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J. Virol., Jul 1997, 5197-5208, Vol 71, No. 7
A Pelletier, F Do, JJ Brisebois, L Lagace and MG Cordingley
The ordered copolymerization of viral proteins to form the herpes simplex
virus (HSV) capsid occurs within the nucleus of the infected cell and is a
complex process involving the products of at least six viral genes. In
common with capsid assembly in double-stranded DNA bacteriophages, HSV
capsid assembly proceeds via the assembly of an outer capsid shell around
an interior scaffold. This capsid intermediate matures through loss of the
scaffold and packaging of the viral genomic DNA. The interior of the HSV
capsid intermediate contains the viral protease and assembly protein which
compose the scaffold. Proteolytic processing of these proteins is essential
for and accompanies capsid maturation. The assembly protein (ICP35) is the
primary component of the scaffold, and previous studies have demonstrated
it to be capable of intermolecular association with itself and with the
major capsid protein, VP5. We have defined structural elements within ICP35
which are responsible for intermolecular self- association and for
interaction with VP5. Yeast (Saccharomyces cerevisiae) two-hybrid assays
and far-Western studies with purified recombinant ICP35 mapped a core
self-association domain between Ser165 and His219. Site-directed mutations
in this domain implicate a putative coiled coil in ICP35 self-association.
This coiled-coil motif is highly conserved within the assembly proteins of
other alpha herpesviruses. In the two-hybrid assay the core
self-association domain was sufficient to mediate stable self-association
only in the presence of additional structural elements in either N- or
C-terminal flanking regions. These regions also contain conserved sequences
which exhibit a high propensity for alpha helicity and may contribute to
self-association by forming additional short coiled coils. Our data
supports a model in which ICP35 molecules have an extended conformation and
associate in parallel orientation through homomeric coiled-coil
interactions. In additional two-hybrid experiments we evaluated ICP35
mutants for association with VP5. We discovered that in addition to the
C-terminal 25 amino acids of ICP35, previously shown to be required for VP5
binding, an additional upstream region was required. This region is between
Ser165 and His234 and contains the core self-association domain.
Site-directed mutations and construction of chimeric molecules in which the
self-association domain of ICP35 was replaced by the GCN4 leucine zipper
indicated that this region contributes to VP5 binding through mediating
self-association of ICP35 and not through direct binding interactions. Our
results suggest that self-association of ICP35 strongly promotes stable
association with VP5 in vivo and are consistent with capsid formation
proceeding via formation of stable subassemblies of ICP35 and VP5 which
subsequently assemble into capsid intermediates in the nucleus.
Copyright © 1997, American Society for Microbiology
Self-association of herpes simplex virus type 1 ICP35 is via coiled- coil interactions and promotes stable interaction with the major capsid protein
Department of Biological Sciences, Bio-Mega Research Division, Boehringer Ingelheim, (Canada) Ltd., Laval, Quebec.
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