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J. Biol. Chem., Vol. 278, Issue 44, 43089-43094, October 31, 2003
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From the
Departments of Clinical Chemistry and
Pediatrics, ¶Emma Children's Hospital, Academic
Medical Center, University of Amsterdam, P. O. Box 22700, 1100 DE Amsterdam,
The Netherlands
Received for publication, June 5, 2003 , and in revised form, August 19, 2003.
| ABSTRACT |
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| INTRODUCTION |
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The TAZ gene (previously known as G4.5), which is mutated in this disorder, is located on Xq28 (4) and is postulated to contain 11 exons based on the alignment of cDNA sequences with genomic data (5). At the transcriptional level, six different mRNAs have been identified containing different combinations of exons 57 because of differential splicing. Two variants containing either all three exons or lacking exon 5 are consistently more abundant in most tissues examined (5). Alternative splicing also has been reported to produce two different 5'-ends of the transcripts leading to two possible translation initiation sites (1st ATG at position 13 and 2nd ATG at position 277279 of the full-length open reading frame). As a consequence, there are at least 12 possible mRNAs, which are present in different amounts in different tissues (5). Whether these splice variants all give rise to functional proteins awaited the identification of the physiological function of the encoded tafazzins, which has not been established so far.
In 1997, Neuwald (6) hypothesized that tafazzins share homology with a family of acyltransferases that are involved in phospholipid metabolism. In line with this hypothesis, our group found abnormal levels of cardiolipin and phosphatidylglycerol in BTHS cells, whereas all other phospholipid classes are normal (7). Cardiolipin is an acidic polyglycerophospholipid, which is almost exclusively found in the inner mitochondrial membrane and is synthesized from phosphatidylglycerol (8). The structure of cardiolipin is shown in Fig. 1. After synthesis of cardiolipin, its fatty acid side chains are remodeled by a deacylation-reacylation cycle to obtain different cardiolipin species, which contain mainly mono-unsaturated and di-unsaturated chains of 1618 carbons in length (8). Cardiolipin is required for optimal functioning of the respiratory chain complexes and several other mitochondrial inner membrane proteins, including the ATP/ADP transporter and the carnitine acylcarnitine translocase (912). Investigations in cultured skin fibroblasts of BTHS patients showed that the rate of biosynthesis of cardiolipin from phosphatidylglycerol is normal but that the cardiolipin pool size is considerably reduced as compared with control cells. Furthermore, the incorporation of linoleic acid, which is the characteristic acyl side chain found in mammalian cardiolipin, into both phosphatidylglycerol and cardiolipin was significantly reduced (7). These results suggest that the TAZ gene encodes one or more acyltransferases involved in the remodeling of cardiolipin or its precursors. Direct evidence that the tafazzins are involved in cardiolipin metabolism was unavailable until now.
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| EXPERIMENTAL PROCEDURES |
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Yeast tafazzin (YTAZ) was amplified from W303 genomic DNA using a KpnI-tagged forward primer 5'-GGT ACC ATG TCT TTT AGG GAT GTC CTA G-3' and a SalI-tagged reverse primer 5'-GTC GAC TCA ATC ATC CTT ACC CTT TGG-3' and cloned into the KpnI and SalI sites of pYPGK18. The insert was sequenced to exclude sequence errors.
Generation of the YPR140w DisruptantTo construct the
taz deletion mutant, the entire YPR140w open reading frame was
replaced by the kanMX4 marker gene
(14). The PCR-derived
construct for disruption consisted of the kanMX4 gene flanked by
short regions of homology corresponding to the YPR140w 3' and 5'
non-coding regions. pKan was used as template with the YPR140w primers
(5'-ATG TCT TTT AGG GAT GTC CTA GAA AGA GGA GAT GAA TTT TTA GAA GCC TAG
CGT ACG CTG CAG GTC GAC-3' and 5'-TCA ATC ATC CTT ACC CTT TGG TTT
ACC CTC TGG AGG CAG AAA CTT TTG ATC GAT GAA TTC GAG CTC G-3'). The
resulting PCR fragments were introduced into Saccharomyces cerevisiae
wild type W303 strain (15).
G418-resistant clones were selected by growth on YPD plates containing G418
(200 mg/liter) (14).
Expression of TAZ Splice Variants and Complementation
AnalysisThe different pYPGK18 constructs were transformed to
taz using the lithium acetate procedure
(16). Transformed yeast cells
were grown on minimal glucose medium (0.67% yeast nitrogen base, 2% glucose)
at 30 °C and harvested in mid-exponential phase. For the complementation
analysis, transformants were streaked on rich ethanol plates (2% peptone, 1%
yeast extract, 2% agar, and 2% ethanol) and incubated at 37 °C for 1
week.
