The TMEM189 gene encodes plasmanylethanolamine
desaturase which introduces the characteristic vinyl
ether double bond into plasmalogens
Ernst R. Wernera,1, Markus A. Kellerb, Sabrina Sailera, Katharina Lacknera, Jakob Kochb, Martin Hermannc,
Stefan Coassind, Georg Golderera, Gabriele Werner-Felmayera, Raphael A. Zoellere, Nicolas Hulof,
Johannes Bergerg, and Katrin Watschingera,1
a
Institute of Biological Chemistry, Biocenter, Medical University of Innsbruck, A-6020 Innsbruck, Austria; bInstitute of Human Genetics, Medical University of
Innsbruck, A-6020 Innsbruck, Austria; cUniversity Clinic for Anesthesiology and General Intensive Care Medicine, Medical University of Innsbruck, A-6020
Innsbruck, Austria; dInstitute of Genetic Epidemiology, Department of Genetics and Pharmacology, Medical University of Innsbruck, A-6020 Innsbruck,
Austria; eDepartment of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118; fInstitute of Genetics and Genomics, University
of Geneva, 1211 Geneva 4, Switzerland; and gDepartment of Pathobiology of the Nervous System, Medical University of Vienna, 1090 Vienna, Austria
Edited by Benjamin F. Cravatt, Scripps Research Institute, La Jolla, CA, and approved February 26, 2020 (received for review October 7, 2019)
A significant fraction of the glycerophospholipids in the human body
is composed of plasmalogens, particularly in the brain, cardiac, and
immune cell membranes. A decline in these lipids has been observed
in such diseases as Alzheimer’s and chronic obstructive pulmonary
disease. Plasmalogens contain a characteristic 1-O-alk-1′-enyl ether
(vinyl ether) double bond that confers special biophysical, biochemical, and chemical properties to these lipids. However, the genetics of
their biosynthesis is not fully understood, since no gene has been
identified that encodes plasmanylethanolamine desaturase (E.C.
1.14.99.19), the enzyme introducing the crucial alk-1′-enyl ether double bond. The present work identifies this gene as transmembrane
protein 189 (TMEM189). Inactivation of the TMEM189 gene in human
HAP1 cells led to a total loss of plasmanylethanolamine desaturase
activity, strongly decreased plasmalogen levels, and accumulation of
plasmanylethanolamine substrates and resulted in an inability of
these cells to form labeled plasmalogens from labeled alkylglycerols.
Transient expression of TMEM189 protein, but not of other selected
desaturases, recovered this deficit. TMEM189 proteins contain a conserved protein motif (pfam10520) with eight conserved histidines
that is shared by an alternative type of plant desaturase but not
by other mammalian proteins. Each of these histidines is essential
for plasmanylethanolamine desaturase activity. Mice homozygous
for an inactivated Tmem189 gene lacked plasmanylethanolamine
desaturase activity and had dramatically lowered plasmalogen levels
in their tissues. These results assign the TMEM189 gene to plasmanylethanolamine desaturase and suggest that the previously characterized phenotype of Tmem189-deficient mice may be caused by a
lack of plasmalogens.
|
plasmalogen transmembrane protein 189
desaturase ether lipid
severe clinical outcomes, including impaired neural development,
bone deformation, and premature death. The initial peroxisomal
steps of human plasmalogen biosynthesis have been well characterized to require the genes GNPAT (11, 12), AGPS (13), and
DHRS7B (14). Further downstream steps occurring in the endoplasmic reticulum are less well understood, however. Enzymes
modifying the sn2 and sn3 positions during the biosynthesis of
ether lipids in the endoplasmic reticulum might be shared to an
unknown extent between ether and ester-linked glycerophospholipids.
Recently, SELENOI, which encodes ethanolamine phosphotransferase
1, has been found to play a crucial role in plasmalogen biosynthesis
(15); however, the gene coding for the enzyme generating the
first plasmalogen in the pathway, plasmanylethanolamine desaturase
(PEDS, E.C. 1.14.99.19), has not yet been identified. This enzymatic reaction is specific for plasmalogen biosynthesis. It introduces the alk-1′-enyl ether double bond (vinyl ether bond)
into plasmanylethanolamines, yielding plasmenylethanolamines
(Fig. 1A), the first plasmalogens formed in the biosynthetic pathway (2). In contrast, plasmenylcholines (plasmalogens of the
Significance
Although sequencing of the human genome was completed years
ago, we still do not know about the physiological significance of
thousands of predicted proteins, particularly of predicted membrane proteins. On the other hand, for approximately 100 human
enzymes, no coding gene is known even though their enzymatic
reaction has been well characterized. In this work, we assign one of
those predicted membrane proteins (transmembrane protein 189;
TMEM189) to one of the enzymatic reactions with an uncharacterized gene (plasmanylethanolamine desaturase). This enzyme
catalyzes the final step in the biosynthesis of plasmalogens, an
abundant class of glycerophospholipids that is depleted in such
diseases as Alzheimer’s. Our findings enable interpretation of the
previously characterized impaired growth phenotype of Tmem189deficient mice.
| plasmanylethanolamine
|
Downloaded by guest on July 24, 2020
P
lasmalogens are a special type of glycerol-based phospholipids
that are abundant in human and animal bodies. Yeast, aerobic
bacteria, plants, and most nonanimal organisms do not contain this
special glycerophospholipid class. Interestingly, some anaerobic bacteria also form plasmalogens using enzymatic reactions different from
those in animals (1). In humans, plasmalogens constitute roughly
one-fifth of all phospholipids, with particularly high concentrations in
brain and in immune cell membranes (2). A decline in plasmalogens
has been associated with Alzheimer’s disease (3–6) and autism
spectrum disorders (7). Plasmalogens are important constituents of
surfactants in the lung and have been shown to effectively reduce the
surface tension of surfactant-like phospholipid mixtures (8). It has
been suggested that decreased plasmalogen levels in smokers could
be involved in the development of smoking-related diseases (9).
Rare inherited disorders have been characterized that impair
the initial peroxisomal steps of plasmalogen biosynthesis, resulting
in rhizomelic chondrodysplasia punctata types 1 to 5 (10) with
7792–7798 | PNAS | April 7, 2020 | vol. 117 | no. 14
Author contributions: E.R.W., M.A.K., G.G., G.W.-F., N.H., J.B., and K.W. designed research; E.R.W., M.A.K., S.S., K.L., J.K., M.H., S.C., N.H., and K.W. performed research;
R.A.Z. contributed new reagents/analytic tools; and E.R.W. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This open access article is distributed under Creative Commons Attribution License 4.0
(CC BY).
1
To whom correspondence may be addressed. Email: ernst.r.werner@i-med.ac.at or
katrin.watschinger@i-med.ac.at.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/
doi:10.1073/pnas.1917461117/-/DCSupplemental.
