Unlike other positive-stranded RNA viruses that use either a 5'-cap structure or an internal ribosome entry site to direct translation of their messenger RNA, calicivirus translation is dependent on the presence of a protein covalently linked to the 5' end of the viral genome (VPg). We have shown a direct interaction of the calicivirus VPg with the cap-binding protein eIF 4 E. This interaction is required for calicivirus mRNA translation, as sequestration of eIF 4 E by 4 E-BP 1 inhibits translation. Functional analysis has shown that VPg does not interfere with the interaction between eIF 4 E and the cap structure or 4 E-BP 1, suggesting that VPg binds to eIF 4 E at a different site from both cap and 4 E-BP 1. This work lends support to the idea that calicivirus VPg acts as a novel 'cap substitute' during initiation of translation on virus mRNA.