Analysis of the mechanism of translation initiation complex formation on feline calicivirus mRNA

When viruses infect cells they need to utilise the host cell's protein synthesis machinery to synthesise new viral proteins. Viruses have therefore evolved a number of novel ways to ensure translation (or decoding) of their messenger RNAs (mRNAs) into protein in order to compete with cellular mRNAs. Members of the calicivirus family are responsible for important diseases of man and animals; noroviruses cause gastroenteritis or 'projectile vomiting' in humans and outbreaks are often seen in hospitals, cruise ships and in military camps, while feline calicivirus (FCV) causes cat 'flu. Currently, little is known about what happens when these viruses infect cells, and how new virus particles are made. We have shown that a viral protein attached to the end of the viral mRNAs (VPg) binds to a key cellular protein synthesis factor, eIF4E. This factor usually binds to a structure on cellular mRNAs (termed the 'cap') and helps recruit the protein synthesis machinery (including the ribosome). This suggests that caliciviruses utilise a novel mechanism to recruit ribosomes to their mRNAs, with VPg acting as a 'cap substitute'. We have also demonstrated that other cellular proteins bind to the FCV mRNA itself and may help to recruit ribosomes. We now aim to define further these interactions and the roles they play in virus protein synthesis and replication. This will aid in the development of novel antiviral therapies for this important group of viruses.

Caliciviruses are important pathogens of man and animals. The human caliciviruses (noroviruses and saporoviruses) are the major cause of non-bacterial gastroenteritis in adults worldwide. In animals these viruses are responsible for a wider range of symptoms and may cause enteric, respiratory, haemorrhagic and vesicular diseases. Feline calicivirus (FCV) is a major cause of upper respiratory tract disease in all feline species and is also associated with stomatitis, conjunctivitis and arthritis. The replication strategy of these viruses is currently little understood. We have recently developed a highly reproducible way of isolating large quantities of VPg-linked viral RNA. This RNA is infectious and translates well in the rabbit reticulocyte lysate system. Using this system we have demonstrated that the genome/linked protein VPg binds to the cap-binding initiation factor eIF4E, suggesting VPg acts as a 'cap substitute'. We have also studied the role of other initiation factors and non-canonical trans-acting factors in the mechanism of translation initiation on FCV mRNA. For example, we have shown binding of eIF4A and the cellular proteins polypyrimidine tract-binding protein (PTB) and La to the 5' end of FCV mRNA. We now wish to further study these interactions and define their role in the novel translation mechanisms employed by caliciviruses.