Multiple sequence alignments and calcula-tions of evolutionary trees revealed a clear subdivision of the eukaryotic MAPEG members, corresponding to the six families of microsomal gluta-t
Trang 1of the MAPEG superfamily
Anders Bresell1,*, Rolf Weinander2,*, Gerd Lundqvist3, Haider Raza3, Miyuki Shimoji3,
Tie-Hua Sun3, Lennart Balk5, Ronney Wiklund6, Jan Eriksson6, Christer Jansson6, Bengt Persson1,4, Per-Johan Jakobsson2and Ralf Morgenstern3
1 IFM Bioinformatics, Linko¨ping University, Sweden
2 Department of Medicine, Division of Rheumatology Unit, Karolinska Institutet, Stockholm, Sweden
3 Institute of Environmental Medicine Karolinska Institutet, Stockholm, Sweden
4 Centre for Genomics and Bioinformatics, Karolinska Institutet, Stockholm, Sweden
5 Stockholm Marine Research Centre, University of Stockholm, Sweden
6 Department of Plant Biology & Forestry Genetics, Swedish Agricultural University, Uppsala, Sweden
Keywords
MAPEG; microsomal glutathione
transferase; prostaglandin; leukotriene
Correspondence
R Morgenstern, Institute of Environmental
Medicine, Karolinska Institutet, S-171 77
Stockholm, Sweden
Fax: +46 8 343849
Tel: +46 8 5248 7574
E-mail: ralf.morgenstern@imm.ki.se
*Both authors contributed equally to this
work
(Received 15 November 2004, revised 27
January 2005, accepted 3 February 2005)
doi:10.1111/j.1742-4658.2005.04596.x
The membrane associated proteins in eicosanoid and glutathione metabo-lism (MAPEG) superfamily includes structurally related membrane proteins with diverse functions of widespread origin A total of 136 proteins belong-ing to the MAPEG superfamily were found in database and genome screenings The members were found in prokaryotes and eukaryotes, but not in any archaeal organism Multiple sequence alignments and calcula-tions of evolutionary trees revealed a clear subdivision of the eukaryotic MAPEG members, corresponding to the six families of microsomal gluta-thione transferases (MGST) 1, 2 and 3, leukotriene C4 synthase (LTC4), 5-lipoxygenase activating protein (FLAP), and prostaglandin E synthase Prokaryotes contain at least two distinct potential ancestral subfamilies, of which one is unique, whereas the other most closely resembles enzymes that belong to the MGST2⁄ FLAP ⁄ LTC4 synthase families The insect members are most similar to MGST1⁄ prostaglandin E synthase With the new data available, we observe that fish enzymes are present in all six families, show-ing an early origin for MAPEG family differentiation Thus, the evolution-ary origins and relationships of the MAPEG superfamily can be defined, including distinct sequence patterns characteristic for each of the sub-families We have further investigated and functionally characterized repre-sentative gene products from Escherichia coli, Synechocystis sp., Arabidopsis thaliana and Drosophila melanogaster, and the fish liver enzyme, purified from pike (Esox lucius) Protein overexpression and enzyme activity ana-lysis demonstrated that all proteins catalyzed the conjugation of 1-chloro-2,4-dinitrobenzene with reduced glutathione The E coli protein displayed glutathione transferase activity of 0.11 lmolÆmin)1Æmg)1 in the membrane fraction from bacteria overexpressing the protein Partial purification of the Synechocystis sp protein yielded an enzyme of the expected molecular mass and an N-terminal amino acid sequence that was at least 50% pure, with a specific activity towards 1-chloro-2,4-dinitrobenzene of
11 lmolÆmin)1Æmg)1 Yeast microsomes expressing the Arabidopsis enzyme
Abbreviations
BSA, bovine serum albumin; CDNB, 1-chloro-2,4-dinitrobenzene; DEAE, diethylaminoethyl; FLAP, 5-lipoxygenase activating protein; LT, leukotriene; MGST, microsomal glutathione transferase; PG, prostaglandin; PGES, prostaglandin E synthase; GST, glutathione S-transferase; GPx, glutathione peroxidase; CuOOH, cumene hydroperoxide.
