Download Structural comparison of three viral fusion proteins

31 25 Biochemical SocietyTransactions ( 1 99 1 ) 19
Structural comparison of three viral "fusion"
proteins
BRUCE H NICHOLSON and MAHMOUD NAASE
Department of Biochemistry & Physiology, AMS
Building, University of Reading, Whiteknights,
P 0 Box 228, Reading RG6 2AJ
Recently the role for the 14 kD protein of
vaccinia has become more clearly defined [l].
Originally thought to be concerned in the
process whereby viral particles are attached
to the target cell it has subsequently been
shown to assist in the passage outwards from
the cell, where it is essential for the
envelopment of intracellular virions by the
fusion of the outermost of the two Golgi
derived membranes with the plasma-membrane,
and release of mature extracellular viral
particles.
Homologues of the vaccinia protein have
been found in capripox [2] and in orf, a
parapox virus [3]. The degree of homology,
when determined by amino acid identity, is
low. Depending on the number of gaps
introduced, it ranges from 26-39% when orf is
compared to either of the other two viruses.
In addition, the proteins are widely divergent
in length, being 148 residues (capripox) 110
residues (vaccinia) and 89 (orf). The
alignment of the sequences to demonstrate
homology can be tested independently using the
core prediction method [4]. Thus for example,
despite the wide disparity in length of
sequences, of the total of 70 core residues
present in the three sequences, over 50% align
with core residues in one or both of the other
sequences.
The conservation of core residues, that is
to say those residues in contact with three
other hydrophobic residues (or two in the case
of ILE or LEU) [ 5 ] is indicative of a
conserved tertiary structure, since the core,
as defined, is a tertiary feature. Thus the
homologous proteins will generally have a
similar tertiary structure, so that the
residues forming the core would be in similar
positions. This is so even though the rules
which predict the core/non-core status may be
different in each case, due to either the core
residue being different, and/or the sequence
either side which forms the basis for the
prediction.
The core volume is lower in the orf
protein, and to a lesser extent the vaccinia
protein, compared to the capripox protein.
This is partly due to the large deletions, but
the numbers of core predicted residues for the
regions where all three sequences overlap are
14 (orf) 18 (vaccinia) and 24 (capripox).
This probably reflects the increasing
difficulty in shielding the core in smaller
molecules. The data base for predictive
algorithms is drawn almost entirely from
globular proteins, and although this may be of
significance in considering membrane bound
proteins as in the present case, it is
noteworthy that integral membrane proteins
show strong helix formation. The combined
secondary structure methods (Leeds suite)
predict essentially four alpha helices for
both capripox and vaccinia. These might be
expected to adopt either a crossed helix
pattern or an antiparallel helix bundle, with
some accommodation for membrane attachment.
In orf only the C-terminal helix remains
intact, and the rest of the structure is
Schematic structure of the homologous
proteins
The solid line represents the predicted
structure of the largest protein (capripox,
148 residues), with side chains of core
residues (
).
The regions deleted are
---- , orf).
shown bypassed (...., vaccinia;
Regions of high antigenic propensity are shown
dotted (capripox), cross-hatched ( / / / ,
vaccinia; \ \ \ , orf) or in combination where
overlapping.
.
drastically modified. A hydrophobic region in
the helix nearest the C-terminal shows a
relatively high (compared to the rest of the
sequence) degree of conservation between the
three species. Leading into this helix are
the two adjacent cysteines (one substituted in
capripox) thought to be essential in the
vaccinia protein, possibly in formation of
trimers. It would seem likely that this
region is exposed, and that the area of
membrane attachment is relatively small,
leaving the rest of the molecule free to
function.
All three sequences were scanned [6] for
potential B-cell epitopes, since the 14 kD
'fusion' protein of vaccinia was first
detected by its reactivity to monoclonal
antibodies produced against vaccinia virus
virions [ 7 ] and polyclonal antibodies raised
in sheep exposed to orf (the gift of Dr Hugh
Reid, Moredun Research Institute, Edinburgh)
cross-reacted with the orf protein [Naase et
al, unpublished observation]. The regions of
antigenic propensity are situated in the
connecting sequences between helices, and at
the N 6 C terminal sequences. The second
helix (deleted in orf) showed a strong
propensity in both vaccinia (res 20-33, 37-41)
and capripox (res 40-44, 51-55) and in general
for all three proteins the highest antigenic
propensity occurs in the N-terminal half of
the molecules.
1. Rodriguez, J.F. & Smith, G.L. (1990)
Nucleic Acids Research 18, 5347-5351.
2. Gershon, P.D., Ansell, D.M. & Black, D.N.
(1989) J. Virol. 63, 4703-4708.
3. Naase, M. & Nicholson, B.H. (1991) J. Gen.
Virol. (in press).
4. Nicholson, B.H. (1982) Biochem. SOC.
Trans. 10, 386-388.
5. Crampin, J., Nicholson, B.H. & Robson, B.
(1978) Nature 272, 558-560.
6. Nicholson, B.H. (1989) Biochem. SOC.
Trans. 17, 398-399.
7. Rodriguez, J.F., Janeczko, R. and Esteban,
M. (1985) J. Virol. 56, 482-488.