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Journal of Virology, December 1998, p. 10126-10137, Vol. 72, No. 12
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
The Vaccinia Virus 14-Kilodalton (A27L) Fusion
Protein Forms a Triple Coiled-Coil Structure and Interacts with the
21-Kilodalton (A17L) Virus Membrane Protein through a
C-Terminal
-Helix
María-Isabel
Vázquez,1
German
Rivas,2
David
Cregut,3
Luis
Serrano,3 and
Mariano
Esteban1,*
Centro Nacional de Biotecnología,
CSIC, Campus Universidad Autónoma, 28049 Madrid,1 and
Centro de Investigaciones
Biológicas, 28006 Madrid,2 Spain, and
European Molecular Biology Laboratory, 69012 Heidelberg,
Germany3
Received 21 July 1998/Accepted 9 September 1998
The vaccinia virus 14-kDa protein (encoded by the A27L gene) plays
an important role in the biology of the virus, acting in virus-to-cell
and cell-to-cell fusions. The protein is located on the surface of the
intracellular mature virus form and is essential for both the release
of extracellular enveloped virus from the cells and virus spread.
Sequence analysis predicts the existence of four regions in this
protein: a structureless region from amino acids 1 to 28, a helical
region from residues 29 to 37, a triple coiled-coil helical region from
residues 44 to 72, and a Leu zipper motif at the C terminus. Circular
dichroism spectroscopy, analytical ultracentrifugation, and chemical
cross-linking studies of the purified wild-type protein and several
mutant forms, lacking one or more of the above regions or with point
mutations, support the above-described structural division of the
14-kDa protein. The two contiguous cysteine residues at positions 71 and 72 are not responsible for the formation of 14-kDa protein trimers.
The location of hydrophobic residues at the a and d positions on a helical wheel and of charged amino acids in adjacent positions, e and
g, suggests that the hydrophobic and ionic interactions in the triple
coiled-coil helical region are involved in oligomer formation. This
conjecture was supported by the construction of a three-helix bundle
model and molecular dynamics. Binding assays with purified proteins
expressed in Escherichia coli and cytoplasmic extracts from
cells infected with a virus that does not produce the 14-kDa protein
during infection (VVindA27L) show that the 21-kDa protein (encoded by
the A17L gene) is the specific viral binding partner and identify the
putative Leu zipper, the predicted third
-helix on the C terminus of
the 14-kDa protein, as the region involved in protein binding. These
findings were confirmed in vivo, following transfection of animal cells
with plasmid vectors expressing mutant forms of the 14-kDa protein and
infected with VVindA27L. We find the structural organization of 14kDa
to be similar to that of other fusion proteins, such as hemagglutinin of influenza virus and gp41 of human immunodeficiency virus, except for
the presence of a protein-anchoring domain instead of a transmembrane domain. Based on our observations, we have established a structural model of the 14-kDa protein.
*
Corresponding author. Mailing address: Centro Nacional
de Biotecnología, CSIC, Campus Universidad Autónoma,
28049 Madrid, Spain. Phone: 34-91-585-4503. Fax: 34-91-585-4506. E-mail: mesteban{at}cnb.uam.es.
Journal of Virology, December 1998, p. 10126-10137, Vol. 72, No. 12
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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