Biology Reference
In-Depth Information
replication complexes associated with intracellular membranes. For
HAstV, it has been suggested that RNA replication is associated to
membranes derived from the endoplasmic reticulum, at the perinu-
clear region, and ultrastructural analysis has revealed the presence of
viral aggregates close to the nuclear periphery surrounded by a high
number of double-membrane vacuoles.
47
Although the exact compo-
sition of these replication complexes is not known, it is likely that all
nonstructural proteins are required and that the temporary regulation
of protein processing of the replicase complex may play a role in the
regulation of the minus- and plus-strand RNA synthesis processes. Of
interest, variability within the protein encoded at the C-terminal
region of ORF1a has been shown to affect the RNA replication pat-
tern, and a certain functional effect of phosphorylation of the protein
has been suggested.
51
Although some nonstructural proteins have
been detected by immunofluorescence within the nucleus of infected
cells,
48
the involvement of the nuclear phase in the replication cycle of
HAstV is not fully understood.
On the other hand, the coordination of genome packaging into
maturing particles has not yet been defined, but the structural deter-
minants for particle assembly seem to be carried out by the structural
proteins alone, since it is possible to make VLPs even without the first
70 amino acids of the capsid polyprotein. The fact that structural
proteins colocalize with nonstructural proteins suggests the possibility
that the membranes might form a scaffold for packaging of the viral
RNA into assembling capsids.
Finally, transient expression experiments showed a direct link
between HAstV ORF1a encoded proteins and apoptosis induction.
28
It seems that astrovirus replication is tightly linked to an apoptotic
host cell response, since cellular caspases seem to be responsible
for the first step of capsid maturation, before the trypsin cleavages
that occur extracellularly.
23
While a premature apoptotic response
usually impairs viral infectivity, when enough virus progeny has been
generated, apoptosis may facilitate the release of virus to bystander
cells, evading the inflammatory response. Kinetic analysis of viral
replication, indicated by the synthesis of structural proteins from the
subgenomic RNA, and onset of apoptosis showed that host cell
