Biology Reference
In-Depth Information
that began 49 nucleotides downstream assembled into VLPs, which
included both small and native-size VLPs (Fig. 3B). Small virions have
not yet been described in clinical stool specimens, which may suggest
that these smaller VLPs were exclusive to insect cell expression.
Capsid Structure
All caliciviruses process a single capsid protein (VP1) with apparent
molecular weights ranging from 59 to 65 kDa. The VP1 is responsible for
assembly, host interactions, and immunogenicity. Capsids that are made
of a single-structure protein are unusual among animal viruses, but are
common among plant viruses. The cryo-EM structure of the Norwalk
virus VLPs was determined in 1994.
27
The Norwalk virus capsid was
found to exhibit a T
=
3 icosahedral symmetry with 180 molecules of the
VP1 organized into 90 dimeric capsomers. The X-ray crystallographic
structure of Norwalk virus capsid was determined in 1999.
28
The
Norwalk virus capsid was found to have two principal domains: a shell
(S) domain and a protrusion (P) domain, the latter of which is further
divided into two subdomains called P1 and P2 (Fig. 4). The S domain is
well conserved among the caliciviruses, whereas the P1 domain is mod-
erately conserved and the P2 domain is highly variable. The P domain is
thought to be exclusively involved in dimeric interactions for all cali-
civiruses; furthermore, the modular S-P1-P2 domain organization of the
capsid protein and the tertiary structure of the P1 subdomain appear to
be conserved in all caliciviruses.
29
Particle assembly is not linked to pro-
tein-RNA interactions because VLP formation occurs without the
genomic RNA. The 3D structure of vesivirus virions (containing RNA),
empty lagovirus VLPs, and empty SaV VLPs have been determined by
cryo-EM to a resolution of ~22 Å.
29-31
In addition, NoV GII VLPs were
determined. Comparative studies found notable surface differences,
though similar general features were also observed. The P2 subdomain
had the most variation. In all caliciviruses, the shell is formed by the S
domain, which is located between radii 100 and 150 Å. However, the
vesivirus contains another an inner contiguous shell (IS) domain, though
this may be associated with RNA. For NoV, the P2 subdomain was
described as a rectangular platform; for SaV, the P2 subdomain was