First published March 24, 2020.
www.pnas.org/cgi/doi/10.1073/pnas.1917461117
we looked for a candidate gene to express or inactivate it in cultured
cells and monitor the enzymatic activity using a fluorescence-based
enzyme activity assay that we developed recently (22). In previous
work, we found that the enzyme has properties similar to non–
heme-containing di-iron desaturases (22), which are characterized
by a motif of eight conserved histidines (23). The enzymatic activity
was found in the microsomal fraction of tissues or cells (21,
22), thus being membrane-bound and likely originating from
the endoplasmic reticulum. In our previous characterization of
the gene for another ether lipid-metabolizing enzyme, alkylglycerol monooxygenase, we found good correlation of mRNA
abundance with enzymatic activity in cells and tissues (24).
Thus, for the selection of candidate genes, we followed the
hypotheses that (i) the messenger RNA amount of the candidate would correlate with the enzymatic activity of the respective cell or tissue, (ii) the candidate should have features
predicting it as a membrane protein, (iii) the candidate protein might have a kind of histidine motif characteristic for lipid
desaturases, and (iv) the candidate gene should occur only in
species synthetizing plasmalogens—that is, in animals but not in
Escherichia coli, plants, yeast, or fungi.
In murine RAW264.7 cells, which have distinctive PEDS activity (22, 25) we found 9,619 of 24,421 genes clearly expressed by
mRNA sequencing. We compiled these data with mRNA sequencing data obtained from the National Center for Biotechnology Information (NCBI) Gene Expression Omnibus (GEO) database for
Results
Selection of Candidate Gene. PEDS has never been purified but is
thought to be a labile integral membrane protein (21). Therefore,
O
plasmanylplasmenylethanolamine [II] ethanolamine [III]
D
HxxxxH (7- 41) HxxxHH
(56-71)
(61-189)
F
3
2
1
0
HxxxHH
G
2.0
1.5
1.0
0.5
0.0
+ GFP
+ DEGS1
+ DEGS2
+ FADS1
+ FADS2
+ FADS3
+ FADS6
+ FA2H
+ SCD1
+ SCD2
+ TMEM189
Downloaded by guest on July 24, 2020
HxxxH (24-26) HxxHH
PEDS activity
(pmol mg-1 min-1 )
PEDS activity
(pmol mg-1 min-1 )
E
HH
H
2.0
1.5
1.0
0.5
0.0
20
15
10
5
0
wild type
∆TMEM189
H
plasmalogen (nmol mg-1)
C
(23)
B
R1
HO
R2
O
+
O O
NH3
-O P O
O
wild type
∆TMEM189
+ GFP
+ DEGS1
+ DEGS2
+ FADS1
+ FADS2
+ FADS3
+ FADS6
+ FA2H
+ SCD1
+ SCD2
+ TMEM189
phosphatidylethanolamine [I]
R1
PEDS
HO
R2
O
+
O O
NH3
-O P O
O
PEDS activity
(pmol mg-1 min-1 )
R1
HO
R2
O
+
O O
NH3
-O P O
O
si non target
si TMEM189
A
Fig. 1. PEDS gene identification. (A) Formulas of phosphatidylethanolamine [I], plasmanylethanolamine [II], and plasmenylethanolamine [III], which differ in
their bonding type at sn1 (red). The PEDS reaction that generates plasmalogens by introduction of the 1-O-alk-1′-enyl double bond is indicated by an arrow
from [II] to [III]. R1 and R2 denote the typical hydrocarbon linear side chains of mammalian lipids comprising (in addition to the two side chain carbons shown
in the formula) typically 14 or 16 carbon atoms and zero or one double bond (R1) and approximately 14 to 22 carbon atoms and one to six double bonds (R2).
(B) Pearson correlation coefficients of PEDS enzymatic activities (22) with normalized mRNA sequence counts for 7,382 genes commonly expressed in cells or
tissues with PEDS activity downloaded from NCBI GEO datasets for seven mouse tissues (y-axis) and 11 human cell lines (x-axis). Black solid circles, genes with
gene symbols with Tm as the first two letters, which are mostly transmembrane proteins with unknown function; gray open circles, other genes. The arrow
indicates TMEM189. (C) Schematic representation of the conserved histidines deduced from a sequence alignment of the 10 most diverse members of the
pfam10520 protein family as displayed in the NCBI conserved domain database (CCD) (43). (D) Schematic representation of the classical eight-histidine motif
of stearoyl-CoA desaturase and related desaturases (23). (E) PEDS activities of human HEK293T cells at 48 h posttransfection with selected expression plasmids
for desaturase and hydroxylase proteins compared with a TMEM189 protein expression plasmid. GFP (green fluorescent protein) served as a transfection
efficiency control. Data are mean ± SEM of four independent experiments. (F) PEDS activities in human A431 cells treated for 72 h with siRNA pools. Data are
mean ± SEM of five independent experiments. (G) PEDS activities in WT HAP1 cells compared with TMEM189-deficient HAP1 cells (ΔTMEM189) at 48 h
posttransfection with expression plasmids for GFP, for selected desaturase and hydroxylase proteins, and for TMEM189 protein. Data are mean ± SEM for
three independent experiments. (H) Plasmalogen content of WT HAP1 cells compared with TMEM189-deficient HAP1 cells. Data are mean ± SEM for three
independent experiments.
Werner et al.
PNAS | April 7, 2020 | vol. 117 | no. 14 | 7793
BIOCHEMISTRY
phosphocholine head group class) are not substrates of the enzyme
and thus must be synthesized from plasmenylethanolamines (2).
Importantly, the alk-1′-enyl ether double bond confers crucial
biophysical, biochemical, and chemical properties to plasmalogens
(6, 16). Plasmenylethanolamine has been shown to have a dramatic impact on the structure of phospholipid bilayers (17) and to
facilitate rapid membrane fusion (18). Both properties are not
shared by the corresponding ester-linked phosphatidylethanolamines (Fig. 1A shows chemical structures). Due to the alk-1′-enyl
ether double bond, plasmalogens become sensitive to chemical
cleavage by low concentrations of ozone (19) or by hydrochloric
acid (6), resulting in formation of an aldehyde. The liberation of
an aldehyde on acid treatment resulted in the detection of these
compounds, giving rise to their designation as plasmalogens
(aldehyde-releasing compounds found in cell plasma) (20).
Here we present evidence that the transmembrane protein 189
gene (TMEM189; Tmem189 for the murine gene) encodes
PEDS activity, that the conserved eight histidines found in a
conserved motif occurring in TMEM189 proteins are essential
for PEDS enzymatic activity, and that mice homozygous for an
inactivated Tmem189 gene have dramatically lower plasmalogen
levels.
