The interaction of host-cell nucleic acid-binding proteins with the genomes of
positive-stranded RNA viruses is known to play a role in the translation and
replication of many viruses. To date, however, the characterisation of similar
interactions with the genomes of members of the Caliciviridae family has been
limited to in vitro binding analysis. In this study, feline calicivirus (FCV) and murine
norovirus (MNV) have been used as model systems to identify and characterise the
role of host-cell factors that interact with the viral RNA and RNA structures that
regulate virus replication. It was demonstrated that RNA-binding proteins such as
polypyrimidine tract-binding protein (PTB), poly(C)-binding proteins (PCBPs) and
La protein interact with the extremities of MNV and FCV genomic and subgenomic
RNAs. PTB acted as a negative-regulator in FCV translation and is possibly involved
in the switch between translation and replication during the late stages of the
infection, as PTB is exported from the nucleus to the cytoplasm, where calicivirus
replication takes place. Furthermore, using the MNV reverse-genetics system,
disruption of 5' end stem-loops reduced infectivity ~15-20 fold, while disruption of an
RNA structure that is suspected to be part of the subgenomic RNA synthesis promoter
and an RNA structure at the 3' end completely inhibited virus replication. Restoration
of infectivity by repair mutations in the subgenomic promoter region and the recovery
of viruses that contained repressor mutations within the disrupted structures, in both
the subgenomic promoter region and the 3’ end, confirmed a functional role for these
RNA secondary structures. Overall this study has yielded new insights into the role of
RNA structures and RNA-protein interactions in the calicivirus life cycle.