J. gen. Virol. (I98O), 49, 333-34I
333
Printed in Great Britain
Ebola Virus: Identification o f Virion Structural Proteins
By M I C H A E L P. K I L E Y , R U S S E L L L. R E G N E R Y
AND K A R L M. J O H N S O N
Special Pathogens Branch, Virology Division, Bureau of Laboratories,
Center for Disease Control, Public Health Service,
U.S. Health, Education, and Welfare, Atlanta, Georgia 3o333, U.S.A.
(Accepted 6 March I98O)
SUMMARY
Polyacrylamide gel electrophoresis of purified Ebola virus revealed the presence
of four major virion structural proteins which we have designated VPI, VP2,
VP3 and VP4. Vesicular stomatitis virus (VSV) proteins were used as mol. wt.
markers, and the virion proteins were found to have tool. wt. of 125ooo (VPI),
IO4OOO (VP2), 40000 (VP3) and 26ooo (VP4). VPI was labelled with glucosamine
and is probably a glycoprotein. The density of the Ebola virion was approx.
I'I4 g/ml in potassium tartrate. Virus nucleocapsids with a density of I-32 g/ml
in caesium chloride were released when virions were treated with detergents.
Proteins VP2 and VP3 were consistently associated with released nucleocapsids and
are probably the major structural nucleocapsid proteins analogous to the N
protein of VSV. Protein VP4 was reduced or absent in released nucleocapsids and
is probably analogous to the membrane (M) protein of VSV and similar viruses.
The glycoprotein (VPI) is larger than the glycoprotein of any known negativestrand RNA virus and is not labelled well with 35S-methionine. VPI is solubilized
by detergent treatment, suggesting that it is a component of the virion spikes and
analogous to the G protein of VSV. Our results, in conjunction with analysis of
Ebola virion RNA (Regnery et al. I98O), strongly suggest that the virus is a
negative-strand RNA virus and, along with Marburg virus, may constitute a new
taxon within this group.
INTRODUCTION
Ebola virus, the aetiological agent of an acute haemorrhagic fever of man, was first
isolated in northern Zaire and southern Sudan in I977 (Bowen et al. I977; Johnson et al.
1977). Initial characterization of the agent indicated that its morphology is very similar to,
if not identical with, that of Marburg virus, which was first isolated in I967 (Siegert et aL
t967; Bowen et al. t977; Pattyn et al. t977). Although the two viruses are very similar in
morphology, they do not appear to be related antigenically, at least as measured by the
indirect fluorescent antibody test (Johnson et al. I977; Webb et al. 1978). To characterize
these agents further we have begun a programme to delineate the chemical composition of
the viruses and to establish structure-function relationships of the various virus components.
This information may shed light on the mechanism of virus pathogenesis and aid in the
development of a rapid, safe and sensitive diagnostic test.
The present report concerns the purification and preliminary biochemical characterization of one of the Zaire strains of Ebola virus. These studies were all conducted in the
recently opened Maximum
Containment
Laboratory at the Center
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M.P.
K I L E Y , R. L, R E G N E R Y A N D K. M. J O H N S O N
METHODS
Cell cultures. The centinuous line of green monkey kidney cells (Vero; Yasumura &
Kawatika, 1963) was used to prepare stock virus and for experimental procedures. Cells
were grown in minimal essential medium (MEM) containing I O% heat-inactivated foetal
calf serum (FCS) and were maintained on the same medium containing 20/0 FCS. S W I 3
cells, derived from a human adenocarcinoma, were obtained from the American Type
Culture Collection. They were grown and maintained on the same media used for Vero cells.
Virus source and assay. The Mayinga strain of Ebola virus, which was isolated from a
patient in Zaire, was used throughout. To obtain the experimental pool, virus was passaged
twice in Vero cells and assayed for infectivity in the following manner. Serial Io-fold
dilutions of virus were each inoculated into four test tube cultures of Vero cells. At 7 days
p.i., medium was removed and cells were scraped and fixed on to Teflon-coated slides with
acetone. The presence of virus antigen was determined by the indirect immunofluorescent
technique described previously (Wulff & Lange, 1975). Fifty % endpoints were determined
by the method of Reed & Muench (1938). The titre of the experimental pool was IO8"5
TCIDso/ml.
Plaque assay. Ebola plaque assays were done in SWI 3 cells. Monolayer cultures in 6-well
panels (Falcon) were infected in duplicate with serial Io-fold dilutions of virus. Infection
inoculum was in a vol. of o.I ml. After a 3o min adsorption period, cells were overlaid with
4 ml of Eagle's basal medium (BME) containing 1% FCS and 1% agarose. On day 5,
1"5 ml of Hanks' balanced salt solution containing 1% agarose and I mg/ml neutral red
were added to each well. Pinpoint plaques were seen on day 6 and were easily counted on
day 7.
