised 15 May 2002,
accepted 25 June 2002)
Trang 2therefore be encoded by a distinct non-HKD family gene
which is likely to be a member of a novel PLD gene family,
but the gene that encodes it has not been identified yet In
the present study we have further characterized the cytosolic
and membrane-bound forms of yeast PtdEtn-PLD and
examined the regulation of PtdEtn-PLD activity under
certain growth, nutritional and stress conditions
M A T E R I A L S A N D M E T H O D S
Chemicals
1-Acyl-2-[6-N-(7-nitrobenzo-2-O-1,3-diazol-4-yl)-amino]-caproyl-glycero-3-phosphorylcholine (C6-NBD-PtdCho)
and
1-acyl-2-[6-N-(7-nitrobenzo-2-O-1,3-diazol-4-yl)-ami-no]-caproyl-glycero-3-phosphorylethanolamine (C6
-NBD-PtdEtn) were from Avanti Polar Lipids (Alabaster, AL,
USA) TLC glass-backed plates precoated with silica gel
60A were from Whatman Yeast Nitrogen Base (YNB)
lacking amino acids and ammonium sulfate were from
Difco Dioleoyl-PtdEtn, PtdInsP2 and all other reagents
were from Sigma
Yeast strains
The wild-type yeast strain utilized for preparation of total
cell lysates and subcellular fractions was W303–1B (MATa
ade2-1 his3-11,15 leu2-3,112 ura3-1 trp1-1) [19] The spo14D
strain used was the strain designated pld1-FS-1 (MATa
ade2-1 leu2-3,112 ura3-1 trp1-1 pld1::HIS3) [10] The diploid
wild-type strain utilized in the carbon source experiments
was W303-1D (MATa/MATa ade2-1/ade2-1 his3-11,15/
his3-11,15 leu2-3,112, leu-2-3112 ura3-1/ura3-1 trp1-1/trp1–
1) sec mutants included: RSY979 (MATa ura3-52 sec7-5),
RSY961 (MATa ura3-52 leu2-3,112 sec12-1), RSY314
(MATa ura3-52 sec13-3), RSY1010 (MATa ura3-52
leu2-3112 sec21-1) and RSY324 (MATa ura3-52 sec22-2) [20]
The sec14-1tsstrain used here was CTY1-1A (MATa
ura3-52 hi 3-200 lys2-801 sec14-1ts) [21]
Media
Wild-type yeast cells were maintained on synthetic complete
minimal medium (SC) Spo14D cells were maintained on SC
drop-out medium lacking histidine SC media were
pre-pared from YNB essentially according to Rose et al [22]
Where indicated, SC medium was supplemented with 75 lM
inositol (I+) and/or 1 mMcholine (C+) Other amino
acid-rich media included: YPD [yeast extract and Bactopeptone
(YP) containing 2% dextrose]; YPA (YP containing 0.05%
glucose and 2% potassium acetate); and YPG (YP
containing 3.5% galactose)
Phospholipase D assays
Spo14p/Pld1p and PtdEtn-PLD activities can be assayed
separately from the same samples, with PtdCho as substrate
in the presence of EGTA and PtdInsP2(Spo14p/Pld1p) or
with PtdEtn in the presence of Ca2+(PtdEtn-PLD) [16]
Total cell lysates were prepared as described previously [10]
To solubilize C6-NBD-PtdEtn, 1.5 mM Triton X-100 was
added The final concentration of Triton X-100 in assay
reactions containing C-NBD-PtdEtn was 0.25 mM The
hydrolysis of C6-NBD-PtdEtn was monitored by the production of C6-NBD-PtdA, essentially as described by Danin et al [23] The Spo14p/Pld1p reaction mixture contained 0.3 mgÆmL)1 yeast protein, 35 mM Na-Hepes
pH 7.4, 150 mM NaCl, 400 lM C6-NBD-PtdCho, 1 mM EDTA, 5 mM EGTA and 4 mol% PtdInsP2 (Note: the surface concentration of PtdInsP2 is expressed as a percentage of the total lipid concentration.) The standard PtdEtn-PLD reaction mixture contained 0.3 mgÆmL)1 pro-tein, 35 mM Na-Hepes pH 7.4, 150 mM NaCl, 40 lM
C6-NBD-PtdEtn, 1 mMEDTA, 5 mMEGTA, 7 mMCaCl2 and no PtdInsP2 In experiments in which the free Ca2+ concentration in the presence of EGTA and EDTA was modified it was calculated utilizing theCALCON software (Version 4.0, for MS-DOS) The reaction mixtures were incubated at 30C for 30 min in a final volume of 120 lL Termination of the reaction, TLC separation and quanti-fication of the fluorescent lipid products were conducted as described [10,23] Activity is expressed as the mean of two duplicate samples measured in arbitrary fluorescence units Where indicated, specific activity is expressed as the PtdA-derived fluorescence units per mg or lg protein
