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Fig. 2.
(
a
) Turnip yellow mosaic virus (TYMV) particles embedded in the
surface of a virus crystal exhibit an icosahedral array of capsomeres. (
b
) Brome
mosaic virus (BMV) crystal. Individual virions show capsomeres with a depres-
sion in their center (
arrow
).
T
=
3 Icosahedral Viruses
In work on turnip yellow mosaic virus (TYMV), illustrated in Fig. 2a,
it was demonstrated for the first time that AFM allowed the struc-
ture of a virion to be visualized to such high resolution that indi-
vidual protein capsomeres on the virion surface could be clearly
resolved.
8
The capsid of the T
3 icosahedral TYMV virion is 28 nm in
diameter and is composed of 180 identical protein subunits, organ-
ized into 12 pentameric and 20 hexameric capsomeres.
9
In AFM
images, pentameric and hexameric clusters, which are both roughly
60 Å across, are clearly discriminated. The difference between the
highest and lowest points on the capsid surface, about 45 Å,
9
was
accurately reflected by AFM measurements. Furthermore, it was
demonstrated that viruses of different, but closely related, virus fami-
lies could be discriminated from each other by AFM on the basis of
differences in capsid structure.
6
Thus, while both are 28 nm diameter
T
=
3 icosahedral viruses, comparison of the capsomere structures of
TYMV (
Tymovirus
=
family) and BMV (brome mosaic virus,