Infection of cells. Confluent monolayers of Vero cells in 49o cm ~ roller bottles were
infected with a 5 ml suspension of virus in M E M - 2 % FCS. The input m.o.i, was approx.
Io -a TCIDs0/cell, and adsorption was for i h. After adsorption, fresh maintenance medium
was added and cells were incubated at 36 °C.
Isotopic labelling of virus. At an appropriate time after infection, the culture medium was
removed and replaced with medium containing either 35S-methionine, all- or 14C-amino
acid or aH-glucosamine. The medium for 35S-methionine labelling was MEM minus unlabelled methionine, for 3H-amino acid labelling was MEM with I o % of the normal
amino acids and for aH-glucosamine labelling was MEM with one-half the normal glucose
concentration. Normal FCS was replaced with dialysed FCS when radioactive amino acids
were used for labelling. Exact times of labelling are included in figure legends.
Purification of virus. Virus was purified by a modification of the technique described by
Obijeski et al. (I974). At an appropriate time after isotopes were added, culture fluid was
removed and clarified by centrifugation at 65oo g for 5 min at 4 °C. To this clarified solution
NaC1 and polyethylene glycol 6ooo (PEG) were added at the rate of 23 g/1 and 7o g/l,
respectively. This solution was stirred slowly overnight at 4 °C and then centrifuged at
I OOOOg for 3o rain in a Beckman J-2i centrifuge. The resultant pellet was resuspended in
TSE (o.oi M-tris, pH 7"4, o'I5 M-NaC1 and 2 mM-EDTA) and 3 ml amounts were layered
on 13 ml o to 5o ~o (w/w) potassium tartrate (KT) gradients prepared in TE buffer (2 mMtris, pH 7"4, 2 mM-EDTA). After these were centrifuged for ~8 h at i 6 o o o o g and 4 °C in
an SW4o rotor, an opalescent band was found about half way down the gradient, just above
a layer of cellular debris. The virus band was removed, diluted I : I with TSE and layered
on a I3 ml 20 to 7o % (w/w) sucrose gradient. This gradient was centrifuged at i6oooo g
for 2o h at 4 °C in an SW4o rotor. The single visible gradient band was removed, diluted at
least l : to with TSE and the virus pelleted at ~6oooo g for x h at 4 °C. The virus pellet was
resuspended in a small volume of o-ox M-PO4 buffer and stored at - 7 o °C.
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Ebola virion proteins
335
Polyacrylamide gel electrophoresis (PAGE). Virion proteins were analysed by the continuous SDS-phosphate gel system described by Maizel (I 970- Samples of purified labelled
virus were made 2"5 % (w/v) with respect to SDS and 5 % (v/v) with respect to z-mercaptoethanol. Samples were then boiled for 2 rain and sucrose and bromophenol blue were
added to final concentrations of 1o % and o.ool %, respectively. Samples (usually I oo/~1)
were then loaded on 7"5 % cylindrical acrylamide gels containing o.z % SDS and I M-urea.
The electrode buffer was o.I M-sodium phosphate buffer, pH 7"2, containing o.1% SDS and
i M-urea. Electrophoresis was usually at 3"5 mg/gel for 2o h.
After electrophoresis, gels were extruded from the tubes and fixed in 2o % trichloroacetic
acid for 24 h. This solution was decanted and replaced with 7 % acetic acid. At this time the
test tubes containing the gels were sealed and removed from the laboratory through a dunk
tank containing a I o % sodium hypochlorite solution. The gels were then removed from the
test tubes, frozen and sliced into i mm sections with a Mickle gel slicer (Mickle Laboratory
Engineering, Co., Guildford, U.K.). Slices were placed in a vial with 6 ml of a xylenebased counting fluid containing 4 % TS-I solubilizer and were incubated overnight at
37 °C. Radioactivity of each sample was determined with a liquid scintillation counter.
Release of virion internal structure. T o release virus cores, labelled virions were treated
with Triton X-Ioo and I M-KC1 as described by Emerson & Wagner (1972). The volume
of this mixture was adjusted to o'5 ml with TSE and this sample was mixed with 4"5 ml of a
solution containing 5"5 g CsC1 per 15 ml of TSE. These gradients were then centrifuged to
equilibrium for at least 24 h at 15ooo0 g at 4 °C in an SW5o. r rotor. After centrifugation,
gradients were fractionated and acid-insoluble radioactivity of a sample of each fraction
was determined as described previously (Kiley et al. 1974). The density of each fraction was
determined by measuring its refractive index with an Abb6 refractometer.