Subcellular fractionation and size-exclusion column chromatography
Total cell lysates were prepared as described previously [10] The lysate was centrifuged at 8000 g for 10 min to remove cell wall debris The supernatant was collected and ultra-centrifuged at 100 000 g for 90 min The supernatant (cytosol) was collected and the resultant pellet (total membranes) was washed as above and resuspended in salt extraction buffer (2M NaCl, 35 mM Na-Hepes buffer
pH 7.4, 10 lgÆmL)1aprotinin and 10 lgÆmL)1leupeptin) The membranes were salt-extracted for 1 h at 4C while shaking and then were sedimented again by ultracentrifu-gation at 100 000 g for 90 min The supernatant containing the salt-extracted peripheral membrane proteins was col-lected
The partially purified cytosolic PtdEtn-PLD was pre-pared as follows: the cytosolic fraction was applied to a Q-Sepharose column (KR26/24, Pharmacia) equilibrated with buffer A (50 mM NaCl, 35 mM Na-Hepes pH 7.4) After washing with buffer A, enzyme was eluted in 5-mL fractions with an NaCl gradient (0.1–1M) in buffer A Eluates containing activity were collected and loaded onto a Reactive Green-19-agarose column (HR16/5, Pharmacia) equilibrated with buffer A containing 0.3M NaCl The column was then eluted with a NaCl gradient (0.3–3M) in buffer A Active fractions were combined and concentrated
to 2 mL by using an Amicon PM5 filter Aliquots of the crude cytosol, salt extracted membranes and partially purified cytosolic fraction (2 mL) were applied to a Superdex-75 size-exclusion chromatography column (HiLoadTM16/60, Pharmacia) equilibrated with buffer A Proteins were eluted with the same buffer at a flow rate of 0.3 mLÆmin)1at 4C Fractions (2 mL) were collected and assayed for PtdEtn-PLD activity Molecular weight mark-ers (albumin, 67 kDa; ovalbumin, 43 kDaA; chymotrypsi-nogen A, 25 kDa; ribonuclease A, 14 kDa) were run separately under identical conditions Further purification
of the cytosolic PtdEtn-PLD resulted in rapid loss of activity
Trang 3PtdEtn-polyacrylamide affinity chromatography
A PtdEtn-polyacrylamide affinity column was prepared
essentially as described in [24] except that PtdEtn was used
instead of PtdSer The PtdEtn-polyacrylamide particles
(2 mL) were loaded onto a small Poly Prep column
(0.8· 4 cm, Bio-Rad) and equilibrated with loading buffer
containing 0.4M NaCl, 35 mM Na-Hepes pH 7.4, 5 mM
dithiothreitol and 15 mM CaCl2 A salt extract of yeast
membranes was diluted in the above buffer and loaded onto
the column After incubating at 4C for 30 min with gentle
shaking, the column was washed once with loading buffer,
followed by a two-step wash with the same buffer
contain-ing 5 mM CaCl2 and then 0.1 mM CaCl2 E lution was
carried out using a buffer containing 2 mMEGTA in place
of CaCl2 Samples of each fraction were assayed for
PtdEtn-PLD activity under standard conditions, with the final free
Ca2+concentration in the assay adjusted to 1 mM
R E S U L T S
Previous work has demonstrated the existence in yeast of a
Ca2+-dependent PLD activity that hydrolyses PtdEtn and
PtdSer [15,16] Both membrane-bound and cytosolic
activ-ities were observed, but the relationship between these two
forms remains unknown Therefore, we have compared
some of the properties of membrane-bound and cytosolic