Trang 2Microsomal glutathione transferases (MGSTs)
repre-sent a recently recognized superfamily of enzymes
involved in detoxification, but also in specific
biosyn-thetic pathways of arachidonic acid metabolism The
superfamily was termed the membrane associated
proteins in eicosanoid and glutathione metabolism
(MAPEG) and consists of proteins from mammals,
plants, fungi and bacteria [1] The six members in
humans include 5-lipoxygenase activating protein
(FLAP) and leukotriene (LT) C4 synthase, which are
both involved in leukotriene biosynthesis [2,3];
MGST1, MGST2 and MGST3, which all are
gluta-thione transferases as well as glutagluta-thione dependent
peroxidases [4–7]; and finally, prostaglandin (PG) E
synthase (PGES), earlier referred to as MGST1-L1 [8]
PGES catalyzes the formation of PGE2 from PGH2,
which in turn is generated from arachidonic acid by
the prostaglandin endoperoxide synthase systems
PGES has also been referred to as p53 induced gene
12 (PIG12) because the gene expression was found to
increase extensively following p53 expression [9] The
relationships and other functional aspects of the
MAPEG enzymes have been reviewed [10]
Two groups of bacteria, purple bacteria and
cyano-bacteria, have been found to produce and maintain
significant levels of glutathione [11] and, interestingly,
also contain MAPEG members [1] Glutathione was
observed in various species within the two groups,
among those in Escherichia coli, one of the most well
characterized species of purple bacteria [11] The
func-tion of glutathione metabolism in bacteria may be
pro-tection against xenobiotics and⁄ or oxidative stress but
also as part of specific biosynthetic pathways [12]
Cyanobacteria produce oxygen by photosynthesis and
purple bacteria can use oxygen as a terminal electron
acceptor Glutathione production in bacteria is thus
closely associated with those bacteria that generate or
utilize oxygen in specific biochemical pathways
indica-ting that glutathione metabolism originated in bacteria
at the time when an oxygen-containing atmosphere
developed on earth [11,12]
A low level of glutathione S-transferase (GST) activ-ity has been demonstrated in E coli but not in cyano-bacteria [11] Cytosolic GSTs have been identified in various strains of bacteria [12] and in a few studies, including those on Proteus mirabilis and E coli, cyto-solic GSTs have been purified and further character-ized [13–15] The three-dimensional structure of the
P mirabilis cytosolic GST has also been determined [16] In Synechocystis sp a gene homologous to cyto-solic GST exists but has not been characterized further [17] In general, the enzymes involved in glutathione metabolism in prokaryotes have not been so exten-sively studied and therefore less is known about their properties as compared to the corresponding proteins
in eukaryotes Microsomal GST activity has not been demonstrated in any prokaryotic organism
Expressed sequence tag (EST) clones with open reading frames (ORFs) similar to MAPEG proteins have been found in E coli, Synechocystis sp and Vibrio cholerae [1] The Synechocystis sp ORF dis-played sequence similarity to the MAPEG subfamily consisting of FLAP, LTC4 synthase and MGST2, and also to the MGST3 subfamily but it could not be sig-nificantly grouped to any of those two subfamilies, whereas the E coli and V cholerae sequences form a separate group [1] Nothing is known, however, about the enzymatic properties of any prokaryotic MAPEG protein
As the number of sequenced bacterial genomes has increased considerably during recent years, we de-signed this study to search further for MAPEG pro-teins and functionally characterize representative gene products Database searches revealed various new gene products, in some cases coexisting, with homologies to the two MAPEG subfamilies (described above and in [1]) We investigated representative gene products from the E coli and Synechocystis sp bacteria further, to gain insight into the function of these proteins and the evolution of the MAPEG superfamily Cloning and overexpression demonstrated that both are membrane-bound glutathione transferases
