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Paramecium Bursaria Chlorella Virus Type 1
High-resolution AFM imaging of Paramecium bursaria chlorella
virus type 1 (PBCV-1) allowed visualization of the virion's surface
lattice, as illustrated in Fig. 3d. 12 The icosahedral PBCV-1 can be
deconstructed into 20 triangular and 12 pentagonal plates known as
trisymmetrons and pentasymmetrons, 13 which are centered around
threefold and fivefold symmetry axes, respectively. Higher-resolution
images, as seen in Fig. 3e, reveal the individual trimeric capsid pro-
teins that form a honeycomb structure, as well as the pentagonal
vertices (Fig. 3f ). These vertices clearly exhibit a single apical glob-
ular protein in their centers and five other unique globular proteins
that together form an exact pentamer. These structures are consistent
with the model of the PBCV-1 virion as derived from cryo-EM. 13 In
addition, AFM studies provide detailed structural information on
both the arrangements of trimeric capsid proteins and the pentagonal
vertices. Thus, from AFM images, the small hole in the trimer center
has a distinctive triangular shape that is more accentuated and angu-
lar than the “doughnut” shape deduced from cryo-EM. 14
In the cryo-EM model a hole was assigned to the pentagonal
apex, while AFM images clearly demonstrate the presence of a unique
protein in its center (Fig. 4a). The arrangement of a trisymmetron
lattice appears not to be affected by regular AFM scanning force.
However, purposely higher AFM tip pressure was found to press
the apical protein into the vertex, leaving in its place a distinct hole
(Fig. 4b). The central protein reappears in the vertex upon decrease of
the tip pressure to its original value. This demonstrates the apparent
lesser integration of the apex protein in the lattice structure, resulting
in the ability to move freely perpendicular to the virion surface.
Ty3 Retrotransposon Virus-like Particles
AFM was successfully utilized for the first determination of the
molecular structure of the Ty3 retrotransposon, virus-like particles
(VLPs) from Saccharomyces cerevisiae . 15 Transmission EM 16 pro-
vided the information on the size range of Ty3; however, no detailed
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