For cardiolipin and immunoblot analysis, spheroplasts were prepared using zymolyase according to Franzusoff et al. (17) and lysed in a 10 mM sodium phosphate buffer, pH 7.4, containing 140 mM NaCl and protease inhibitor mixture (Roche Applied Science). After sonication, protein determination was performed by the method of Bradford (18).
Cardiolipin Analysis by High Pressure Liquid Chromatography Tandem Mass SpectrometryLipids were extracted using a modification of the method of Folch et al. (19). Briefly, 0.4 nmol of CL(C14: 0)4 (Avanti Polar Lipids, Alabaster, AL) was added to the equivalent of 1 mg of protein in a total volume of 1 ml of distilled water, and this aqueous phase was extracted twice with 3 ml of chloroform/methanol (2:1 v/v) followed by centrifugation at 1000 x g. The combined organic phases were evaporated under a nitrogen stream, and the residue was taken up in 100 µl of chloroform/methanol/water (50:45:5 v/v/v) containing 2.5 ml/liter NH4OH. Five microliters of this lipid extract was injected into a straight-phase high pressure liquid chromatography system, and cardiolipin analysis was performed by on-line electrospray tandem mass spectrometry as described previously (20). By using bovine heart monolysocardiolipin (consisting mostly of trilinoleylmonolysocardiolipin, Avanti Polar Lipids, Alabaster, AL), we determined that with this method monolysocardiolipins elute about 0.2 min later than the cardiolipins. For the acquisition of the cardiolipin spectra of both cardiolipins and monolysocardiolipins, a continuous negative scan was made from 400 to 1000 m/z in a time window of 2.5 min, during which both types of compounds elute from the column.
In a separate analysis, using the same extract, single ion monitoring was used to quantify selected monolysocardiolipins and cardiolipins. The area of each (monolyso)cardiolipin peak (A(ML)CL) and that of the added internal standard (AIS) was quantified using MassLynx 3.3 (Micromass, Manchester, UK). The amount of (monolyso)cardiolipins was calculated by the formula: (A(ML)CL/AIS) x 0.4 nmol (the amount of internal standard added). These data were used to calculate the relative cardiolipin and monolysocardiolipin levels (Fig. 4) in comparison to wild type levels.
Tafazzin Antibody GenerationThe portion of the TAZ cDNA
corresponding to exons 811 was amplified by PCR from human fibroblast
cDNA using Taq polymerase and the following primers: a
BamHI-tagged forward primer 5'-GGA TCC ATC GGG CGC CTG ATT GCT
GAG-3' and an HindIII-tagged reverse primer 5'-AAG CTT
CTA TCT CCC AGG CTG GAG GTG-3'. The PCR product was cloned downstream of
the isopropyl-1-thio-
-D-galactopyranoside-inducible
PTAC promoter into the BamHI and HindIII sites of
the bacterial expression vector pMAL-C2X, to express the last 98 amino acids
of tafazzin fused to maltose-binding protein. This protein sequence is present
in all of the protein products of the putative splice variants of the
TAZ gene. The insert was sequenced to exclude sequence errors
introduced by PCR, after which the construct was transformed to the
Escherichia coli strain BL21. Protein expression, purification, and
antibody generation was performed as described previously
(21). For affinity
purification of the crude antiserum, the same tafazzin fragment was cloned
downstream of the
isopropyl-1-thio-
-D-galactopyranoside-inducible
PTAC promoter into the BamHI and HindIII sites of
the bacterial expression vector pGEX-4T in order to express it as a fusion to
glutathione S-transferase. This protein was expressed, purified,
coupled to CNBr-activated Sepharose (Amersham Biosciences), and used for
affinity purification of the
-TAZ antibodies as described previously
(22). This resulting antibody
solution was diluted 1:100 for immunoblotting, which was performed according
to Vaz et al.
(21).
| RESULTS AND DISCUSSION |
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taz
displays an abnormal cardiolipin profile.
Fig. 2A shows the
cardiolipin profile of wild type W303 strain and that of the disruption mutant
taz. In each cardiolipin cluster, the peak with the lowest
m/z represents tetra-unsaturated cardiolipin. For example,
the ion with m/z 727.5 represents a cardiolipin containing
four oleic acid (C18:1) acyl side chains. To interpret the cardiolipin spectra
shown in Fig. 2, one should
keep in mind that cardiolipins are measured as doubly charged ions, which
implies that the mass over charge ratio (m/z) corresponds to
half of the actual cardiolipin mass and that the distance between the
molecular ion peak and its first isotope peak is 0.5 Da. Therefore, the next
cardiolipin within the 727.5 cluster has an m/z of 728.5.
This cardiolipin has one unsaturation less and represents tri-unsaturated
cardiolipin with three oleic acid (C18:1) acyl side chain and one stearic acid
(C18:0) acyl side chain. The next cluster, which contains an ion with
m/z 713.5, consists of cardiolipins with one C16 and three
C18 fatty acyl side chains, where m/z 713.5 represents
cardiolipin containing one palmitoleic acid (C16:1) and three oleic acid
(C18:1) acyl side chains. Fig.
