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The surface architecture of isolated MuLV virions (Fig. 7d)
appears to be similar to that seen on the virions budding from the
surfaces of cells (Fig. 7e).
28
A common structural feature of isolated
MuLV virions was a knob-like protrusion, which more likely repre-
sents the “bud scar” resulting from the pinching off of the cell mem-
brane. Partially damaged particles lacking portions of viral envelope
also allowed visualization of intact viral cores with diameters of 65 nm,
and enabled an estimate of the viral shell thickness of 35-40 nm.
NIH 3T3 cells infected with wild-type MuLV virus exhibit a
number of budding viruses on their surfaces (Fig. 7e). In contrast,
cells infected with virus carrying the gPr80gag mutations were found
to be covered with tubular structures (Fig. 7f ).
24
The diameter of
tubular structures was found to be comparable to ones measured for
wild-type MuLV virions. Similarly, the surface features of the tubu-
lar gPr80gag mutation structures appear to be similar to ones seen
on the surfaces of wild-type virions. It was concluded that the
observed tubular structures on the cell surfaces are a result of the
defective morphogenesis of the mutant virions and their subsequent
failure in budding from the cell's surface.
Human Immunodeficiency Virus-infected Cells
The first AFM images of isolated human immunodeficiency virus
(HIV) and HIV-infected lymphocytes in culture have been recently
reported by Kuznetsov
et al
.
29
(Fig. 8a). It was demonstrated that
HIV virions are roughly spherical and have a pleomorphic exterior
with a dense packing of protruding protein units. The average diam-
eter of virions was found to be about 120
25 nm, which is ~ 10%
smaller than that determined from cryo-EM.
30
±
Similar particles
30 nm were observed on
infected cell surfaces. Morphologies of these particles varied more
than they do for free virions, which is presumably so because the cell
surface virions are in various stages of the budding process.
The ~ 20 nm tufts visualized on the surface of HIV particles repre-
sent aggregates of the receptor-binding protein gp120, which is com-
monly assumed to be a symmetrical trimer.
31
However, variations in size
(Figs. 8b and 8c) having diameters of 127
±