PtdEtn-PLD activities Our studies demonstrate that their
dependence on free Ca2+ concentration is quite similar,
both being stimulated in a biphasic manner, with an initial
activation phase at concentrations of 10)6to 10)5 and a
second phase between 10)3 and 10)2M (Fig 1) The difference between PtdEtn-PLD activity at 10 lM and
10 mM free Ca2+was statistically significant (P < 0.001, Student’s t-test) Next, we examined the effects of different chloride salts of divalent cations on the membrane-bound and cytosolic PtdEtn-PLD activities assayed in the absence
of added EDTA and EGTA, i.e in the presence of 10)5M
of ambient free Ca2+ The divalent cations tested (at a concentration of 1 mM) affected membrane-bound and cytosolic PtdEtn-PLD activities in a similar manner While
Ca2+ ions further stimulated PtdEtn-PLD activity as expected, Mg2+ions had no effect on the activity, whereas the other divalent cations inhibited basal PtdEtn-PLD in the following potency order: Co2+> Mn2+¼ Zn2+> Ba2+ (Table 1) These data indicate that the pattern and extent
of stimulation of the membrane and soluble yeast PtdEtn-PLD activity by Ca2+and their inhibition by other divalent cations is highly comparable
The mechanism of action of Ca2+ions in PtdEtn-PLD activation may involve facilitation of substrate interaction, stimulation of substrate hydrolysis, or both To establish whether the interaction of PtdEtn-PLD with its substrate PtdEtn is stimulated by Ca2+ions, we examined its ability
to interact with PtdEtn, immobilized within polyacrylamide beads, in a Ca2+-dependent manner, as previously demon-strated for protein kinase C [24] As shown in Fig 2, loading
a yeast salt extract (see Materials and methods) on a column containing immobilized PtdEtn in the presence of a high
Ca2+ concentration (15 mM) resulted in retention of a fraction of total yeast PtdEtn-PLD activity on the column, which could then be released by adding EGTA Thus, yeast PtdEtn-PLD activity is able to interact with immobilized PtdEtn in a Ca2+-dependent manner
Soluble enzymes that utilize membrane phospholipids as substrates or cofactors are often translocated to a mem-brane compartment upon activation or during homogeni-zation [25] Their similar response to Ca2+ and other divalent cations, and the Ca2+-dependent interaction of yeast PtdEtn-PLD with its PtdEtn substrate, raised the possibility that the membrane PtdEtn-PLD activity repre-sents a fraction of the cytosolic form that becomes bound to membrane PtdEtn upon cell lysis To determine if the soluble PtdEtn-PLD activity may translocate to membranes
Fig 1 Effect of increasing Ca2+ concentration on membrane and
cytosolic PtdEtn-PLD activity Cytosolic and membrane-bound
fractions were prepared as described in Materials and methods.
PtdEtn-PLD activity was measured with the indicated free Ca 2+
concentrations The amount of cytosolic protein included in the assay
was 32 lg per reaction and the amount of membrane protein was
0.4 lg per reaction Results (mean ± SD) are from four (cytosol) and
two (membrane-bound) replicates carried out in duplicate The lack of
an error bar indicates an SD smaller than the size of the symbols.
Table 1 Effect of different divalent cations on cytosolic and membrane-bound PtdEtn-PLD activity Cytosolic and membrane-membrane-bound fractions were prepared as described in Materials and methods PtdEtn-PLD activity measured without addition of EDTA, EGTA and any divalent cations was considered as 100% Different cation chloride salts were added at a concentration of 1 m M Results are from a representative experiment carried out in duplicate and repeated twice.
