Biology Reference
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high-resolution EM-reconstruction analysis. This was demonstrated
in AFM studies of structure and assembly of intracellular mature vac-
cinia (IMV) virus.
7
Vaccinia virus, a homologue of smallpox and the
basis of the smallpox vaccine, is one of the largest viruses to replicate
in humans. Among the human-tropic viruses, vaccinia has one of the
most structurally complex, markedly asymmetric virions. The struc-
ture of the vaccinia virion has been extensively studied for several
decades by conventional EM.
17-19
The virion was initially described as
brick-shaped with dimensions of approximately 300
120 nm,
and with inner and outer membranes sandwiching a pair of lateral
bodies and enclosing a central core containing the genomic DNA.
17
Conventional EM studies revealed membranous surface tubules and
a dumbbell shape of the core. These structural attributes were not
visible in cryo-EM images,
20
which suggested that these features may
have been artifacts of virion dehydration and non-isotropic collapse
during conventional EM sample preparation. Thus, because of these
inconsistencies, the combination of conventional and cryo-EM did
not provide a definitive model for the structure of the vaccinia virion.
AFM images of fully hydrated vaccinia virions (Fig. 1d) demon-
strate that the majority of virions have an ellipsoidal shape with
major and minor axial dimensions in the range 320-380 nm and
260-340 nm, respectively, and a virion height of 240-290 nm. While
the lateral dimensions derived from AFM images are consistent with
those from cryo-EM studies, the AFM virion height is approxi-
mately double that of the cryo-SEM- and EM-estimated heights of
110 nm and 150 nm, respectively (Griffiths
et al
., 1994).
21
Note that
while height information from AFM is very precise, with a resolution
approaching 0.1 nm, estimation of vertical dimensions by EM tech-
niques is not straightforward and depends on the stage tilt angle.
Fully hydrated IMV particles, as revealed by AFM, have a “rounded
barrel” shape, which differs from the classical “brick-shaped” mor-
phologies for pox viruses described in the literature.
17
It is expected that due to its thickness and electron-translucent
properties, visualization of the high-resolution surface morphology of
the vaccinia virion by cryo-EM becomes difficult, since EM images
provide a projection of the entire thickness of the sample. Indeed,
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