Reagents. Uniformly labelled L-~4C- or 3H-amino acid mixtures, 3H-uridine (ZO Ci/mmol),
~H-glucosamine (86 mCi/mg) and ~sS-methionine (559 Ci/mmol) were obtained from New
England Nuclear, Boston, Mass., U.S.A. Caesium chloride, density gradient grade, was
obtained from BDH, Poole, U.K. Density gradient grade sucrose was obtained from
Schwarz/Mann, Orangeburg, N.Y., U.S.A. Triton X-too was obtained from Eastman
Chemicals, Rochester, N.Y., U.S.A. and TS-I solubilizer was purchased from Research
Products International, Elk Grove, Ill., U.S.A.
RESULTS
Virus growth and purification
When roller-bottle cultures of Vero cells were infected with Ebola virus at an m.o.i, of
approx, to -3, progeny virus was detected within 24 h. Virus production continued for at
least 9 days with a peak titre of approx. Io s'5 TCID~0/ml being reached on day 4 and
continuing at that level until day 9. Polyethylene glycol precipitation of virus from supernatant fluids resulted in approx, loo-fold concentration of infectivity. To purify Ebola virions
and to determine their density, PEG-precipitated 35S-methionine-labelled virions were
banded in a o to 5o % K T gradient. Electron microscopic examination of this band demonstrated typical Ebola virus morphology as previously described (Murphy et al. I978). The
infectivity in this band, as determined by plaque titration, was ~ io 1° p.f.u./ml. This virus
band was then diluted and centrifuged to equilibrium in a 20 to 70 % sucrose gradient. The
virus was then pelleted and recentrifuged on a o to 5o % K T gradient for density determination. The results of recentrifuging purified virions in a K T gradient are shown in Fig. I. It
can be seen that the maximum peak of radioactivity was recovered at a buoyant density of
about ~. 14 g/ml, approx, the same density at which vesicular stomatitis virus (VSV) was
recovered. Cell supernates from uninfected cells did not produce any bands similar to the
virus bands when carried through this purification procedure.
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M. P. KILEY, R. L. REGNERY AND K. M. JOHNSON
? 31
'
1.20
!
,-,°'!
"~
1 "05
°
ij
..........
1 i
3
5
7
9
11'
13
15
17
Fraction number
Fig. I. Potassium tartrate equilibrium sedimentation analysis of ssS-Ebola and 3H-VSV virions.
Fractions were assayed for TCA-precipitable radioactivity and refractive index. O--O, 3sSmethionine; (3--(3, SH-amino acids; at--A, density.
--
I
I
I
I
I
I
I
I
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90
100
(a)
? 4
o
)<
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B
~g
14
7
-
I
10
I
20
i
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Gel slicenumber
Fig. 2. (a) Polyacrylarnide gel electrophoresis of 3nS-methionine-labelledEbola virus and (b) coelectrophoresis of 3H-amino acid-labelled Ebola ((3--(3) and 3sS-methionine-labelled VSV
(O--O).
Virion polypeptides
To determine the nature o f the Ebola virion structural polypeptides, purified virions
labelled with either a 3H-amino acid mixture, or 85S-methionine were treated with SDS and
mercaptoethanol and analysed on 7"5 % polyacrylamide gels. The electropherogram in
Fig. 2 (a) illustrates the polypeptide pattern observed when virions were purified from the
supernatant fluid of infected cells grown in the presence of ~sS-methionine from day 2 to
day 5 p.i. The profile demonstrates three major polypeptides (fractions 33, 74, and 95) and
two minor polypeptides (fractions io and 82). The minor polypeptides were not consistently
present and may represent aggregates or breakdown products. The purification procedures
have apparently eliminated most host cell proteins since the background is quite low.
Electrophoresis of PEG-pelleted unpurified virus (data not shown) showed significant
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Ebola virion proteins
I
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I
lO s
,.d 5 X 104
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t
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Fig. 3. S e m i - l o g a r i t h m i c plot o f d i s t a n c e m i g r a t e d v e r s u s m o l . wt. t o e s t a b l i s h the mol. wt. o f
E b o l a v i r i o n proteins. V S V p r o t e i n s L, G, N , N S a n d M were u s e d as m o l . wt. m a r k e r s .
radioactivity near the top of the gel and approx. IO peaks of radioactivity elsewhere in the
gel. When virions were grown in the presence of aH-amino acids and co-electrophoresed
with 35S-methionine-labelled VSV the results depicted in Fig. 2(b) were obtained. An
additional polypeptide not seen in asS-labelled virions was evident at fraction 20 in this
figure. We have designated the four major proteins as VP I to VP 4, with VP I being the
slowest migrating protein and VP 4 the fastest migrating protein.