Cation added
PtdEtn-PLD activity (% of control) Cytosolic Membrane-bound
Trang 4in the presence of Ca2+ we lysed the yeast cells in the
presence of Ca2+(10 mM) or E GTA (1 mM) and examined
PtdEtn-PLD activity in the 100 000 g pellet (membranes)
and the 100 000 g supernatant (cytosol) Cell lysis in the
presence of Ca2+resulted in a marked decrease in
PtdEtn-PLD activity in the cytosol; however, there was no
corresponding increase in the activity found in the pellet
(Fig 3) To rule out the possibility that the decrease in
cytosolic PtdEtn-PLD resulted from a Ca2+-dependent
membrane translocation of an essential cofactor, an EGTA
wash of the Ca2+-lysed membranes was reconstituted with
the Ca2+-lysed cytosol However, the normal cytosolic
PtdEtn-PLD activity was not recovered even after
reconsti-tution (data not shown) The possibility that the
translo-cated enzyme might be masked by the presence of a
membrane-bound inhibitor is also excluded by this
exper-iment These results indicate that the decrease in cytosolic
PtdEtn-PLD is not due to translocation to the membrane
The decrease in cytosolic PtdEtn-PLD activity upon lysis in
the presence of Ca2+ may occur because of stimulated
proteolytic degradation of the enzyme This possibility was
not examined further
To further elucidate the relationship between the
mem-brane-bound and cytosolic PtdEtn-PLD activities we
compared their chromatographic properties Size-exclusion
column chromatography of a salt-extracted membrane
PtdEtn-PLD and the crude cytosolic PtdEtn-PLD activities
on Superdex-75 revealed that they exhibit a different
elution pattern Whereas membrane-bound PLD eluted as
two major peaks, one of high apparent molecular mass
(peaking in fraction 6) and another of very low apparent
molecular mass (peaking in fraction 34) (Fig 4A), the
crude cytosolic PtdEtn-PLD eluted as a single high
apparent molecular weight peak that paralleled the
corre-sponding peak of membrane PtdEtn-PLD (Fig 4B)
However, after partial purification by Q-Sepharose and Reactive Green-19-agarose, the partially purified cytosolic PtdEtn-PLD eluted as a single low apparent molecular weight peak that paralleled the corresponding peak of membrane PtdEtn-PLD (Fig 4C) In conclusion, it seems that the two forms may share a common low apparent molecular weight catalytic subunit, that mediates PtdEtn-PLD response to Ca2+and other cations and may interact with other component(s) in the high apparent molecular weight peaks that determine their differential size and subcellular localization Only the future cloning of yeast PtdEtn-PLD and its isozymes will confirm or refute this conjecture
To gain insight into the possible physiological role(s) of yeast PtdEtn-PLD we examined the regulation of its activity under different environmental and physiological conditions First, the effect of growth in media containing different carbon sources (YPD, YPG and YPA, supplemented with glucose, galactose and acetate, respectively) on Spo14p/ Pld1p activity and PtdEtn-PLD activity in vitro was determined in parallel throughout culture growth Dilution
of stationary phase diploid W303-1D wild-type cells in fresh YPD media resulted in a 4.5-fold increase in Spo14p/Pld1p activity within 30 min, which was followed by a second peak of activation after 70 min The activity then declined gradually to near basal levels after 2, 4 and 8 h (Fig 5A) PtdEtn-PLD activity similarly exhibited a transient 3.5-fold activation which seemed to be biphasic, although the first peak of activation was not as pronounced (Fig 5A) Spo14p/Pld1p activity was stimulated also upon exit from
Fig 3 Effect of the presence of Ca2+during lysis on membrane and cytosolic PtdEtn-PLD activity Yeast cells were lysed in the presence of EGTA (1 m M ; left) or CaCl 2 (10 m M ; right) and the membrane and cytosol fractions were separated by centrifugation (100 000 g, 60 min) The fractions were then assayed for PtdEtn-PLD activity under stan-dard conditions, with final free Ca2+ concentration in the assay adjusted to 1 m M Results are from a representative experiment carried out in duplicate and repeated twice.
Fig 2 Ca2+-dependent retention of PtdEtn-PLD on a
polyacrylamide-immobilized PtdEtn affinity column The PtdEtn-affinity column was
prepared as described in Materials and methods A salt extract of yeast
membranes was then loaded onto the column (equilibrated with
15 m M CaCl 2 ) A two-step wash with buffer containing 5 m M and
0.1 m M CaCl 2 was followed by elution with 2 m M EGTA Fractions
were assayed for PtdEtn-PLD activity under standard conditions, with
final free Ca 2+ concentration in the assay adjusted to 1 m M Results
are from a representative experiment carried out in duplicate and
repeated three times.