Downloaded by guest on July 24, 2020
11 human cell lines and seven mouse tissues. This resulted in 7,382
genes commonly expressed RAW 264.7 in 11 human cell lines and in
seven mouse tissues. We then correlated for each of these genes,
separately for human cell lines and for mouse tissues, the normalized
mRNA sequencing counts with PEDS enzymatic activity that we had
observed previously (22) (Fig. 1B). We ranked these 7,382 genes
according to their mean correlation of cell and tissue mRNA sequencing counts with PEDS activity. TMEM189 was the very top hit
of all 7,382 genes examined, and both the gene symbol and the
protein name defined it as a transmembrane protein. TMEM189
proteins contain a conserved motif (pfam10520) with eight conserved
histidines (Fig. 1C). This motif had also been found in an alternative
class of plant desaturases designated fatty acid desaturase type 4
(FAD4) (26). These desaturases introduce a trans double bond at
position Δ3 into a hexadecanoyl residue at sn2 of phosphatidylglycerol in photosynthetic membranes of plants (26). The eighthistidine motif of pfam10520 resembles the classical eight-histidine
stearoyl CoA desaturase motif (Fig. 1D) (23) but is different in the
amino acid distances and the grouping of the histidines. In addition,
one TMEM189 protein homolog per species was found throughout
the animal kingdom, but not in E. coli, Saccharomyces cerevisiae, or
Arabidopsis thaliana. Thus, the TMEM189 gene best met all four of
our criteria, and we tested this gene for its impact on PEDS activity.
the enzymatic activity levels off (22). Only an expression plasmid
for TMEM189 protein, but not for selected other desaturase proteins, was able to yield the formation of pyrene-labeled plasmalogens.
All other desaturase proteins tested did not catalyze formation of the
alk-1′-enyl ether bond (<0.1% of TMEM189 transfection). Also with
myc-tagged proteins, only transfection of TMEM189-6x myc resulted
in the formation of alk-1′-enyl lipids (SI Appendix, Fig. S2), although
all proteins were robustly expressed (SI Appendix, Fig. S1).
TMEM189 Encodes for PEDS. For the following experiments, we
used different cell types chosen according to their suitability for
the respective assays—for example, HEK293T for transfection of
expression plasmids, A431 for siRNA experiments, and human
haploid HAP1 (27) for generating a TMEM189 knockout cell
line. In HEK-293T cells, transfection of an expression plasmid
for TMEM189 protein, but not for selected desaturases included
as controls, resulted in significantly increased PEDS activity
(F(10, 33) = 6.801, P < 0.0001, one way ANOVA; n = 4) (Fig. 1E).
Repetition of this experiment with C-terminally 6x myc-tagged
proteins and investigation of protein expression by Western blot
analysis with an anti myc-tag antibody confirmed expression of
all recombinant proteins (SI Appendix, Fig. S1). Comparable to
the results with the untagged proteins shown in Fig. 1E, also for
the transfection of the myc-tagged versions of the proteins only
TMEM189-6x myc led to a mean 2.37 ± 0.18-fold increase in
PEDS activity (n = 4) compared with green fluorescent protein
(GFP)-transfected cells. Knockdown of TMEM189 mRNA by
siRNA in A431 cells (Fig. 1F) led to significantly lower PEDS
activity (P < 0.032, two-tailed, unpaired t test; n = 5).
We next obtained HAP1 cells with an inactivated TMEM189
gene generated by specific gene editing with the CRISPR/Cas9
system. A 61-bp deletion of exon 5 resulted in a truncated protein lacking five of the eight conserved histidines of pfam10520
(SI Appendix). In contrast to wild-type (WT) HAP1 cells,
TMEM189-deficient HAP1 cells had no detectable PEDS activity
(Fig. 1G). Only an expression plasmid for TMEM189 protein,
but not for other selected desaturase proteins, was able to restore
PEDS activity in TMEM189-deficient HAP1 cells (Fig. 1G). Repetition of this experiment with C-terminally 6x myc-tagged proteins
and investigation of protein expression by Western blot analysis
with an anti-myc antibody confirmed expression of all recombinant
proteins (SI Appendix, Fig. S1). Again, only TMEM189-6x myc
yielded PEDS activity (mean, 0.39 ± 0.16 pmol mg−1 min−1; n = 3).
TMEM189-deficient HAP1 cells had dramatically lower plasmalogen content compared with WT HAP1 cells (17-fold lower; P =
0.0015, two-tailed, unpaired t test) (Fig. 1H).
To measure de novo formation of the alk-1′-enyl ether double
bond in intact cells, we fed WT HAP1 and TMEM189-deficient
HAP1 cells for 24 h with 1-O-pyrenedecyl-sn-glycerol, which is
taken up by cells and incorporated into plasmalogens (22).
Owing to the accumulation of the metabolites in cells over 24 h,
this gives a more sensitive readout than the enzymatic activity
assay, in which the incubation time is limited to 30 min before
histidines found in the pfam10250 motif for PEDS enzymatic activity
by site-directed mutagenesis to alanine. Fig. 3A shows an alignment
of two conserved parts of TMEM189 proteins of mice, humans, and
selected model organisms compared with three selected plant FAD4
proteins. In the murine TMEM189 protein, the eight histidines
conserved in the pfam10250 motif carry numbers 96, 121, 122, 187,
191, 215, 218, and 219 (black arrows in Fig. 3A). In addition, we
included histidine 131, which is conserved in all TMEM189 proteins
but not in FAD4 plant desaturase proteins (gray arrow in Fig. 3A).
We transfected expression plasmids carrying mutations of either of
these histidines to TMEM189-deficient HAP1 cells and monitored
the amount of pyrene-labeled plasmalogens formed on feeding of
the transfected cells for 24 h with 1-O-pyrenedecyl-sn-glycerol.
Pyrene-labeled plasmalogens were quantified in lipid extracts by
HPLC with fluorescence detection of the amount of pyrenedecanal
dimethylacetal formed from plasmalogens by treatment with HCl
in methanol (Fig. 3B). Expression of the transfected proteins in
the cells was confirmed by Western blot to the C-terminal 6x myc
tag (Fig. 3C), with equal loading of cellular protein to all lanes by
staining with β-actin (Fig. 3D). Each of the eight histidines conserved in pfam10250 was absolutely essential for PEDS activity as
monitored by labeled plasmalogen formation in intact cells,
whereas mutation of histidine 131 to alanine resulted in a strongly
reduced but still detectable PEDS activity (Fig. 3B).
7794 | www.pnas.org/cgi/doi/10.1073/pnas.1917461117
TMEM189-Deficient HAP1 Cells Selectively Accumulate Plasmanylethanolamines. An analysis of glycerophosphoethanolamines and glyc-
erophosphocholines by liquid chromatography tandem-mass
spectrometry (LC-MS/MS) revealed that plasmanylethanolamines, the substrates of PEDS, accumulated in TMEM189deficient HAP1 cells displaying the same side chain pattern as
the plasmenylethanolamines, which were found only in WT
HAP1 cells (Fig. 2). Plasmenylcholines formed by the cells from
plasmenylethanolamines (2) were less abundant and also absent
in TMEM189-deficient cells. The ester-linked phosphatidylcholines and phosphatidylethanolamines, as well as the plasmanylcholine species, remained unchanged (Fig. 2).
