Polypyrimidine tract binding protein is required for feline calicivirus replication in a temperature-dependent manner

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. We have now used feline calicivirus (FCV) as a model system to identify and characterise the role of host cell factors that interact with the viral RNA. We demonstrate that polypyrimidine tract-binding protein (PTB) interacts specifically with the 5' sequences of the FCV genomic and subgenomic RNAs. Using RNA interference we demonstrate that PTB is required for efficient FCV replication in a temperature-dependent manner. siRNA-mediated knockdown of PTB resulted in a 15-100 fold reduction in virus titre, as well as a concomitant reduction in viral RNA and protein synthesis at 32°C. In addition, virus-induced cytopathic effect was significantly delayed as a result of a siRNA-mediated reduction in PTB levels. A role for PTB in the calicivirus life cycle was more apparent at temperatures above and below 37°C, fitting with the hypothesis that PTB functions as an RNA chaperone, potentially aiding the folding of RNA into functional structures. This is the first functional demonstration of a host cell protein interacting with a calicivirus RNA.