Trang 5stationary phase in YPG, but the second sixfold activation
peak was delayed somewhat and occurred after 120 min of
incubation (Fig 5B) Here, the activation of PtdEtn-PLD
was smaller in magnitude (1.5-fold to twofold) but more
persistent (up to 4 h; Fig 5B) In YPA, the pattern of
Spo14p/Pld1p activity was similar to that observed in YPD
PtdEtn-PLD activity was stimulated rapidly nearly
three-fold and this was followed by a second, smaller activation
peak at 70 min of incubation (Fig 5C) A biphasic
activa-tion of PtdEtn-PLD upon diluactiva-tion (similar in terms of
magnitude and timing) was observed also in haploid
wild-type W303-1B cells (data not shown) These data clearly
indicate that both Spo14p/Pld1p and PtdEtn-PLD are
highly regulated enzymes that are turned on upon yeast
entry into the cell cycle
Different lines of evidence support a biological role for
mammalian PLDs during vesicle formation, budding,
transport, docking and fusion to target membranes [2] In
yeast, SPO14/PLD1 is required for SEC14-independent
vesicle transport (i.e under sec14-bypass conditions) [13,14]
To explore the involvement of PtdEtn-PLD in secretion, we screened 16 different secretion mutants, bearing mutations
at the early and late stages of the secretory pathway, for changes in PtdEtn-PLD activity at room temperature and at the restrictive temperature of 37C (at which the temper-ature-sensitive secretion phenotype is manifested) Fig 6 shows PtdEtn-PLD activity in a selected subset of six secretion mutants sec14tsis the only one among 16 secretion
Fig 5 Effect of carbon source on Spo14p and PtdEtn-PLD activity in diploid cells during culture growth PLD activity was determined at different stages of growth in culture A 48-h-old stationary phase culture of W303-1D diploid cells was diluted in fresh YPD (A), YPG (B) or YPA (C) media to 0.65 · 10 6 cellsÆmL)1and grown at 30 C Samples were taken at the indicated times and Spo14p/Pld1p (d) and PtdEtn-PLD activity (s) were assayed in duplicate Results are expressed as the percentage of the specific PLD activity at time 0 and are taken from representative experiments that were repeated at least twice.
Fig 4 Size-exclusion chromatography of membrane (A), cytosolic (B)
and partially purified cytosolic (C) PtdEtn-PLD activities on
Superdex-75 Salt-extracted membrane, crude cytosolic, and cytosolic
PtdEtn-PLD partially purified on Q-Sepharose and Reactive Green-19
aga-rose, were prepared and chromatographed on a Superdex-75 column
(see Materials and methods for details) Samples from each column
fraction were then assayed for PtdEtn-PLD activity in duplicate under
standard conditions Molecular mass markers (arrows) were run
sep-arately under identical conditions Results are from representative
experiments that were repeated at least twice.
Trang 6mutants in which PtdEtn-PLD activity is elevated (by 37%)
at the restrictive temperature All of the other secretion
mutants that we checked, and four wild-type cells that
served as additional controls, showed either a slight decrease
in PtdEtn-PLD activity at 37C or were unaffected by the
change in temperature, as compared with the room
temperature controls (data not shown) The activation in
sec14tsmutants suggests that Sec14p is involved, directly or
indirectly, in negative regulation of PtdEtn-PLD Thus,
Sec14p may be a common negative regulator of both
Spo14p- and PtdEtn-PLD-mediated PtdA accumulation in
yeast It should be noted that the effect on PtdEtn-PLD is
evident within 1 h of temperature elevation, indicating that
it may reflect a change in PtdEtn-PLD stability or in its
activation state rather than a change at the transcriptional
level
Recent results indicate that SPO14/PLD1 may be involved in regulating the expression of genes that are part
of the INO1 regulon [13] Therefore, we examined the effect
of the presence of inositol and choline in the medium on yeast PtdEtn-PLD activity The results indicate that Spo14p/Pld1p activity in wild-type cells (Fig 7, left) and PtdEtn-PLD activity in either wild-type yeast or spo14D mutants (Fig 7, right) are decreased by 30–40% upon addition of inositol (75 lM) to the medium This effect is seen regardless of the presence of 1 mM choline in the medium The results suggest that under conditions of repression of the INO1 regulon, both Spo14p/Pld1p and PtdEtn-PLD activities are down-regulated, further impli-cating both of these enzymes in regulating phospholipid biosynthesis in yeast [13]
Mammalian PLD isoforms are activated upon exposure
to oxidative stress signals [26–29] This prompted us to examine the effect of H2O2 on yeast Spo14p/Pld1p and PtdEtn-PLD activities A short exposure of yeast cells to
H2O2 (30 min) caused a rapid but limited (25–30%) stimulation of PtdEtn-PLD activity (Fig 8A) Under these conditions Spo14p/Pld1p activity was not significantly stimulated When yeast cells were exposed to H2O2 for
2 h there was a profound decrease in PtdEtn-PLD activity (up to 90%), that was evident at concentration of‡ 1 mM (Fig 8B) Interestingly, although Spo14p/Pld1p activity is also reduced by long exposure to H2O2it was affected less, being reduced by 50% (Fig 8B) The time course of the changes in PtdEtn-PLD and Spo14p/Pld1p activity in response to 2 mMH2O2demonstrates the biphasic nature (i.e a brief initial stimulation followed by a prolonged inhibition) of the response of PtdEtn-PLD activity to this oxidative stress (Fig 8C)
D I S C U S S I O N Yeast PtdEtn-PLD is an unconventional PLD that differs from prokaryotic and eukaryotic HKD family PLDs in its inability to catalyse a transphosphatidylation reaction with
Fig 6 PtdEtn-PLD activity in various sec mutants Different strains
carrying mutations in various genes involved in secretion were grown
to stationary phase and then diluted in YPD and grown to mid
log-phase (6 h) The cultures were then divided into two portions and
further incubated either at room temperature or at 37 C for 1 h The
cells were then harvested and whole cell lysates were prepared and
assayed in duplicate for PtdEtn-PLD activity Results are mean ± SD
of three independent experiments.
