Capture of eukaryotic mRNA cap structures by the coat protein of the yeast L-A dsRNA virus

The eukaryotic mRNA 5$ sp prime$ cap structure m$ sp7$GpppX (where X is any nucleotide) is of crucial importance in translation initiation and protects messenger RNAs (mRNAs) against exonucleolytic degradation. L-A and L-BC are double-stranded RNA viruses which are persistently maintained in the cytoplasm of many Saccharomyces cerevisiae strains. Unexpectedly, their coat proteins exhibited a strong cap-binding activity. Unlike any other cap-binding protein, the coat protein (gag) of the L-A virus was shown to attach covalently to the cap structure of mRNAs, resulting in the cleavage of the cap structure and in the capture of m$ sp7$Gp. The linkage was determined to be a phosphoroimidazole bond between the $ alpha$ phosphate of the cap structure and a nitrogen ($N sp1$ or $N sp3$) in the gag His154 imidazole side-chain. Mutation of His154 abrogated the ability of gag to covalently link to the cap structure, without affecting cap recognition, viral particle formation from an L-A cDNA clone in vivo, or specific binding and replication of (+) single-stranded RNA in vitro. However, genetic analysis demonstrated that His154 was essential for the expression of viral proteins, possibly because efficient translation of the viral mRNA requires capping of the initially ppG-terminated viral transcripts by transfer of the m$ sp7$Gp captured by gag