Biology Reference
In-Depth Information
boundary of the inner shell (112 Å) of MGNNV and PaV are nearly
coincident, and this defines the interior limit of the
-sandwich
domain of PaV. Density with a radius of less than 112 Å is probably
predominately RNA. The outer shell of MGNNV is composed mainly
of the large protrusions located at the quasi-threefold axes. These
protrusions are much more prominent than those of PaV. The inner
shell is relatively uniform, and the protrusions are separated from it by
a void of nearly 12 Å, indicating that the connection between the
domains is probably a single, extended polypeptide chain that is not
visible in the cryoEM map. The crystal structures of PaV and Flock
house virus showed that the protrusions at the highest radius are
formed by three two-stranded
β
β
-sheets related to each other by quasi-
-sheets are formed by insertions between
strands of the canonical viral
threefold symmetry. These
β
-sandwich that forms the contiguous
protein shell of the insect nodaviruses, and the strands are twisted
together about the quasi-threefold axes to form the surface protru-
sions. However, the inner and outer regions of the insect nodaviruses
display continuous density in contrast to the density gap between the
outer and inner shells of protein in MGNNV. Superposition of the
cryoEM map of MGNNV with the atomic model of PaV derived from
the crystal structure showed relatively good agreement for the region
occupied by the contiguous
β
-sandwich shell in PaV. However, there
was poor agreement in the outer radial region. The protrusions at the
quasi-threefold axes in MGNNV have much larger volumes than
could be accounted for by the three twisted
β
-sheets at the surface of
the PaV structure. Thus, these protrusions must contain more protein
that may form individual domains. Moreover, the PaV model could
not account for differences in density observed in the MGNNV
reconstruction at the icosahedral and quasi-twofold symmetry axes.
Significant density exists at the icosahedral two-fold axes between
protrusions. However, there is only weak density at the correspon-
ding regions between protrusions related by the quasi-twofold sym-
metry. This density difference implies different patterns of contacts
between protrusions at icosahedral and quasi-twofold axes. Both the
cryoEM reconstruction and the crystal structure of PaV revealed that
an ordered portion of the viral RNA forms a dodecahedral cage
β