The Conserved Eight Histidines Are Absolutely Essential for PEDS
Activity. We next checked the importance of the eight conserved
A TMEM189-GFP Fusion Protein Localizes to the Endoplasmic Reticulum.
We next checked localization of the TMEM189 protein by expressing a TMEM189-GFP fusion protein in HEK293T cells and observed the localization of the fluorescent fusion protein by confocal
microscopy. The expression pattern showed localization in the endoplasmic reticulum by an overlay of the green fluorescence signal
with ER Tracker Red (SI Appendix, Fig. S3). Curiously, red fluorescence also appeared in vesicle-like structures that were devoid of
GFP fluorescence. While mutation of the nine histidines to alanine
strongly affected PEDS enzymatic activity (Fig. 3), localization of the
GFP fusion proteins was not altered (SI Appendix, Fig. S3).
Tmem189-Deficient Mice Lack Both PEDS Activity and Plasmalogens.
Tmem189-deficient mice were generated as part of the international mouse phenotype project (28) and subjected to a systematic
phenotyping protocol (29). In these mice, we measured plasmalogen levels and PEDS activity in the kidneys, the organ with the
highest PEDS activity (22). PEDS activities were diminished in
heterozygotes compared with WT (two-tailed P = 0.0056 for males,
Werner et al.
PE
1-O-alk-1'-enyl-PE
PE(36:2)
10
PE(P-38:3)
PE(P-34:1)
PE(P-38:6)
PE(P-36:1)
PE(P-40:6)
PE(38:4)
PE(38:3)
PE(38:5) PE(40:6)
PE(34:1)
5
wild type
15
0
ΔTMEM189
abundance (%)
1-O-alkyl-PE
PE(36:1)
15
PE(O-38:3)
PE(O-38:6)
PE(O-34:1)
PE(O-36:1)
PE(O-40:6)
10
5
0
32 33 34
35
36
37
38
40
42
34
36
PC
38
40
34
1-O-alkyl-PC
36
38
40
(DB)
(Carb)
1-O-alk-1'-enyl-PC
PC(34:1)
PC(36:2)
PC(36:1)
PC(34:2)
PC(30:0)
PC(O-34:1)
PC(P-34:1)
PC(38:2)
0
20
10
0
<30 30 31 32 33
34
35
36
37
38
40
32 34 36
38
32 34
36
38
40
(DB)
42 (Carb)
Fig. 2. Glycerophosphoethanolamines and glycerophosphocholines in WT and TMEM189-deficient human HAP1 cells. TMEM189-deficient HAP1 cells
(ΔTMEM189) contain a frame-shift mutation introduced by a CRISPR-Cas9 protocol, leading to a truncated, inactive protein (SI Appendix). Cells were cultured
under standard conditions, and lipids were extracted and analyzed by LC-MS/MS as described in Materials and Methods. Mean ± SD of three independent
experiments is shown. PC, phosphatidylcholines (ester bond at sn1); PE, phosphatidylethanolamines (ester bond at sn1); 1-O-alkyl PC, plasmanylcholines [ether
bond at sn1, PC(O−)]; 1-O-alk-1′-enyl PC, plasmenylcholines [vinyl ether bond at sn1, i.e., plasmalogens, choline type, PC(P-)] 1-O-alkyl PE, plasmanylethanolamines [ether bond at sn1, PE(O-)]; 1-O-alk-1′-enyl PE, plasmenylethanolamines [vinyl ether bond at sn1, i.e., plasmalogens, ethanolamine type, PE(P-)].
Numbers in parentheses give the total number of side chain carbon atoms (Carb), followed by the number of double bonds (DB). To facilitate location of
species, selected bars are labeled.
0.3
0.2
0.1
55
anti-myc
kDa
C
GFP
wild type
H96A
H121A
H122A
H131A
H187A
H191A
H215A
H218A
H219A
IHKWSH-TYLGLPYWVTVLQDWHVILPRKHHRIHHVAPHE
IHKWSH-TYFGLPRWVTLLQDWHVILPRKHHRIHHVSPHE
IHKWSH-TYFGLPRWVIFLQDWHVILPRKHHRIHHVSPHE
IHKWSH-TYFGLPRWVVLLQDCHIILPRKHHRVHHVAPRE
IHKWSH-TYFGLPTWVVFLQKAHIILPRSHHKIHHISPHA
IHKWSH-TYNVHP-FVGFLQKSGIILSKRDHAIHHRNPFDK
IHKWSH-QAKQSR-IVRKAMDMDILLSPIAHRKHHKDPFDR
FHAWAHGTKSKLPPLVVALQDMGLLVSRRQHAEHHRAPYNN
FHSWAHGTKSKLPPLVVALQDAGILVSRSQHAAHHRPPYNN
FHAWAHGNPRRLPPGVGAMQRAGVLVSRAQHGAHHRAPYDN
labeled plasmalogen
(nmol mg-1)
186
185
189
189
234
268
188
228
241
187
0.4
0.0
43
D
34
kDa
TMEM189
FAD4
Downloaded by guest on July 24, 2020
131
M. musculus
H. sapiens
G. gallus
D. rerio
C. elegans
D. discoideum
L. major
A. thaliana
N. tabacum
O. sativa
B
218
219
VHWGADTWGSVDLPIVGKAFIRPFREHHIDPTAITRHDF
VHWGADTWGSVELPIVGKAFIRPFREHHIDPTAITRHDF
FHWGADTWGSVELPIVGKAFIRPFREHHIDPTAITRHDF
VHWGADTWGSVDLPIVGKAFIRPFREHHIDPTAITRHDF
VHWAADTFGSVET-WFGRSFIRPFREHHVDPTAITRHDI
VHWAADTWGSLDTPLVGNSFIRSFREHHVVPTQMTHHDV
VHWGMDTWGTPDTPIFG-TFIRSFREHHVDQTAMCKHDF
YHWAIDNYGDESTPVVG-TQIEAFQGHHKWPWTITRRQF
YHWGIDNYGSAKTPVFG-SQIDAFQGHHKWPWTITRREF
YHWLVDNYGDADTPVLG-PQIAAFQGHHRHPSTITRREP
215
95
94
98
98
144
180
91
147
160
102
187
M. musculus
H. sapiens
G. gallus
D. rerio
C. elegans
D. discoideum
L. major
A. thaliana
N. tabacum
O. sativa
191
TMEM189
FAD4
96
A
one WT allele with the inactive tg (two-tailed P = 0.577 for males
and 0.333 for females; n = 3 each), whereas homozygous animals
with both alleles as tg had almost completely lost their plasmalogens (two-tailed P, WT/WT vs tg/tg = 0.0021 for males, <0.0001
for females, unpaired t test; n = 3 each) (Fig. 4B). Body weight was
reduced in homozygous male (F(2, 6) = 9.305, P = 0.0145, two-way
121
122
0.0044 for females, unpaired t test), and were below the detection
limit of 0.2 pmol mg−1 min−1 (22) in animals homozygous for the
transgene (tg) (Fig. 4A). In the tg construct, a galactosidase reporter
is spliced downstream exon 2 to the TMEM189 protein, thus
truncating the protein and inactivating the enzymatic activity (28).