showed an activity of 0.02 lmolÆmin)1Æmg)1, whereas the Drosophila enzyme expressed in E coli was highly active at 3.6 lmolÆmin)1Æmg)1 The purified pike enzyme is the most active MGST described so far with a spe-cific activity of 285 lmolÆmin)1Æmg)1 Drosophila and pike enzymes also displayed glutathione peroxidase activity towards cumene hydroperoxide (0.4 and 2.2 lmolÆmin)1Æmg)1, respectively) Glutathione transferase activity can thus be regarded as a common denominator for a majority of MAPEG members throughout the kingdoms of life whereas glutathione peroxidase activity occurs in representatives from the MGST1, 2 and 3 and PGES sub-families
Trang 3To understand the evolutionary relationships better
on a more global scale we also cloned and expressed
(or purified) MAPEG representatives from plant,
insect and fish Together with earlier data on the frog
enzyme [18] these data define glutathione transferase
activity as a central property of MAPEG members
from a wide range of organisms and suggest ancestral
MAPEG members
Results
MAPEG members from complete genomes
Over 130 MAPEG members were retrieved from
sequence databases and completed genomes, of which
less than half (56) were previously known members
according to the PF01124 entry in Pfam release 11 [19]
Multiple sequence alignments and hydrophobicity plots
were calculated (for a full alignment see supplementary
Fig 1) Even though several members are distantly
related, all exhibit the typical MAPEG properties of
150 residue subunits with four hydrophobic regions,
compatible with four transmembrane regions [20,21]
Using information from completed genomes, we
have traced the evolutionary relationships of the
MAPEG members The general relationships are
depicted in Fig 1 MGST1, PGES and insect forms
have a common branch, compatible with their
overlap-ping substrate-specificities [22] Likewise, MGST2,
FLAP and LTC4 synthase also show somewhat closer
relationships, indicating properties in common
MGST3 forms a separate branch The bacterial E coli
and Synechocystis variants are found on separate
bran-ches A detailed dendrogram is shown in Fig 2
The bacterial forms show distant relationships and
their exact grouping is not significant at all sites, as
indicated from their low bootstrap values (no asterisks
in Fig 2) Furthermore, the bacterial forms are present
at three sites in the dendrogram However, the
group-ing of the families MGST1, MGST2, MGST3, PGES,
FLAP and LTC4 synthase is significant In a
dendro-gram without the bacterial forms, the grouping of these
families becomes even more evident (not shown)
Among the MAPEG sequences from fish, we find
members from all six branches (MGST1, MGST2,
MGST3, PGES, FLAP and LTC4 synthase), suggest-ing that the origin of these forms dates back to before the occurrence of vertebrates, i.e more than 500 mya This dates the differentiation of the MAPEG forms back to the late Cambrian multiplicity of eukaryotic species Notably, in the screenings we have not found any members from the archaea kingdom, indicating that the enzymatic activities of the MAPEG family are not present in these species or that these activities are catalysed by other enzymes The absence of MAPEG members in archaea is certainly consistent with the lack of GSH in these organisms
Cloning, expression and characterization
of selected MAPEG members MGST homologues from Synechocystis and E coli After identifying MAPEG members in several bacterial strains, the E coli and Synechocystis sp proteins were
Fig 1 Schematic evolutionary tree of the MAPEG superfamily The evolutionary tree shows the relationships between the six MAPEG families and three further groups (Insect, E.coliMGST cluster and SynMGST cluster) A major subgrouping is visible with MGST1, PGES and Insect in the upper part of the tree and the remaining families ⁄ groups in the lower part In the lower part, MGST2, FLAP and LTC4 synthase have a close relationship, as judged by the short branches between these enzymes.
Fig 2 Detailed dendrogram of the MAPEG superfamily The tree shows all presently known MAPEG forms, excluding species variants which differ at only a single position In the tree, the six families are clearly distinguished The prokaryotic forms are found at three sites – the E coli cluster, the Synechocystis cluster, and the group of remaining forms, denoted Bacteria Two further groups are marked, denoted Insects and Waterliving The branch lengths are proportional to the number of residue differences, with the scale bar indicating a 5% amino acid difference The fish forms, having representatives for all six MAPEG families, are marked with a fish symbol Accession numbers refer
to the databases Uniprot, NCBI or ENSEMBL.
