Biology Reference
In-Depth Information
associated with these events. Currently,
in situ
high-resolution AFM
imaging of virus-infected cells is rather challenging due to the inher-
ited softness of cell membranes, and their subsequent deformation.
However, these problems were successfully overcome using standard
histological procedures (fixing the cells with glutaraldehyde and
osmium tetroxide), which resulted in the preservation of virus and cell
membrane structures.
24
Mouse Leukemic Virus-infected Cells
In vitro
AFM analysis of mouse leukemic virus (MuLV) infected NIH
3T3 cells
24
allowed visualization of the emergence of viruses in all
stages of the budding process. The distribution of sizes of emerging
virions was found to be broad, with an average particle diameter of
145 nm. The surfaces of the virions (Figs. 7a and 7b) were found to
be studded with densely packed “tufts” of protein, having sizes of
11-12 nm.
25
The number of “tufts” on the virion surfaces was esti-
mated to be ~100
20. AFM-based immuno-labeling experiments
25
demonstrated binding of monoclonal antibodies directed against the
SU domain of the envelope protein to these “tufts” on the virion sur-
face. This indicates that the “tufts” are formed by aggregates of the
SU protein. The absence of a regular arrangement of the “tufts” on
the virion surface as revealed by AFM is consistent with the cryo-EM
observed pleiomorhic appearance of MuLV virions.
26
In particular, the
absence of trimeric symmetry in the tufts is rather intriguing, since it
is commonly assumed that the SU protein is organized into threefold
symmetrical trimers.
27
AFM studies of M-MuLV virions lacking the gene for the enve-
lope glycoprotein (env
−
) demonstrated that the surface architectures
of env
−
particles (Fig. 7c ) is profoundly different compared with that
for wild-type virions.
25
In particular, the protein “tufts,” which
appear to be a structural attribute of wild-type virions, are not seen
on the surface of env
−
MuLV virions. Furthermore, it was found
that the surfaces of env
−
virions exhibit less robust mechanical prop-
erties compared to the surfaces of wild-type virions. Thus, the sur-
face architecture of env
−
virions was found to be strongly dependent
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