Plasmalogen levels remained largely unchanged by replacement of
anti-actin
43
Fig. 3. Importance of the eight-histidine motif for plasmalogen formation by transiently expressed TMEM189 protein. (A) Amino acid sequence comparison
of pfam10520 motif containing proteins. Two regions containing the conserved histidines are shown. Black arrows indicate histidines conserved in all proteins; gray arrow, histidine conserved in TMEM189 proteins only. Numbers at arrows correspond to the position in the murine TMEM189 protein sequence. (B)
Ability of plasmids expressing WT and mutated murine TMEM189 proteins to enable formation of pyrene-labeled plasmalogens on feeding with 1-Opyrenedecyl-sn glycerol in TMEM189-deficient human HAP1 cells. At 24 h posttransfection with plasmids for TMEM189 proteins containing C-terminal 6x myc
tags, cells were fed for another 24 h with 5 μM 1-O-pyrenedecyl-sn-glycerol. A plasmid for GFP served as a control. Values were related to the amount of
cellular protein. Data are mean ± SEM of three experiments. (C) Expression of the recombinant proteins was monitored at 48 h posttransfection by Western
blot analysis using an antibody against the C-terminal 6x myc tag. A representative example of three independent experiments is shown. (D) Western blots
against β-actin to monitor cellular protein loading.
Werner et al.
PNAS | April 7, 2020 | vol. 117 | no. 14 | 7795
BIOCHEMISTRY
ΔTMEM189
abundance (%)
10
PC(32:1)
wild type
20
ANOVA; n = 3) (Fig. 4C) and female (F(2, 6) = 9.653, P = 0.0133,
two-way ANOVA; n = 3) (Fig. 4D) animals compared with heterozygous or WT animals. In addition to the kidneys (Fig. 4B),
plasmalogens were also found to be strongly reduced in 12 other
tissues of homozygous tg-carrying mice (SI Appendix, Fig. S4).
Discussion
In this work, we show that the TMEM189 gene is essential for the
introduction of the alk-1′-enyl ether bond to form plasmalogens.
TMEM189 proteins contain a conserved motif comprising eight
conserved histidines that had previously been found in an alternative type of plant fatty acid desaturase (26). This motif differs in
structure somewhat from the eight-histidine motif found in classical
membrane bound desaturases, such as stearoyl CoA desaturase
(23). The conserved histidines of stearoyl CoA desaturase have
been shown to coordinate a di-metal center in crystal structures (30,
31) that is thought to be involved in catalysis of the enzymatic reaction. We found that each of the eight histidines conserved in both
the TMEM189 proteins and the plant FAD4 alternative desaturase
proteins is essential for PEDS activity. These data strongly suggest
that the TMEM189 protein is the PEDS enzyme itself rather than
an accessory protein required for the enzymatic reaction.
TMEM189 protein is currently annotated as an ubiquitin ligase
in databases, and the pfam10520 protein motif is annotated as the
localization B domain of TMEM189. This is based on findings that
in human cells, a read-through transcript of TMEM189 with the
adjacent gene, the ubiquitin-conjugating enzyme E2 variant 1
(UBE2V1), was found (32). The authors characterized TMEM189
as a conserved class of proteins with a histidine-rich motif. When
fused to the downstream gene UBE2V1, overexpression of the
TMEM189 fusion protein altered localization of the UBE2V1
protein from the nucleus (32) to the endoplasmic reticulum (33),
where unfused TMEM189 protein was also found (33). This gave
rise to the designation of the pfam10520 domain as a localization
domain. The read-through transcript was rare, however, and the
two genes adjacent in humans were found to be separate in
Caenorhabditis elegans as well as in Drosophila melanogaster (32),
30
20
10
4
6
age (weeks)
10
5
8
30
wt/wt
wt/tg
tg/tg
plasmalogen
(nmol mg-1)
D
male
female
15
0
wt/wt
wt/tg
tg/tg
0
20 male
wt/wt
wt/tg
tg/tg
B
female
1
0
2
Downloaded by guest on July 24, 2020
male
body weight (g)
body weight (g)
C
2
wt/wt
wt/tg
tg/tg
PEDS activity
(pmol mg-1min -1)
A
female
20
10
0
2
4
6
age (weeks)
8
Fig. 4. PEDS activities, plasmalogen levels, and body weight in mice depending
on the Tmem189 gene locus. Tmem189tm1a(KOMP)Wtsi mice were housed
under standard conditions and weighed weekly, and kidneys were collected at 8
wk for PEDS activity and plasmalogen measurements as described in Materials
and Methods. The tg was a knockout-first allele leading to an inactive truncated
TMEM189 protein by artificially splicing a LacZ reporter downstream of exon 2
(28). (A) PEDS activity in kidney samples. (B) Plasmalogen levels in kidney samples.
(C) Body weight in male animals. (D) Body weight in female animals. Open circles, dashed line: homozygous tg animals (tg/tg); solid squares, solid line: heterozygous animals (wt/tg); solid triangles, solid line: WT animals (wt/wt). Data are
mean ± SEM for three animals each.
7796 | www.pnas.org/cgi/doi/10.1073/pnas.1917461117
indicating that formation of the read-through transcript with
UBE2V1 might not be the major role of TMEM189 proteins.
Since we find here that the TMEM189 encodes PEDS, we suggest
naming the gene PEDS and the protein plasmanylethanolamine
desaturase. Since the pfam10520 domain is also found in the
FAD4 alternative type of plant fatty acid desaturase proteins, and
the conserved histidines of this domain are essential for PEDS
enzymatic activity, we further suggest annotating pfam10520 as a
lipid desaturase domain.