Trang 5selected for functional characterization of bacterial
MGST homologues These homologues represent two
different groups of prokaryotic MAPEG members found
The E coli ORF, which we refer to as E.coliMGST,
encodes a 141 amino acid residue polypeptide with a
cal-culated molecular mass of 16.2 kDa The Synechocystis
sp ORF (from strain PCC6803 [23]) encodes a 137
resi-due polypeptide with a predicted molecular mass of
15.4 kDa, which we refer to as SynMGST
The ORFs encoding E.coliMGST and SynMGST
were amplified by PCR, the products cloned into an
expression vector and the DNA sequences were verified
against the EMBL database entries Following
hetero-logous expression in E coli, the membrane fractions
were assayed for enzyme activities The membrane
fraction from cells overexpressing E.coliMGST
cata-lyzed the conjugation of 1-chloro-2,4-dinitrobenzene
(CDNB) with reduced glutathione with a specific
activity of 0.11 lmolÆmin)1Æmg)1 When a shorter
con-struct beginning from the alternative translation start
site of the E.coliMGST was expressed no activity was
detected Incubation with N-ethylmaleimide (which
activates mammalian MGST1) did not affect the
activity of E.coliMGST Membranes from cells
over-expressing the SynMGST also displayed glutathione
transferase activity The glutathione conjugating
activ-ity towards CDNB was 1.7 lmolÆmin)1Æmg)1 for the
SynMGST membrane fraction Neither LTC4 synthase
activity, nor any glutathione-dependent peroxidase
activity (towards cumene hydroperoxide or
5-hydrope-roxy-eicosatetraenoic acid) could be observed in any of
the fractions No activity could be detected with these
enzymes towards
1,2-epoxy-3-para-nitrophenoxypro-pane or trans-phenylbut-3-en-2-one as substrates (sum-marised in Table 1)
Partial purification of SynMGST
To characterize bacterial MGSTs further we concen-trated on SynMGST RT-PCR was used to confirm that SynMGST is indeed expressed in the cyanobac-teria (Fig 3)
Having established gene expression of SynMGST in the cyanobacteria and a functional overexpression of recombinant protein in E coli we made an attempt to purify the protein for further characterization Bacterial membranes isolated from cells overexpressing recom-binant SynMGST were solubilized in Triton X-100 The recombinant SynMGST was also enzymatically active upon detergent solubilization and the CDNB conjugating activity was used to monitor subsequent purification steps The SynMGST is basic (the cal-culated isoelectric point being 9.9) and could therefore
be expected to yield a purified product using meth-ods developed for MGST1 [24] However, although the enzyme behaved in a predictable manner upon hydroxyapatite batch chromatography, in cation exchange chromatography the enzyme was recovered in the flow-through fractions Diethylaminoethyl (DEAE) columns, likewise, did not retain the enzyme Because cation and anion exchange chromatography, in concert, did retain most of the contaminating proteins, a parti-ally purified protein was nevertheless recovered In fact, SDS⁄ PAGE (Fig 4) shows that the protein is nearly homogeneous Furthermore, N-terminal amino acid
Table 1 Comparison of glutathione transferase and peroxidase
activity of MAPEG members expressed ⁄ purified from prokaryotes,
plant, nonmammalian and mammalian species ND, not detectable.
Species
CDNB activity (lmolÆmin)1Æmg)1)
CuOOH GPx activity (lmolÆmin)1Æmg)1) Activity of purified enzyme
Xenopus laevis, frog [18] 210 2.1
(partially purified)
Activity in membrane fraction
after heterologous expression
800
400 200 100
Fig 3 RT-PCR To demonstrate that the SynMGST gene was expressed in Synechocystis 6803, total RNA was isolated and amplified by PCR with SynMGST-specific primers, in the presence (lane 4) or absence (lane 3) of reverse transcriptase PCR amplifica-tion from isolated total DNA, using the same primers (lane 2) served as a positive control Sizes in bp, deduced from a 100 bp ladder (lane 1) are indicated.
