Biology Reference
In-Depth Information
and transmethylase activities and an RNA-binding protein. The core
particle is surrounded by one or two layers of outer capsid proteins.
During studies of the structural organization of
Rice dwarf virus
(RDV), a member of the genus
Phytoreovirus
in the family
Reoviridae
,
we have identified possible structural mechanisms that allow creation
of a large cavity inside a double-layered spherical particle that con-
sists of heterologous proteins with different lattices at the atomic
level. The viral particle seems to be created in a genetically econom-
ical manner, with the sealing of joints between inner-layer proteins by
a second layer of proteins, suggesting the organization of a rigid
protein layer that is separate from the interior of the virus and prob-
ably protects the interior from the cytoplasmic environment within
infected cells.
The Core Capsid Protein of RDV has
Conformational Properties that Allow
Creation of a Large Cavity Inside the Core
The physical properties of RDV can be summarized as follows. The
molecular mass of the virus is 7.5
10
7
Da, and dsRNA accounts
for approximately 20% of the total mass. In the inner core there are
12 segments of dsRNA, each of which encodes a single protein. The
genome is 25.7 kilo base pairs (kbp) long, with a molecular mass of
1.64
×
10
7
Da. Each viral particle consists of two concentric layers of
proteins. The core of the virus is composed of P3 proteins that encap-
sidate the 12 segments of dsRNA plus the proteins that are required
for transcription. The core is surrounded by the outer capsid layer,
and the major component of this layer is the P8 protein.
Studies of the three-dimensional structure, determined at 3.5 Å
resolution,
2
revealed the mechanism of assembly of RDV, with its
double-layered capsid composed of two kinds of protein. The inner
layer of the capsid is composed of 120 copies of P3 protein. It has
T
×
1 symmetry and, as found in other viruses in
Reoviridae
, it is
composed of two icosahedrally independent P3 subunits, P3A and
P3B (Fig. 1(a)). In this respect it resembles bluetongue virus (BTV)
and orthoreovirus, which are animal reoviruses.
3,4
The outer, second
layer of proteins exhibits T
=
=
13
l
icosahedral symmetry
5,6
and is