Fig 7 Changes in Spo14p and PtdEtn-PLD activity in response to inositol and/or choline in the medium Wild-type and spo14D mutant strains were grown to mid log-phase in SC medium in the absence or presence of choline (1 m M ) and inositol (75 l M ) as indicated The cells were then harvested and whole cell lysates were prepared and assayed in duplicate for Spo14p/Pld1p and PtdEtn-PLD activity Results are from a representative experiment that was repeated three times.
Trang 7short-chain primary alcohols This difference is meaningful because it implies that the yeast PtdEtn-PLD and HKD family PLDs use different catalytic mechanisms In HKD family enzymes catalysis involves the formation of a covalent phospho-enzyme intermediate that is formed on the highly conserved active site histidine which is part of the HKD family signature motif, HXKXXXXD [30] It is assumed that in HKD family PLDs the phosphatidyl-histidine intermediate is attacked by an activated water molecule to release PtdA, and that alcohols can compete with water to form a phosphatidylalcohol product [31] The yeast genome includes only one HKD-family PLD gene, namely, SPO14 Another HKD family gene found in the yeast genome is PEL1/PGS1, encoding phosphatidylglyc-erol phosphate synthase [32] It is therefore highly likely that yeast PtdEtn-PLD is encoded by a non-HKD gene and may thus represent a novel PLD gene family A prokaryotic PLD activity similar to yeast PtdEtn-PLD that was identified recently in Sterptoverticillium cinnamoneum and was partially purified and characterized, may be another member of this putative gene family [17] With the exception
of alcohols (that act as competitive substrates) there are no known active site-directed inhibitors of HKD-family PLDs Hence the existence of a distinct catalytic site in PtdEtn-PLD cannot be tested directly at this time Obviously, identification of the gene that encodes yeast PtdEtn-PLD is
an important goal So far, our earnest attempts to identify this elusive gene, by using numerous genetic, genomic and biochemical approaches, have proved unsuccessful (X Tang & M Liscovitch, unpublished data) Therefore, the present work was undertaken in order to obtain more information about the yeast PtdEtn-PLD activity, its properties and regulation
In previous work we have shown that PtdEtn-PLD activity can be found in both cytosolic and membrane-bound forms [16] The relationship between these two forms was examined here in various ways One of the characteristic features of yeast PtdEtn-PLD is its almost absolute dependence on Ca2+[15,16] Our data indicate that the response of the cytosolic and membrane-bound PtdEtn-PLDs to increasing free Ca2+ concentrations is almost identical, both forms being activated in a biphasic manner Also, the two forms are similarly inhibited by the divalent cations tested This similarity suggests that the two forms are catalytically related Size exclusion column chromatog-raphy of the membrane bound PtdEtn-PLD, solubilized by treatment with high salt concentration, revealed that it eluted as two major peaks Intriguingly, the crude cytosolic PtdEtn-PLD eluted as a single peak that corresponded to the high apparent molecular weight peak of the membrane form However, following its partial purification, the cytosol PtdEtn-PLD eluted as a single peak that corresponded to the low apparent molecular weight peak of the membrane form These data are consistent with the hypothesis that the cytosol and membrane forms of yeast PtdEtn-PLD share a common catalytic subunit of low apparent molecular weight that may interact with one or more subunits which could determine their different cellular localization
The stimulation of PtdEtn-PLD by Ca2+ions is biphasic This pattern raises the possibility that Ca2+may have a dual mechanism of action in activating PtdEtn-PLD, e.g Ca2+ may participate in catalysis as well as facilitate enzyme– substrate interaction Our data, showing that PtdEtn-PLD
Fig 8 Changes in Spo14p and PtdEtn-PLD activity in response to
oxidative stress Wild-type cells were grown to mid log-phase in SC
medium The cells were then aliquoted and incubated in the absence or
in the presence of H 2 O 2 at the indicated concentrations for 30 min (A)
and 2 h (B), or cells were incubated at the same concentration of H 2 O 2
(2 m M ) for different times (C) The cells were then harvested and whole
cell lysates were prepared and assayed in duplicate for Spo14p/Pld1p
and PtdEtn-PLD activity Results are from a representative
experi-ment that was repeated three times.