Trang 6sequence analysis of the predominant band displaying
the correct molecular mass, purified from the gel,
yielded the expected sequence The partially purified
protein constitutes a major part of the preparation and
therefore the specific activities measured will be close to
those of the pure enzyme
The enzyme is more active than its mammalian
counterparts and expressed extremely well Assuming
that the protein was at least 50% pure, the purification
factor (12-fold) indicates that expressed SynMGST
constituted about 8% of the E coli membrane
asso-ciated proteins The specific activity of the partially
purified enzyme with 1-chloro-2,4-dinitrobenzene was
11 ± 0.4 lmolÆmin)1Æmg)1 (mean ± SD, n¼ 3) The
activity was not affected by incubation with the
sulf-hydryl reagent N-ethylmaleimide in contrast to
mam-malian MGST1, which is activated several-fold by this
reagent
MGST3 from Arabidopsis
When plant MGST3 was cloned and overexpressed
in a yeast expression system, the yeast microsomes
displayed a low glutathione transferase activity with
CDNB (0.02 lmolÆmin)1Æmg)1) that was not
activa-ted⁄ inhibited by N-ethylmaleimide Glutathione
peroxi-dase activity was not altered compared to that in
microsomes from yeast expressing the pYeDP60 vector
only (the negative control)
MGST1/PGES-like enzyme from Drosophila
The MGST from Drosophila was cloned and
over-expressed in E coli where the isolated membrane
fraction displayed a high glutathione transferase
acti-vity (3.6 lmolÆmin)1Æmg)1) and glutathione peroxidase activity (0.4 lmolÆmin)1Æmg)1) Addition of 1% (v⁄ v) Triton X-100 to the membrane fraction resulted in a slight increase in activity, whereas N-ethylmaleimide had no effect on enzyme activity The enzyme did not display PGES activity
MGST1/PGES-like enzyme from pike MGST was successfully purified to apparent homogen-eity (Fig 4) from pike liver using protocols developed for the rat enzyme The N-terminal sequence of the purified pike enzyme was determined using Edman de-gradation Sequence comparisons reveal that the pike form purified is closely related to the MGST1⁄ PGES branch (Fig 5) Of the N-terminal 47 residues, 22–28 residues are identical to fish MGST1 sequences, while only 2–12 residues are identical to the fish sequences of other MAPEG families
The enzymatic properties of the pike MGST1-like enzyme were extensively characterised (Table 1) dem-onstrating that the protein has the highest glutathione transferase activity of any MAPEG member detected
so far As the enzyme displays similar substrate speci-ficity to MGST1, including glutathione peroxidase activity, the assignment to the MGST1⁄ PGES sub-family appears well founded
Sequence patterns of the MAPEG members For the MGST1–3, FLAP, LTC4 synthase, PGES and Insect family clusters we generated sequence patterns, shown in Table 2 These patterns are all 100% unambiguous when scanned against Swiss-Prot and TrEMBL, i.e no nonmembers are ranked higher than
Syn MGST
Rat Pike
1mg/lane 1mg/lane
MGST1 Rat
75 ng 25 ng kDa
Mr markers
kDa
Mr markers
45 31
21.5
14.4 10
20 15
150 ng
Fig 4 SDS ⁄ PAGE analysis of purified
SynMGST and pike MGST The protein was
fractionated on SDS⁄ PAGE (15%) and
visualized by silver staining Major proteins
were detected that comigrated with purified
RatMGST1 (17 kDa).
Trang 7the lowest ranked true member These patterns are
more specific then the existing PROSITE pattern
PS01297 (FLAP⁄ GST2 ⁄ LTC4S:
G-x(3)-F-E-R-V-[FY]-x-A-[NQ]-x-N-C) [25] The patterns are selected based
on conserved regions in the sequence Notably, the
PGES pattern is located at the beginning of loop one
and for FLAP it is located in the third hydrophobic
seg-ment All of the remaining patterns are located at the
end of first loop (Fig 6) Both the first and third loop
are located on the cytosolic side of the membrane and
are regions earlier postulated to host the active site
[21,26] Furthermore, the patterns of the two very
sim-ilar families of PGES (earlier denoted MGST1-like) and
MGST1 do not overlap, even though they both are
located in the first loop
For the classical FERV pattern, which is a part of
PS01297, we note that it is still included in the two
new and more specific patterns of MGST2 and LTC4
synthase The last member of PS01297 is FLAP for
which the novel pattern is located in the third loop The reason for the similarity and location of these pat-terns could be a result of short evolutionary time rather than gain of new features as FLAP, MGST2 and LTC4 synthase have been detected only in higher eukaryotes to date However, all patterns in Table 2 will be useful in genome characterizations and func-tional annotations
Discussion The MAPEG family
We have characterized the MAPEG family and found the eukaryotic forms to consist of six families, while the prokaryotic forms are clustered at two sites or more, depending upon whether the E coli cluster (top) and the bacterial cluster (bottom) are separated or not (Fig 2) The SynMGST branches with the cluster of
Fig 5 Alignment of pike MGST1 with homologous forms The N-terminal fragment of pike MGST1 is multiply aligned with other MAPEG fish members Positions identical between the pike form and any other fish form are shown in bold It can be seen that most of the bold amino acid residues are found within the MGST1 family, supporting evidence for the pike form to belong here A dendrogram is shown to the left of the alignment, calculated from the aligned sequences.
