RNA capping: progress and prospects

The 5′ m7GpppN cap, co-discovered by Shatkin, Furuichi, and Moss in 1975, is the signature feature of eukaryal cellular and viral messenger RNA that confers mRNA stability and efficient translation. Cap formation entails three sequential enzymatic modifications targeted to nascent pre-mRNAs synthesized by cellular or viral RNA polymerases. First, the 5′ triphosphate end of the pre-mRNA is hydrolyzed to a diphosphate by RNA 5′ triphosphatase (RTPase). Second, the diphosphate RNA is capped with GMP by RNA guanylyltransferase (GTase) via a two-step mechanism: (i) reaction of GTase with GTP to form a covalent enzyme-(lysyl-Nζ)-GMP intermediate and PPi and (ii) transfer of GMP from GTase to the ppRNA end to form GpppRNA. Third, the GpppRNA cap is converted to m7GpppRNA by AdoMet:RNA(guanine-N7)-methyltransferase (MTase). This pathway was elucidated between 1975 and 1984 via the analysis of the purified vaccinia virus capping enzyme, a heterodimer of 97 kDa and 33 kDa subunits that catalyzes all three steps in cap formation. The same biochemical pathway (though not the same organization of the capping apparatus) is conserved in all eukaryal taxa.