2A shows that, as observed for BTHS, tetra-unsaturated
cardiolipin species are either completely absent (m/z 727.5)
or present in decreased levels (m/z 685.5, 699.5, and 713.5)
indicating that this yeast disruptant is a good model for BTHS. In addition,
there is accumulation of monolysocardiolipins (m/z 567.5/
568.5 and 581.5/582.5), which thus far has not been observed in BTHS.
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taz, including pYPGK18 without insert, which
was used as a negative control. Immunoblot analysis using an affinity-purified
polyclonal antibody that we raised against the invariant C-terminal part of
human tafazzin confirmed that the splice variants were correctly expressed
(Fig. 3A).
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taz. Second,
cardiolipin analysis was performed in cell homogenates, and this showed that
all transformants had the same cardiolipin profile as
taz,
except for those expressing YTAZ and HTAZ-1stATG-ex5 and HTAZ-1stATG-full,
which show either a complete or partial normalization of the cardiolipin
profile (Fig. 2). By virtue of
the added internal standard, the amount of the most abundant
monolysocardiolipins (m/z 568.5 and 582.5) and that of the
tetra-unsaturated cardiolipins containing C16 and C18 acyl side chains
(m/z 685.5, 699.5, 713.5, and 727.5) was determined and
related to the levels found in the wild type strain
(Fig. 4). The quantitative
results presented in this figure confirm that the growth-phenotype correction
also corresponds to a complete restoration of the cardiolipin profile
for YTAZ and HTAZ-1stATG-ex5. Interestingly, in contrast to these two latter
strains, the transformant expressing HTAZ-1stATG-full displays a partial
restoration of the cardiolipin profile, i.e. cardiolipin species
containing a majority of C18 fatty acids (cardiolipin clusters
m/z 727.5 and m/z 713.5) are still
deficient, whereas cardiolipin species containing mostly C16 fatty acids
(m/z 699.5 and m/z 685.5) display a
pattern similar to that of wild type (Fig.
4A). This partial restoration of the cardiolipin profile
is also reflected by the monolysocardiolipin levels, which in the
transformants expressing YTAZ or HTAZ-1stATG-ex5 are comparable with wild type
but in the transformant expressing HTAZ-1stATG-full are still elevated
(Fig. 4B). Taken
together, our results suggest that only HTAZ-1stATG-ex5 and HTAZ-1stATG-full
are functional splice variants, which are directly involved in cardiolipin
metabolism. There are several reasons, however, to question whether
HTAZ-1stATG-full really represents a physiologically relevant mRNA.
Fig. 5 shows a ClustalW
alignment of amino acids residues 94172 of HTAZ-1stATG-full with
tafazzin orthologues of very distinct organisms, including mammals, flies,
yeast, and plant. This sequence region includes amino acids residues
123153 encoded by exon 5. Note that none of the orthologous sequences
have any sequence homology to this region, whereas in the sequences flanking
the amino acids encoded by exon 5 a high homology with all orthologues is
observed (Fig. 5). This is also
true at the genomic DNA level, as concluded from translated blast analysis
(tBlastn) using the full-length human tafazzin as query in the mouse and rat
genome data base, which identifies all exons, except exon 5 (not shown). This
lack of homology with both related and distant organisms in conjunction with
the inability of HTAZ-1stATG-full to restore the growth phenotype on selective
conditions and the partial restoration of the cardiolipin profile strongly
suggests that exon 5, the existence of which was postulated on the basis of
its presence in some human mRNA splice variants, is a result of aberrant
splicing and in fact does not lead to a functionally relevant protein. This is
also supported by the observation that of the 42 different mutations that
currently have been identified in the TAZ gene, no mutation is
located in exon 5 (Fig. 6)
(2,
3,
5,
2328).
The results from the complementation analysis also show that none of the
tafazzins produced from the second translation initiation site were able to
complement the phenotype, not even HTAZ-2ndATG-ex5. This is in agreement with
the observation that patients with mutations in exons 1 and 2, i.e.
before the second translation initiation site which is supposed to be used for
the production of the shorter tafazzins, also suffer from BTHS and do not
display a milder phenotype when compared with patients with mutations in exons
310 (3,
5,
24,
25).
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| FOOTNOTES |
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To whom correspondence should be addressed: University of Amsterdam, Academic
Medical Center, Depts. of Clinical Chemistry and Pediatrics, Laboratory
Genetic Metabolic Diseases (F0-224), P. O. Box 22700, 1100 DE Amsterdam, The
Netherlands. Tel.: 31-20-5665958; Fax: 31-20-6962596; E-mail:
f.m.vaz{at}amc.uva.nl.
1 The abbreviations used are: BTHS, Barth syndrome; HTAZ, human tafazzin;
YTAZ, yeast tafazzin. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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