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(Fig. 2). Wang
et al
. further subdivided region II into three regions
with different degrees of similarity.
36
Consistent with the capsid
polyprotein domain organization, studies using antibodies revealed
that VP34 would express conserved epitopes shared by all serotypes,
while VP29 and VP26 contained the serotype-specific neutralizing
epitopes.
24,37,38
Thus, it is reasonable to hypothesize that the hyper-
variable C-terminal region would be located on the surface of the viral
particle contributing to the strain-specific tropism of the virus, while
the conserved region would constitute the “assembly domain,” a
building block for capsid formation and encapsidation of the viral
genome.
27,35
Interestingly, during the construction of the HAstV-1
infectious clone it was found that amino acid residue Thr
227,
within
region I of the capsid polyprotein, was essential for proper assembly
of viral capsids.
19
Finally, within the first 70 residues of the ORF2 polyprotein,
region I includes a well-conserved domain rich in basic amino acids,
which has been associated with the viral RNA packaging process due
to its potential RNA-binding properties and its similarities to other
well-documented icosahedral RNA viruses, such as Sindbis virus and
some plant viruses.
39-41
Consistent with this observation, it has been
shown that the first 70 amino acid residues of HAstV-1 are dispensa-
ble for virus-like particle formation using a recombinant baculovirus
expression system.
27
Jonassen
et al
. also related this arginine-rich
region to gene expression regulation processes in other viruses such as
coronaviruses, papillomavirus and baculovirus.
59
Detailed ultrastructure studies of trypsin-treated HAstV-2 virions
revealed icosahedral particles with an array of spikes protruding from
the surface of the virion.
42
A low-resolution cryoelectron microscopy
image for trypsin-treated HAstV-1 particles showed a solid icosahedral
capsid shell 330 Å in diameter and decorated with 30 dimeric spikes
extending 50 Å from the surface.
4
Using the three-dimensional position-specific scoring matrix (3D-
PSSM)
43
to identify recognizable protein-folding motifs within the
astrovirus capsid proteins, meaningful structural predictions are cur-
rently providing insights into different functional aspects of the viral
