Structural requirements for the specific recognition of an m(7)G mRNA cap

7-Methylguanosine (m(7)G), also known as the mRNA "cap", is used as a molec ular tag in eukaryotic cells to mark the 5' end of messenger RNAs. The mRNA cap is required for several key events in gene expression in which the m(7 )G moiety is specifically recognized by cellular proteins. The configuratio ns of the m(7)G-binding pockets of a cellular (eIF4E) and a viral (VP39) ca p-binding protein have been determined by X-ray crystallography. The bindin g energy has been hypothesized to result from a pi-pi stacking interaction between aromatic residues sandwiching the m(7)G base in addition to hydroge n bonds between the base and acidic protein side chains. To further underst and the structural requirements for the specific recognition of an m(7)G mR NA cap, we determined the effects of amino acid substitutions in eIF4E and VP39 cap-binding sites on their affinity for m(7)GDP. The requirements for residues suggested to pi-pi stack and hydrogen bond with the m(7)G base wer e examined in each protein by measuring their affinities for m(7)GDP by flu orimetry. The results suggest that both eIF4E and VP39 require a complicate d pattern of both orientation and identity of the stacking aromatic residue s to permit the selective binding of m(7)GDP.