His-154 Is Involved in the Linkage of the Saccharomyces cerevisiae L-A Double-Stranded RNA Virus Gag Protein to the Cap Structure of mRNAs and Is Essential for M₁ Satellite Virus Expression

Abstract

The coat protein (Gag) of the double-stranded RNA virus L-A was previously shown to form a covalent bond with the cap structure of eukaryotic mRNAs. Here, we identify the linkage as a phosphoroimidazole bond between the alpha phosphate of the cap structure and a nitrogen in the Gag protein His-154 imidazole side chain. Mutations of His-154 abrogate the ability of Gag to bind to the cap structure, without affecting cap recognition, in vivo virus particle formation from an L-A cDNA clone, or in vitro specific binding and replication of plus-stranded single-stranded RNA. However, genetic analyses demonstrate that His-154 is essential for M₁ satellite virus expression.