Table 2 Sequence patterns for the different MAPEG families.
Trang 8FLAP, MGST2 and LTC4 synthase, while
E.coli-MGST branches earlier, before the divergence of
MGST3 from the previous cluster However, it should
be kept in mind that the early branches have low
boot-strap values and that the order might change when
more data become available Interestingly, several
bac-terial species contain multiple MAPEG forms For
example, the Caulobacter crescentus has three different
forms – one in the SynMGST cluster, one in the
Ecoli-MGST cluster and one in the large bacterial cluster
(Fig 2) We checked whether any of the MAPEG
members were encoded by plasmids, but we did not
find any MAPEG members among known plasmid
sequences
Mutiplicity of MAPEG members is also seen in
insects Both Drosophila and Anopheles show multiple
forms, but these forms are more closely related than
the multiple forms of bacterial species As judged from
sequence comparisons, the insect multiple forms have
appeared independently in each species, probably
reflecting adoption to the environment Interestingly,
Drosophila also has multiple gene families of cytosolic
GSTs [27]
Extensive searches in archaea only revealed possible
homologues related to transport proteins If these
rela-tionships are real they might give a link to ancestors
with different functions, which were later recruited as
detoxification enzymes
Upon examination of the eukaryotic MAPEG
forms, we found that the subdivision into six different
families is present already in fish, dating this diver-gence to 500 mya These findings agree in general with the known well developed capacity of fish xenobiotic metabolism [28] and raises the possibility of arachi-donic acid based signalling Zebrafish express both cyclooxygenase (cox)-1 and -2 and the primary prostaglandin end product is PGE2 [29] Furthermore, the bleeding time as a measurement of platelet activa-tion was sensitive to inhibiactiva-tion of cox-1 but not of cox-2, i.e similar to the situation in humans Incuba-tion of whole blood from rainbow trout with calcium ionophore resulted in the biosynthesis of leuko-triene B4 suggesting an intact leukotriene pathway including phospholipase, 5-lipoxygenase and LTA4 hydrolase in this species [30] Thus, the fish kingdom seems to contain a similar biosynthetic capacity to humans to oxidize arachidonic acid In plants, leuko-triene B4 has been demonstrated in nettles [31] prob-ably as part of its defence mechanism In various species of corals, large amounts of rela-ted compounds are found [32] Here the prostaglandin-like compounds may constitute structural elements of the organism or be part of their chemical defence Recently, two coral (Gersemia fruticosa) cyclooxygen-ases were cloned and functionally characterized, and found to catalyze the formation of PGF2a, PGE2 and PGD2 (presumably through nonenzymatic conversion
of PGH2) as well as unspecified hydroxyeicosatetra-enoic acids [33] It is also suggested that an ancestral gene coding for cyclooxygenase was duplicated before
A
B
Fig 6 Hydrophobicity plot (A) The plot
shows the mean value of hydrophobicity
(solid lines) and standard deviation (dashed
lines) Values are calculated according to
Kyte and Doolittle [53] using an 11-residue
window The positional numbers follow a
multiple sequence alignment of all MAPEG
members (B) The plot shows the number
of sequences present at each position The
four hydrophobic segments, corresponding
to the transmembrane regions are visible as
peaks in (A).
Trang 9the divergence of the modern cyclooxygenase-1 and -2.
