Biology Reference
In-Depth Information
Thus, RDV produces two types of P3 capsid protein that bind
tightly to each other and form dimmers, which are the building blocks
of the core particle. It seems that only one gene (for P3) is required
for the synthesis of the two types of molecule that are needed for cre-
ation of the large cavity. This system is rather economical when
viewed from a genetic perspective. It will be of interest to examine the
events that are responsible for the post-translational discrimination
between the two types of P3 molecule in the dimer.
Structural Mechanism that Creates Symmetry
Mismatch Between the Inner and Outer Layers and
Results in the T
=
13
l
Structure of the Outer Layer
The next task in the formation of RDV is the construction of the
second layer of the capsid. The double-layered capsid of RDV has
different icosahedral lattice symmetries in the inner (T
=
1) and
13
l
) capsid. In the outer capsid, five icosahedrally unique
P8-trimers (designated P-, Q-, R-, S- and T-trimers) are located in dif-
ferent environments on the surface of the inner core particle (Fig. 1).
The concept of T number has provided an index for the display of
quasi-equivalent lattice systems of viral capsid structures.
14
The struc-
ture described above seems to be common to viruses in
Reoviridae
,
such as rotavirus (Prasad
et al
., 1996),
15
BTV
3
and RDV.
As described above, there are two kinds of interaction in the cap-
sid: one type between homologous proteins (P3-P3 and P8-P8) in the
intra-layer, with lateral interactions; and one type between heterolo-
gous proteins (P3-P8) in the inter-layer, with vertical interactions.
Both types of interaction are necessary for the formation of the double-
layered capsid.
When studies of the atomic structure of RDV focused on the bind-
ing between heterologous proteins P3 and P8, the strongest interactions
between the P8-trimer and P3 proteins were found in the three-fold
symmetry region of the viral core. Total energies of interaction were
53.7, 110.3, 68.5, 49.3 and 153.3 kcal/mol for the P-, Q-, R-, S- and
T- trimers, respectively (Fig. 1(b)). This result supported the obser-
vation, made by cryo-electron microscopy,
16
that only the T-trimers
remain on the surface of the core after treatment of intact particles
outer (T
=