Mol. wt. estimations of virus polypeptides
The approximate tool. wt. of Ebola virion polypeptides was estimated by the procedure
of Weber & Osborn (I969) using VSV proteins as mol. wt. markers. The VSV mol. wt.
used in our calculations were those reported by Obijeski et al. (I974), namely, I6OOOO for
the L protein, 65ooo for the G protein, 54oo0 for the N protein, 42ooo for the NS protein
and 27o0o for the M protein. In our gel system a plot of the logarithm of tool. wt. versus
the relative electrophoretic mobility also yielded a straight line.
Using data similar to that presented in Fig. 2(b) the mol. wt. versus relative migration
plot depicted in Fig. 3 was constructed. With VSV proteins as markers the mol. wt. of the
Ebola proteins were found to be I25OOO (VP~), IO4OOO (VP2), 4ooeo (VP3) and 26o0o
(VP4).
Incorporation of glucosamine
To determine if the virion contained any glycoprotein the virus was grown in the presence
of 3H-glucosamine and co-electrophoresed with Ebola virus labelled with 14C-amino acids.
The result of such an experiment is presented in Fig. 4 and demonstrates that VPI is a
glycoprotein. Since this protein was not labelled in Fig. 2 (a) it appears to be a glycoprotein
containing few or no methionine residues. Further experiments (data not shown) have
indicated that even when VPI is heavily labelled with glucosamine, label cannot be detected
in the other virion proteins.
Release of nucleocapsids from Ebola virions
To obtain more information as to the location of the virion proteins, virions were disrupted with detergent and centrifuged to equilibrium in CsC1. Fig. 5 presents the results of
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338
M. P.
3
K1LEY,
R. L. R E G N E R Y
I
I
I
I
10
20
30
AND
K.
I
M. J O H N S O N
I
I
I
60
70
80
O
X
40
50
Gel slice number
Fig. 4- Co-electrophoresis of purified Ebola virions grown in the presence of either 8H-gLucosamine
( ( 3 - - 0 ) or "C-amino acids ( 0 - - 0 ) .
1-4
~1
1.1 ~
1
3
7
9
11
Fraction number
5
13
15
17
Fig. 5. Caesium chloride equilibrium density gradient analysis of nucleocapsids released by
detergent treatment of a mixture of ~sS-methionine-labelled VSV ( 0 - - 0 ) and 3H-amino acidlabelled Ebola virions ( © - - © ) . A - - A , Density.
10
I
1
I
I
i
I
I
I
l
20
I
i
40
I
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I
I
I
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O
~X 8
~6
24
-
60
G~sliee number
80
1
I
100
+
Fig. 6. Electrophoretic analysis of the polypeptides in the peak fraction (fraction I6) from the CsCl
gradient in Fig. 5- © - - O , 3H-Ebola; 0 - - 0 , nsS-VSV.
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Ebola virion p r o t e i n s
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an experiment in which Ebola virions labelled with 3H-amino acids were mixed with 35Smethionine-labelled VSV treated with Triton X-Ioo-KC1 and then centrifuged to equilibrium in a CsC1 gradient. This treatment released a structure with a density of approx.
1.32 g/ml from both viruses.
To determine the polypeptide composition of these 1.32 g/ml virus nucleocapsid structures an aliquot of the 3H-Ebola-35S-VSV virion mixture depicted in Fig. 2 (b) was detergent
treated and centrifuged to equilibrium in CsC1 as in Fig. 5- Peak fractions from the resultant
1.32 g/ml peak were pooled, concentrated by TCA and electrophoresed. The results
presented in Fig. 6 indicate that the structure contains only the N protein of VSV and the
VP2 and VP3 proteins of Ebola. The nature or significance of the 3H peak in fraction I6
has not yet been determined. It may represent a large protein analogous to the L protein of
VSV or it may be an aggregate. If it were analogous to the L protein of VSV it should not be
present with the nucleocapsids after detergent-high salt treatment (Kiley & Wagner, I972).
DISCUSSION
Because Ebola virus (and Marburg virus) closely resembles the rhabdovirus group in
structure (Murphy et al. I978), it seems appropriate to discuss our findings in relation to
members of this and related virus groups.
We have determined that the Ebola virion contains four major proteins which we have
designated VPI, VP2, VP3 and VP4. Two additional minor peaks of radioactivity appeared
inconsistently and have not been conclusively identified as virion structural proteins. They
may be aggregates or breakdown products. By using VSV virion proteins as markers, the
mol. wt. of the major proteins were found to be I25OOO, to4ooo, 4ooo0 and 26ooo, respectively. The largest protein (VPI) was the only one labelled by glucosamine.