It would be interesting to know at what time during
development the MAPEG proteins (specifically PGE
synthase and FLAP⁄ LTC4 synthase) were associated
with the cyclooxygenase and lipoxygenase protein
families, respectively At the introduction of these
MAPEG proteins a more specialized level of product
control must have occurred, allowing for the specific
metabolism of the products derived from
cyclooxygen-ases and lipoxygencyclooxygen-ases into the end products known
today
Structural implications
Now that over 100 different MAPEG forms are
avail-able, a limited number of conserved residues have
appeared Two of these, Glu81 and Arg114 (human
MGST1 positional numbering), are found in the
puta-tive transmembrane segments 2 and 3, respecputa-tively
According to electron crystallographic structure
deter-mination of MGST1 [34] and LTC4 synthase [35] and
hydrophobicity properties, the MAPEG forms all
appear to contain four transmembrane regions MGST1,
LTC4 synthase and PGES [22] are all trimeric
proteins At the tight border between transmembrane
region 2 and 3, some of the sequences have a Gly-Pro
sequence, typical of a sharp bend Interestingly, the
almost strictly conserved charged residues mentioned
above are both spaced by exactly 15 residues from the
Gly-Pro bend, strengthening a role for structural
charge interactions In addition, Asn78 is conserved in
almost all MAPEG members This residue faces the
cytosol, positioned just before the second
transmem-brane segment, and is probably involved at the active
site In fact, mutation of these residues in MGST1
seriously affects activity (unpublished observations)
Mutation of the residue corresponding to Arg114
(Arg110) in human mPGES-1 also abolishes activity
[36] Similarly Arg130, facing the cytosol and adjacent
to the fourth transmembrane segment, is conserved in
nearly all members The sequence patterns diagnostic
for the PGES and FLAP families are both found
in regions facing the cytosol, thus implying that they
represent family specific regions of the active site
and⁄ or substrate-binding areas
Observations on the proteins
E.coliMGST and SynMGST represent the first
charac-terized prokaryotic members of the MAPEG
super-family It was therefore of strong interest to determine
their catalytic properties Both enzymes efficiently
cata-lyze a glutathione transferase reaction and
conse-quently may be involved in detoxification In contrast
to human MGSTs 1, 2 and 3, no glutathione peroxi-dase activity could be detected Our results thus demonstrate that both of these highly divergent pro-karyotic MAPEG members indeed are microsomal glutathione transferases
SynMGST, MGST2, and LTC4 synthase to some extent, align with a postulated lipid binding site of FLAP (amino acids 48–61) [37–39] In addition, Syn-MGST contains conserved arginine and tyrosine resi-dues implicated in LTC4 synthase activity [40] However no such activity could be detected, logically coinciding with the fact that 5-lipoxygenase (forming the substrate) as well as other lipoxygenases are found later in evolution [41] However, recently a 15-lipoxy-genase was characterized as a secretable enzyme
in Pseudomonas aeruginosa [42] and is, to the best of our knowledge, the first example of a lipoxygenase in bacteria
The cyanobacteria, Synechocystis spp., represent an interesting model system for further studies of the bio-logical functions of SynMGST Knock out experi-ments, as well as studies of the effects caused by environmental factors such as light and oxygen on SynMGST gene expression, will provide important information about the biological function Moreover,
if the MGSTs represent common bacterial components involved in glutathione metabolism mediating cell sur-vival, they may constitute possible targets for the development of novel antibiotics
N-ethylmaleimide, activity and activation Mammalian MGST1 is activated by sulfhydryl rea-gents and its relatively modest activity towards CDNB
is increased by 20-fold (from 3 lmolÆmin)1Æmg)1 to
60 lmolÆmin)1Æmg)1) [43] An MGST has been purified from Xenopus laevis that was extremely active (200 lmol min)1Æmg)1) but on the other hand very sen-sitive to sulfhydryl reagents [44] The pike enzyme is also inactivated by N-ethylmaleimide (not shown) Synechocystis, Arabidopsis and Drosophila MGSTs appear to represent a third category, namely enzymes that are insensitive to sulfhydryl reagents In the case
of Synechocystis and Drosophila enzymes, this is accounted for by the fact that no cysteine residues are present and probably explains why SynMGST is an exceptionally stable protein (in our experience) The catalytically active form of E.coliMGST contains three cysteine residues but was not activated by N-ethylmaleimide Instead a slight inhibition of the activity towards CDNB was observed Apparently, none of the cysteines is situated at an accessible
Trang 10posi-tion that is critical for enzyme activity of the