Trang 8may bind to immobilized PtdEtn in Ca2+-dependent
manner, strongly suggest that one mechanism of Ca2+
action is to stimulate the interaction of PtdEtn-PLD with its
phospholipid substrate It is not yet clear how Ca2+ions
promote enzyme–PtdEtn association One possibility is that
yeast PtdEtn-PLD amino acid sequence contains a C2/
CaLB domain, homologous to the domains found in
mammalian proteins including protein kinase C, cytosolic
phospholipase A2and synaptotagmin [33] We have
there-fore screened the yeast genome and identified three ORFs
encoding hypothetical unknown proteins with a distinct C2
domain Specific disruptants of these ORFs, namely
YLR019w, YLL010c, and YOR086c, were obtained from
Research Genetics Inc Lysates were prepared from these
disruption strains and assayed for PtdEtn-PLD activity In
all three cases, PtdEtn-PLD levels were comparable to those
found in the wild-type strain (data not shown), indicating
that these ORFs do not encode PtdEtn-PLD nor any other
protein required for PtdEtn-PLD expression or activity
Obviously, definitive verification of this conjecture must
await the identification of the yeast PtdEtn-PLD gene Be
that as it may, the ability of PtdEtn-PLD to bind to
immobilized PtdEtn may facilitate the future use of a
PtdEtn-affinity matrix for purification of PtdEtn-PLD by
phospholipid-affinity chromatography
In the second part of this work we aimed to study the
regulation of PtdEtn-PLD activity and, in particular, to
compare it with the other yeast PLD, Spo14p/Pld1p We
have taken advantage of the fact that Spo14p/Pld1p and
PtdEtn-PLD activities can be measured independently in
the same samples Spo14p/Pld1p is Ca2+-independent and
thus may be assayed in the presence of EGTA and
EDTA, conditions under which PtdEtn-PLD is inactive
On the other hand, PtdEtn-PLD hydrolyses PtdEtn and
thus may be assayed with this phospholipid as substrate,
conditions under which Spo14p/Pld1p is inactive [15,16]
We have previously shown that initiation of yeast cell
proliferation upon transfer of stationary cultures to fresh
medium is associated with stimulation of Spo14p/Pld1p
activity [10] Here we confirm these results and further
show that the activation of Spo14p/Pld1p is biphasic and
occurs, albeit with different kinetics, regardless of whether
the yeast cultures are initiated in YPD, YPG or YPA
Interestingly, further analysis has shown that PtdEtn-PLD
activity is also stimulated upon exit of yeast cells from
stationary phase in either YPD, YPG or YPA However,
the activation of PtdEtn-PLD was not as pronounced as
that of Spo14p/Pld1p, especially in YPG medium The
fact that PtdEtn-PLD activation occurred in all tested
media, albeit to a different extent, suggests that it may
correlate with resumption of mitosis rather than with
glucose repression, induction of sporulation or the carbon
source being utilized It may therefore be speculated that
PtdEtn-PLD shares a regulatory role with Spo14p/Pld1p
in one or more steps of the mitotic cell cycle
Alterna-tively, the activation of the two yeast PLDs may reflect a
generalized stimulation of phospholipid metabolism
asso-ciated with new membrane synthesis coincident with
initiation of cell growth
In mammals, PLD was strongly implicated in vesicular
trafficking, both in the Golgi (formation of nascent
exocytotic vesicles) and at the plasma membrane
(endocy-tosis) (reviewed in [7]) In yeast, recent work has indicated
that Spo14p/Pld1p plays a permissive role in SEC14-independent secretion as evinced by abrogation of growth of sec14ts-bypass mutants upon disruption of SPO14/PLD1 [13,14] In addition, inactivation of sec14tsat its restrictive temperature results in stimulation of Spo14p/Pld1p activity [13,34,35] Our data show that PtdEtn-PLD activity is similarly stimulated upon inactivation of sec14tsby a 60-min incubation at 37C It is important to note that in this experiment, as in all studies of PtdEtn-PLD regulation described here, PLD activities were examined in vitro after cell lysis and under optimal assay conditions The in vitro PtdEtn-PLD activity is therefore not likely to fully reflect the activity in situ and probably underestimates the extent of PtdEtn-PLD activation upon temperature-dependent inac-tivation of sec14ts The role played by Spo14p/Pld1p in regulating normal Golgi transport activity is still not clear, although several possibilities have been suggested High levels of PtdCho in the Golgi were hypothesized to interfere with Golgi secretory activity [36–39] Thus, under sec14ts -bypass conditions, the activated Spo14p/Pld1p may act to reduce Golgi PtdCho to levels that are compatible with normal secretion and its ablation would result in Golgi dysfunction Another suggested function of Spo14p/Pld1p might be to supply critical lipid metabolite(s)