We found that a mouse strain with inactivated Tmem189 gene
had no detectable PEDS activity in the kidneys, the organ with the
otherwise highest PEDS activity (22). Plasmalogen levels were
strongly reduced in all organs of the mouse that we tested, indicating that no isoenzyme encoded by a gene different from
Tmem189 is present in these tissues to carry out the PEDS reaction. Mice with inactivated Tmem189 gene had significantly
lower weight, indicating the importance of the vinyl ether bond for
normal growth. The phenotype of Tmem189-deficient mice has
already been characterized in a systematic phenotyping program
(28), and results have been presented previously (29). Our finding
of a lack of plasmalogens in these mice provides a novel biochemical basis for the interpretation of the phenotype. For example, plasmalogen deficiency accompanies diminished weight
also in mouse strains with plasmalogen deficiency caused by inactivation of Pex7, Gnpat, or Agps genes, all of which are required
for the peroxisomal steps of ether lipid biosynthesis (34). Growth
retardation is also a hallmark of human peroxisomal ether lipid
biosynthesis deficiency (10). In humans, a UK Biobank study
found associations of single nucleotide polymorphisms in the
TMEM189 gene with altered monocyte percentage of white cells
and with altered granulocyte percentage of myeloid white cells
(35). However, alterations of monocyte or granulocyte cell counts
were not evident in Tmem189-deficient mice (29).
Pex7-, Gnpat-, and Agps-deficient mice all show male infertility (34),
whereas male Tmem189-deficient mice are fertile (29). This is consistent with the hypothesis that male infertility in mice with deficient
peroxisomal ether lipid biosynthesis is caused by the lack of seminolipid (36), which has no vinyl ether bond and thus does not require
PEDS to be synthesized. Interestingly, Tmem189-deficient mice have
decreased bone mineral content (29). Humans with plasmalogen deficiency show a characteristic bone phenotype, rhizomelic chondrodysplasia punctata (10). For unknown reasons, this is observed
mostly in proximal, but not in distal bone elements (34).
Mice with one WT allele contained >90% of plasmalogens of
animals with both WT alleles, although the PEDS activity was
even <50% of WT (Fig. 4). This indicates that PEDS might not
be the rate-limiting step in plasmalogen tissue homeostasis,
which is consistent with the suggestion that fatty acyl CoA reductase 1 controls the plasmalogen biosynthesis rate (37). In
TMEM189-deficient human HAP-1 cells, we found accumulation
of plasmanylethanolamines, the substrates of PEDS. The pattern
of these plasmanylethanolamine species with regard to total
carbon chain length and number of double bonds was strikingly
similar to the pattern of plasmenylethanolamines, the products
of the PEDS reaction, observed in WT cells. This shows that PEDS
apparently does not discriminate between individual plasmanylethanolamine species for desaturation.
Our study provides a paradigm for the usefulness of shared public
resources. High-throughput mRNA sequencing data from the
NCBI GEO datasets for 11 human cell lines and seven mouse tissues allowed us to correlate gene expression with PEDS enzyme
activity data that we had measured in our laboratory and directly led
us to the gene for which we had searched. A mouse deficient in our
gene of interest was available from the European Mutant Mouse
Archive, and a systematic study of its phenotype had already been
performed by the International Mouse Phenotyping Consortium,
allowing us to interpret it on the basis of our biochemical findings
regarding the role of the Tmem189 gene in encoding PEDS. Our
Werner et al.
Materials and Methods
More detailed information is provided in SI Appendix, Materials and Methods.
High-Throughput mRNA Sequencing and Candidate Gene Selection. mRNA sequencing of RAW264.7 cells was carried out using total RNA prepared in our
laboratory. Further processing, including a PolyA enrichment step, was done
by Microsynth. High-throughput mRNA sequencing data for seven mouse
tissues and 11 human cell lines was downloaded from the GEO database
(https://www.ncbi.nlm.nih.gov/gds/), and normalized to the same total read
count. Mouse–human gene comparison was done by two independent methods,
using data from the ensembl server (mmusculus(mm10/GRCm38.p1)) -hsapiens
(hg19/GRCh37.p8, https://www.ensembl.org/) (38), and OrthoRetriever (https://
lighthouse.ucsf.edu/orthoretriever/) (39). The results of the two combined mRNA
seq datasets for 7,382 genes were then ranked according to their mean
Pearson correlation, with PEDS activities calculated separately for the respective cells and tissues. The full dataset is available in SI Appendix. PEDS
activity data were taken from our previous work (22) and are also available as
a separate datafile in SI Appendix.
Cultivation of Cells and Generation of a TMEM189-Deficient Human HAP1 Cell
Line. All cell lines were kept at 37 °C in a humidified atmosphere with 5% CO2
in media recommended by the suppliers containing 10% (vol/vol) FBS (F7524;
Sigma-Aldrich), except that we used media free of antibiotics. Cells were
from American Type Culture Collection. WT and TMEM189-deficient HAP1
cells were from Horizon.
Transfection with Expression Plasmids and siRNAs. Expression plasmids from
Dharmacon or Origine and TurboFect (Thermo Fisher Scientific) or siGenome
Smart pools from Dharmacon and ScreenFect were used. HAP1 cells were
transfected with Turbofectin (Origene). Site-directed mutagenesis was performed with the Quikchange Kit (Stratagene).
Real Time Live Confocal Microscopy. Murine Tmem189 cDNA was cloned into
pEGF-N1 (Clontech), and the TMEM189-GFP fusion protein was expressed in
HEK-293T cells. Real-time confocal imaging was performed at 48 h after
transfection with a spinning-disk confocal system (UltraVIEW VoX; PerkinElmer) connected to a Zeiss AxioObserver Z1 microscope.
Feeding of Cells with 1-O-pyrenedecyl-sn-Glycerol, Cell Harvest, and Lipid
Extraction. These steps were performed as described previously (22). 1-Opyrenedecyl-sn-glycerol was obtained from Otava. Lipids were extracted
from cell pellets twice with 500 μL of chloroform/methanol (2/1 vol/vol), and
the combined organic phases were dried. The dried lipid extract was taken up
in 100 μL of acetonitrile/ethanol (1/1 vol/vol) and stored at −20 °C until analysis.
Downloaded by guest on July 24, 2020
Measurement of PEDS Enzymatic Activity. PEDS activity assays were performed
as described previously (22). The fluorescent 1-O-pyrenedecyl-sn-glycero-3phosphoethanolamine substrate was purified from lipid extracts of 1-Opyrenedecyl-sn-glycerol–treated RAW.12 cells (25) with a protocol comprising
cleavage of residual plasmenyl species by HCl, a first reversed-phase HPLC
purification step, cleavage of the 2-acyl side chains by NaOH, and then a
second HPLC purification step (22). After incubation with NADPH and catalase,
the reaction was stopped with acetonitrile/HCl to liberate pyrenedecanal from
the plasmalogen formed, which was quantified by reversed-phase HPLC and
fluorescence detection. Controls with acetic acid/acetonitrile to quantify non–
plasmalogen-derived pyrenedecanal were always negative.
liberate pyrenedecanal from plasmalogens, and that the aldehyde was
quantified as the resulting pyrenedecanal dimethylacetal.