E.coliMGST In conclusion, sulfhydryl reagent
activa-tion⁄ inactivation cannot be used as a criterion to
iden-tify MAPEG MGST1 members as the activation has
been detected so far only with mammalian MGST1
Also, the closest relative of MGST1, PGES, is
inacti-vated by N-ethylmaleimide [22] as well as LTC4
syn-thase [45] It is evident that cysteine is not involved in
the catalytic mechanism of several MAPEG members,
but could well be relevant for PGES and LTC4
syn-thase, which harbour cysteines at unique positions
Conclusion
We have identified several new MAPEG proteins by
sequence homologies with proteins in various databases
The mammalian members can be traced back 500 mya
as all six families can be found in fish, consistent with a
role in eicosanoid signalling The gene products
from two representative bacterial strains, E coli and
Synechocystis sp were cloned and overexpressed in
E coli In addition, plant, insect and fish MAPEG
mem-bers were characterized As a common denominator,
most MAPEG members catalyze glutathione
conjuga-ting activity towards CDNB or a specific substrate such
as LTC4, some with remarkable efficiency The enzymes
represent early MAPEG members in their respective
phylogenetic classes and thus create a defined basis for
understanding this superfamily
Experimental procedures
Materials
Oligonucleotides were synthesized by KEBO, (Stockholm,
Sweden) Pfu DNA polymerase was purchased from
Strata-gene (La Jolla, CA, USA) pGEM T-vector was from
Promega (Madison, WI, USA) Gel extraction and plasmid
isolation kits were from Qiagen (Hilden, Germany) DNA
sequencing kit (ABI PRISM Dye Terminator Cycle
Sequen-cing Ready Reaction Kit) was obtained from Perkin-Elmer
(Boston, MA, USA) Hydroxyapatite (Bio-Gel HTP) was
from Bio-Rad (Hercules, CA, USA)
Sequence comparisons
In the search for new members of the MAPEG superfamily
a set of representative members were selected as seeds The
seeds were the human member proteins of MGST1-3
(Uni-prot-Swissprot identifiers P10620, Q99735 and O14880);
FLAP (P20292); LTC4 synthase (Q16873) and PGES
(O14684) Two bacterial members, SynMGST (P73795) and
E.coliMGST (P64515), were additionally selected to
com-plement the six human forms The eight seeds were used as query sequences in the search for homologues using fasta [46] against Swissprot release 41.24 [47], TrEMBL release 24.13 [47] and 138 completely sequenced genomes Further screenings were performed against the NCBI non-redund-ant protein database using psi-blast [48] Finally, to fetch unverified translations of MAPEG members the NCBI EST database (excluding human and mouse) [49] was searched using tblastn [48] The resulting nucleotide sequences from the EST search were translated using getorf from the embosspackage [50] The open reading frames were filtered
by a minimum size of 100 amino acid residues and flanked
by start and stop codons These homology searches resulted
in nearly 1000 redundant amino acid sequences which were followed by an extensive work of manual filtering to obtain
a non-redundant set of sequences by removing duplicates and non-EST supported alternative splicings
Multiple sequence alignments and dendrograms
To study the relationships between the new members of the superfamily we calculated multiple alignments using clu-stalw [51] on the resulting sequences from the homology searches Dendrograms were obtained using neighbor-join-ing method in the clustalw package and protpars from the phylip package [52] An unrooted tree was generated based on the complete set of sequences of all superfamily members To also visualise the more general relationships
of the families included in MAPEG an unrooted consensus tree was produced The consensus sequences of the families
of MGST1–3, FLAP, LTC4 synthase, PGES, SynMGST cluster, E.coliMGST cluster and Insect cluster were gener-ated by the cons program from the emboss package A hydrophobicity plot was generated to verify the structural similarities of the proteins It was based on the multiple sequence alignment of the complete superfamily and calcu-lated according to Kyte and Doolittle [53] using a window
of 11 residues
Pattern detection
To characterize the MAPEG families further we extracted patterns compatible to the PROSITE database [25,54] These patterns are helpful in annotation of new sequences and model the unique motifs of a family The patterns were generated by the program pratt version 2.1 [55,56] pratt was run on sequences from each of the MGST1-3, FLAP, LTC4 synthase, PGES and Insect families by setting the maximal pattern length parameter to 20 The best ranked patterns of each family, shown in Table 2, were selected and tested for unambiguousness by performing a scan against Swiss-Prot and TrEMBL with the program fuzz-profrom the emboss package The degree of unambiguous-ness was defined as the fraction of member ranked higher than the first occurring non-member