Biology Reference
In-Depth Information
the anchorage of the RNA replication complex to intracellular mem-
branes. Based on the software predictions on the orientation of the
TM helices, it seems likely that the N-terminal region of the helices in
nsP1a is on the cytosolic side of a membrane. Sequence analysis has
also predicted a KKXX-like endoplasmic retention (ER) signal that
suggests an association between the viral RNA replication process and
ER-derived intracellular membranes.
47
In addition, computer predic-
tion led to the identification of a bipartite nuclear localization signal
(NLS).
11
Although some nsP1a-derived products have been shown to
accumulate in the nuclei of infected cells,
48
the significance of a
nuclear involvement in an RNA virus lifecycle is yet to be understood.
Recently, two common motifs for astrovirus and pestivirus RNA heli-
case (GKT and VVIT) have been proposed upstream from the pro-
tease coding region. These motifs would represent motifs I and II of
the seven conserved motifs of SF2 helicases and would be function-
ally important in ATP binding.
17
The presence of an immunoreactive epitope (IRE) close to the
C-terminus of nsP1a was partially characterized some years ago, indi-
cating the high immunogenicity of this region.
49
In addition, a hyper-
variable region (HVR) has been identified close to this epitope by
different authors following genetic characterization of HAstV-1,
HAstV-3, HAstV-4, and HAstV-8.
9,13,47,50
Variability within this HVR
has been associated with different viral RNA replication and growth
properties, as well as with different virus RNA levels in feces from chil-
dren with gastroenteritis, suggesting a relationship between certain
genotypes and some viral properties related to its pathogenic pheno-
type.
51
Variability within this region consists mainly of high rates of
nucleotide and amino acid substitutions, as well as many insertions and
deletions that retain the reading frame. Interestingly, a 15-amino acid
deletion identified some years ago was related to adaptation of HAstV
to certain cell lines,
50
and recently this region has been associated
with a distinctive RNA replication pattern. Although the molecular
mechanisms that regulate the efficient minus and plus RNA strands,
including the synthesis of a subgenomic RNA, remain unclear, muta-
genesis studies have demonstrated that genetic variability of the
C-terminal of nsP1a affects the virus RNA replication phenotype.
51
Additionally, using antibodies against the HVR, it has been shown
