Recognition of mRNA cap structures by viral and cellular proteins.

Most cellular and eukaryotic viral mRNAs have a cap structure at their 5' end that is critical for efficient translation. Cap structures also aid in mRNA transport from nucleus to cytoplasm and, in addition, protect the mRNAs from degradation by 5' exonucleases. Cap function is mediated by cap-binding proteins that play a key role in translational control. Recent structural studies on the cellular cap-binding complex, the eukaryotic translation initiation factor 4E and the vaccinia virus protein 39, suggest that these three evolutionary unrelated cap-binding proteins have evolved a common cap-binding pocket by convergent evolution. In this pocket the positively charged N(7)-methylated guanine ring of the cap structure is stacked between two aromatic amino acids. In this review, the similarities and differences in cap binding by these three different cap-binding proteins are discussed. A comparison with new functional data for another viral cap-binding protein--the polymerase basic protein (PB2) of influenza virus--suggests that a similar cap-binding mechanism has also evolved in influenza virus.