Measurement of Total Unlabeled Plasmalogen. This step was performed as
described previously (22). Lipid extracts were derivatized with dansylhydrazine (Sigma-Aldrich) in acetonitrile in the presence of either HCl (to
quantify plasmalogens plus free aldehydes) or acetic acid (to quantify free
aldehydes only, which were typically <1% of plasmalogens). The resulting
dansylhydrazones were determined by reversed-phase HPLC with fluorescence detection (22).
LC-MS/MS of Glycerophosphocholines and Glycerophosphoethanolamines. This
step was performed as described previously (40), modified and extended to
also allow quantification and fragmentation of glycerophosphocholines and
glycerophosphoethanolamines. Internal standards were added to cell homogenates, and lipids were extracted via the Folch procedure (41). Dried
lipid extracts were dissolved in HPLC starting condition, separated by
reversed-phase HPLC on a Dionex Ultimate 3000 HPLC (Thermo Fisher Scientific), and quantified with a Velos Pro Dual-Pressure Linear Ion Trap Mass
Spectrometer (Thermo Fisher Scientific). Baseline corrected data were integrated in MZmine 2 (42), quantified, and visualized as described previously
(40) using custom-made scripts in R (https://www.R-project.org/). Molecular
PE, PC, plasmanyl-PE and plasmenyl-PE species were identified by their retention time, monoisotopic mass-to-charge ratio, isotope pattern, and
fragmentation behavior (SI Appendix, Fig. S5 and Materials and Methods),
which was cross-validated with single lipid standards commercially available
from Avanti Polar Lipids: C16-18:1 PC, C18(Plasm)-22:6 PC, C16-18:1 PE,
C18(Plasm)-18:1 PC, C18(Plasm)-18:1 PE, and C18(Plasm)-20:4 PC.
Western Blot Analysis. This analysis was performed using standard techniques
and antibodies against the myc tag (ab 9106, Abcam) and β-actin (MAB1501;
Merck Millipore), Cy3- and Cy5-labeled secondary antibodies (GE Healthcare), and a Typhoon 9410 multi-wavelength laser scanner (GE Healthcare).
Harvest of Mouse Tissues for Analysis of PEDS Activity and Plasmalogen
Content. Animal breeding practices were approved by the Austrian Ministry of Education, Science, and Culture (BMBWF-66.011/0100-V/3b/2019).
Tmem189tm1a(KOMP)Wtsi mice (Welcome Sanger Institute) were maintained on C57bl/6N genetic background. The tg was a knockout-first allele
leading to an inactive truncated TMEM189 protein by artificially splicing a
galactosidase reporter downstream of exon 2 (28). The mice were housed in
individual ventilated cages with nesting material, on a 12-h/12-h light/dark
cycle with standard chow and water ad libitum. Mice were weighed weekly
from 3 to 8 wk of age. For tissue harvest, 8-wk-old female and male homozygous Tmem189-deficient mice and their heterozygous and WT littermates were euthanized by cervical dislocation. Tissues were snap-frozen in
liquid nitrogen and stored at −80 °C until further analysis.
Data Availability. The complete dataset of compiled RNA sequencing of
RAW264.7, seven mouse tissues, and 11 human cell lines and their correlation
to PEDS activity, as well as the PEDS activity data used to calculate the correlation, are available as data files in SI Appendix.
Note Added in Proof. A recent study characterizing a bacterial light response
also found that the TMEM189 gene encodes PEDS (44).
Measurement of Labeled Alkyl and Alk-1′-Enyl Lipids. Pyrene-labeled alkyl and
alk-1′-enyl lipids were quantified by reversed-phase HPLC with fluorescence
detection as described (22), with the modification that methanol rather than
acetonitrile was used together with HCl (or acetic acid as a control) to
ACKNOWLEDGMENTS. We thank the Wellcome Trust Sanger Institute Mouse
Genetics Project (Sanger MPG) and its funders for providing the mutant mouse
line Tmem189tm1a(KOMP)Wtsi and INFRAFRONTIER/EMMA (https://www.
infrafrontier.eu/). Funding information may be found at https://www.sanger.ac.
uk/science/collaboration/mouse-resource-portal and associated primary phenotypic information at https://www.mousephenotype.org/. We thank Sanger Mouse
Pipelines for providing mouse phenotyping data on the Tmem189tm1a(KOMP)
Wtsi colony. We also thank Rita Holzknecht, Petra Loitzl, Nina Madl, Nico Schöpf,
and Nadine Heinrich for excellent technical assistance; and Dr. Chantal Rodgarkia-Dara (THP Medical Products) for valuable suggestions. This work was supported by the Austrian Science Fund (Projects P29551, to E.R.W.; P30800, to K.W.;
and I2738, to J.B.), and the Tiroler Wissenschaftsfonds (UNI-0404/1680, to K.W.).
1. H. Goldfine, The appearance, disappearance and reappearance of plasmalogens in
evolution. Prog. Lipid Res. 49, 493–498 (2010).
2. N. Nagan, R. A. Zoeller, Plasmalogens: Biosynthesis and functions. Prog. Lipid Res. 40,
199–229 (2001).
3. L. Ginsberg, S. Rafique, J. H. Xuereb, S. I. Rapoport, N. L. Gershfeld, Disease and anatomic specificity of ethanolamine plasmalogen deficiency in Alzheimer’s disease
brain. Brain Res. 698, 223–226 (1995).
4. T. Onodera et al., Phosphatidylethanolamine plasmalogen enhances the inhibiting
effect of phosphatidylethanolamine on γ-secretase activity. J. Biochem. 157, 301–309
(2015).
5. X. Q. Su, J. Wang, A. J. Sinclair, Plasmalogens and Alzheimer’s disease: A review. Lipids
Health Dis. 18, 100 (2019).
6. N. E. Braverman, A. B. Moser, Functions of plasmalogen lipids in health and disease.
Biochim. Biophys. Acta 1822, 1442–1452 (2012).
Werner et al.
PNAS | April 7, 2020 | vol. 117 | no. 14 | 7797
BIOCHEMISTRY
work will enable future investigations of specific roles of plasmalogens for mouse physiology by studying the Tmem189-deficient
mouse in more detail with regard to known and assumed roles of
this lipid class.
Downloaded by guest on July 24, 2020
7. F. Dorninger, S. Forss-Petter, J. Berger, From peroxisomal disorders to common neurodegenerative diseases: The role of ether phospholipids in the nervous system. FEBS
Lett. 591, 2761–2788 (2017).
8. M. Rüdiger et al., Plasmalogens effectively reduce the surface tension of surfactantlike phospholipid mixtures. Am. J. Physiol. 274, L143–L148 (1998).
9. R. Wang-Sattler et al., Metabolic profiling reveals distinct variations linked to nicotine
consumption in humans–First results from the KORA study. PLoS One 3, e3863 (2008).
10. H. R. Waterham, S. Ferdinandusse, R. J. Wanders, Human disorders of peroxisome
metabolism and biogenesis. Biochim. Biophys. Acta 1863, 922–933 (2016).