The two major proteins present in the virion are VP2 and VP 3. When virions were
detergent-disrupted, these two proteins consistently remained associated with the presumed
virion nucleocapsid which had a density of t'32 g/ml in CsCI. One or both of these proteins
may be analogous to the N protein of VSV, which is the major virion protein and is the
structural protein that is tightly complexed with virus RNA to form the ribonucleoprotein
(Kang & Prevec, I969; Wagner et al. I969). Alternatively, the presence of two proteins
tightly bound to the virus RNA may be analogous to the situation with paramyxoviruses,
in which the nucleocapsid generally contains a protein that has a mol. wt. of about 60 ooo
but is susceptible to some proteolytic enzymes. Under certain conditions this nucleocapsid
protein is cleaved to form a protein with a mol. wt. of 45ooo (Mountcastle et al. 197o);
thus both cleaved and uncleaved forms may be present in a virus population.
Other studies with paramyxoviruses suggest that a second non-glycosylated proteinmay
be bound to the nucleocapsid and that the avidity of this binding may vary among the
different viruses studied (Mountcastle et al. 1971 ; Stone et al. 1972). Tryptic peptide analysis
of Ebola proteins VP2 and VP3 should determine whether they are two distinct proteins or
they share a precursor-product relationship.
Protein VP 4 may be analogous to the M protein of the rhabdovirus group (Wagner
et al. I972). It is non-glycosylated and has a mol. wt. of approx. 26ooo. When virions are
disrupted by treatment with Triton X Ioo-KC1, this protein is solubilized, which indicates
that it is present in the virion envelope. A similar protein of approx, tool. wt. 4oooo is also
present in the envelope of paramyxoviruses (Scheid & Choppin, I973).
The protein which we have designated VPI is a large glycosylated protein with a mol. wt.
of approx, i25ooo. Since rhabdoviruses and other members of the negative-strand viruses
contain an envelope glycoprotein (Obijeski et al. 1974; Choppin & Compans, 1975 ; Compans
& Choppin, I975; Wagner, 1975), the presence of this glycoprotein in Ebola virus is not
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M.P.
KILEY,
R.
L.
REGNERY
AND
K.
M.
JOHNSON
surprising. By analogy with other members of these virus groups, it is most likely the
structural component (G protein) of the projections found on the surface of the Ebola
virion (Murphy et al. I978). Because Ebola is similar in protein composition to the negativestrand viruses the apparent size of this protein is surprising. The largest rhabdovirus
glycoprotein has a mol. wt. of approx. 78 ooo (Knudson & MacLeod, ~972) and the average
mol. wt. of the rhabdovirus G proteins is about 68oo0 (Wagner, I975). The H N glycoprotein of Newcastle disease virus has a mol. wt. of approx. 74 ooo (Mountcastle et al. 197 I)
and the uncleaved haemagglutinin (HA) glycoprotein of influenza virions may be as large
as 80000 (Skehel & Schild, 197~). Members of the Bunyaviridae, which are probably
negative-strand viruses, do possess a glycoprotein of approx, the same mol. wt. as VPI;
but they also contain another smaller glycoprotein, and the single virion nucleocapsid
protein has a mol. wt. of approx. 25ooo (Obijeski & Murphy, I977). The virions of this
group also contain three species of RNA with an aggregate tool. wt. of approx. 5 x ~on
whereas Ebola virus contains only one RNA species.
The fact that Ebola virions contain only a single negative-strand RNA species (Regnery
et al. I98O) and the proteins described above indicates that Ebola virus is probably a
member of the negative-strand RNA group of viruses and closely resembles the rhabdoviruses in certain morphological aspects and in general protein composition. Whether Ebola
and the structurally similar Marburg virus are charter members of a new taxon or are rather
bizarre members of the Rhabdoviridae family has yet to be determined. We would predict
that although certain fish viruses (McAllister & Wagner, I975) and rabies virus (Sokol et al.
I 9 7 0 are included in the rhabdovirus group despite lack of NS protein and presence of two
M proteins, Ebola and Marburg viruses are sufficiently different both morphologically and
chemically to warrant placement in a new taxon.
We thank Drs J. F. Obijeski and A. P. Kendal for their helpful discussions and Ms Donna
Sasso for her excellent technical assistance. We are particularly grateful to Dr E. Palmer for
performing the negative-contrast electron microscopy used to establish the morphology of
purified Ebola virions.
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