(diacylglycer-ol, for example) that may be necessary for normal Golgi activity [40] How might PtdEtn-PLD fit in these schemes is
a matter of conjecture PtdEtn-PLD is capable of hydro-lysing PtdCho in vitro although not as efficiently as it hydrolyses PtdEtn and PtdSer [16] and thus may perhaps participate in regulating Golgi PtdCho levels and generating lipid metabolites required in the Golgi Such metabolites can
be produced also from PtdEtn and/or PtdSer It is noteworthy that defects in PtdEtn methylation effect sec14ts-bypass when PtdCho synthesis via the CDP-choline pathway is abrogated by eliminating uptake of free choline [39] Thus, activation of PtdEtn-PLD may support normal Golgi function also by reducing the levels of the PtdCho precursor PtdEtn In this context, it may be supposed that over-expression of PtdEtn-PLD should rescue the growth defect of the sec14ts cki1 spo14D strain Based on this supposition we attempted to identify the PtdEtn-PLD gene
by multicopy suppression of the triple mutant with a yeast genomic library However, the only genes picked up in this screen were SEC14 and SPO14/PLD1 (X Tang & M Lis-covitch, unpublished data) Obviously, these negative results
do not rule out the possibility that, once identified, the PtdEtn-PLD gene will be found as essential for SEC14-independent secretion as was Spo14/Pld1p
Numerous genes involved in yeast phospholipid biosyn-thesis are repressed when inositol is present in the medium [41–43] The INO1 gene, whose product is inositol-1-phosphate synthase [44], is the prototypic inositol-regulated gene that gave its name to the entire INO1 regulon Repression by inositol is mediated by a repeated element, UASINO, found in the upstream promoter region of INO1 and other genes that are part of the INO1 regulon [45] As regulation of phospholipid biosynthesis and degradation are likely to be coordinated, it was of interest to examine the effect of inositol and choline on the level of PtdEtn-PLD activity Our results clearly show that PtdEtn-PLD activity
is down-regulated in cells grown in the presence of inositol Choline, which sometimes enhances the repressive effect of inositol, had no influence on PtdEtn-PLD activity In
Trang 9addition, the effect of inositol was seen in both wild-type
and spo14D cells Future identification of the PtdEtn-PLD
gene and analysis of its promoter region will allow
examination of its transcriptional regulation by inositol
and the possible existence of UASINOin its 5¢-untranslated
region
Finally, in view of the potential role of mammalian
PLD in the oxidative stress response [26–29], we examined
the changes in PtdEtn-PLD activity upon exposure of
yeast to H2O2 The results were quite striking: Following
a rapid activation seen within 20 min of the oxidative
challenge, there was a gradual decline in activity that was
both time- and dose-dependent, reaching a maximal
decrease of almost 90% after exposure to 15 mM H2O2
for 2 h This result is most intriguing Much additional
work is required to work out the mechanisms involved in
the down-regulation PtdEtn-PLD and its possible role in
the yeast oxidative stress response Identification of the
gene encoding PtdEtn-PLD is an obvious key to progress
in understanding the structure, mechanism of action,
localization, regulation and function of this intriguing
enzyme
A C K N O W L E D G E M E N T S
We thank J Gerst for providing sec mutants and for many helpful
discussions We are grateful to Z Elazar for his kind advice and interest
in this study This work was supported in part by grants from the Israel
Science Foundation administered by the Israel Academy of Science and
Humanities (Jerusalem) and the Minerva Foundation (Munich) M L.
is incumbent of the Harold L Korda Professorial Chair in Biology.
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