11. T. P. Thai et al., Ether lipid biosynthesis: Isolation and molecular characterization of
human dihydroxyacetonephosphate acyltransferase. FEBS Lett. 420, 205–211 (1997).
12. R. Ofman et al., Acyl-CoA:dihydroxyacetonephosphate acyltransferase: Cloning of the
human cDNA and resolution of the molecular basis in rhizomelic Chondrodysplasia
punctata type 2. Hum. Mol. Genet. 7, 847–853 (1998).
13. E. C. de Vet, A. W. Zomer, G. J. Lahaut, H. van den Bosch, Polymerase chain reactionbased cloning of alkyl-dihydroxyacetonephosphate synthase complementary DNA
from guinea pig liver. J. Biol. Chem. 272, 798–803 (1997).
14. I. J. Lodhi et al., Inhibiting adipose tissue lipogenesis reprograms thermogenesis and
PPARγ activation to decrease diet-induced obesity. Cell Metab. 16, 189–201 (2012).
15. Y. Horibata et al., EPT1 (selenoprotein I) is critical for the neural development and
maintenance of plasmalogen in humans. J. Lipid Res. 59, 1015–1026 (2018).
16. A. Koivuniemi, The biophysical properties of plasmalogens originating from their
unique molecular architecture. FEBS Lett. 591, 2700–2713 (2017).
17. K. Lohner, P. Balgavy, A. Hermetter, F. Paltauf, P. Laggner, Stabilization of nonbilayer structures by the etherlipid ethanolamine plasmalogen. Biochim. Biophys.
Acta 1061, 132–140 (1991).
18. P. E. Glaser, R. W. Gross, Plasmenylethanolamine facilitates rapid membrane fusion: A
stopped-flow kinetic investigation correlating the propensity of a major plasma
membrane constituent to adopt an HII phase with its ability to promote membrane
fusion. Biochemistry 33, 5805–5812 (1994).
19. K. M. Wynalda, R. C. Murphy, Low-concentration ozone reacts with plasmalogen
glycerophosphoethanolamine lipids in lung surfactant. Chem. Res. Toxicol. 23, 108–
117 (2010).
20. F. Snyder, The ether lipid trail: A historical perspective. Biochim. Biophys. Acta 1436,
265–278 (1999).
21. M. L. Blank, F. Snyder, Plasmanylethanolamine delta 1-desaturase. Methods Enzymol.
209, 390–396 (1992).
22. E. R. Werner et al., A novel assay for the introduction of the vinyl ether double bond
into plasmalogens using pyrene-labeled substrates. J. Lipid Res. 59, 901–909 (2018).
23. J. Shanklin, E. Whittle, B. G. Fox, Eight histidine residues are catalytically essential in a
membrane-associated iron enzyme, stearoyl-CoA desaturase, and are conserved in alkane hydroxylase and xylene monooxygenase. Biochemistry 33, 12787–12794 (1994).
24. K. Watschinger et al., Identification of the gene encoding alkylglycerol monooxygenase defines a third class of tetrahydrobiopterin-dependent enzymes. Proc.
Natl. Acad. Sci. U.S.A. 107, 13672–13677 (2010).
7798 | www.pnas.org/cgi/doi/10.1073/pnas.1917461117
25. R. A. Zoeller et al., Mutants in a macrophage-like cell line are defective in plasmalogen biosynthesis, but contain functional peroxisomes. J. Biol. Chem. 267, 8299–8306
(1992).
26. J. Gao et al., FATTY ACID DESATURASE4 of Arabidopsis encodes a protein distinct
from characterized fatty acid desaturases. Plant J. 60, 832–839 (2009).
27. V. A. Blomen et al., Gene essentiality and synthetic lethality in haploid human cells.
Science 350, 1092–1096 (2015).
28. J. K. White et al.; Sanger Institute Mouse Genetics Project, Genome-wide generation
and systematic phenotyping of knockout mice reveals new roles for many genes. Cell
154, 452–464 (2013).
29. N. J. Ingham et al., Mouse screen reveals multiple new genes underlying mouse and
human hearing loss. PLoS Biol. 17, e3000194 (2019).
30. Y. Bai et al., X-ray structure of a mammalian stearoyl-CoA desaturase. Nature 524,
252–256 (2015).
31. H. Wang et al., Crystal structure of human stearoyl-coenzyme A desaturase in complex with substrate. Nat. Struct. Mol. Biol. 22, 581–585 (2015).
32. T. M. Thomson et al., Fusion of the human gene for the polyubiquitination coeffector
UEV1 with Kua, a newly identified gene. Genome Res. 10, 1743–1756 (2000).
33. J. E. Duex, M. R. Mullins, A. Sorkin, Recruitment of Uev1B to Hrs-containing endosomes and its effect on endosomal trafficking. Exp. Cell Res. 316, 2136–2151 (2010).
34. T. F. da Silva, V. F. Sousa, A. R. Malheiro, P. Brites, The importance of etherphospholipids: A view from the perspective of mouse models. Biochim. Biophys.
Acta 1822, 1501–1508 (2012).
35. W.J. Astle, H. Elding, T. Jiang, D. Allen, D. Ruklisa et al., The allelic landscape of human blood cell trait variation and links to common complex disease. Cell 167, 1415–
1429 e19 (2016).
36. C. Rodemer et al., Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice. Hum. Mol. Genet. 12,
1881–1895 (2003).
37. M. Honsho, Y. Fujiki, Plasmalogen homeostasis—Regulation of plasmalogen biosynthesis and its physiological consequence in mammals. FEBS Lett. 591, 2720–2729
(2017).
38. Ensembl Genome Browser, A Comprehensive Source for Genomic Data. https://www.
ensembl.org/. Accessed 28 June 2018.
39. University of California, San Francisco, Orthoretriever, A Tool for Finding Orthologous
Genes. https://lighthouse.ucsf.edu/orthoretriever/. Accessed 28 June 2018.
40. G. Oemer et al., Molecular structural diversity of mitochondrial cardiolipins. Proc.
Natl. Acad. Sci. U.S.A. 115, 4158–4163 (2018).
41. J. Folch, M. Lees, G. H. Sloane Stanley, A simple method for the isolation and purification of total lipides from animal tissues. J. Biol. Chem. 226, 497–509 (1957).
42. T. Pluskal, S. Castillo, A. Villar-Briones, M. Oresic, MZmine 2: Modular framework for
processing, visualizing, and analyzing mass spectrometry-based molecular profile
data. BMC Bioinformatics 11, 395 (2010).
43. A. Marchler-Bauer et al., CDD/SPARCLE: Functional classification of proteins via subfamily domain architectures. Nucleic Acids Res. 45, D200–D203 (2017).
44. A. Gallego-Garcia et al., A bacterial light response reveals an orphan desaturase for
human plasmalogen synthesis. Science 366, 128–132 (2019).
Werner et al.