mRNA ageing shapes the Cap2 methylome in mammalian mRNA

The mRNA cap structure is a major site of dynamic mRNA methylation. mRNA caps exist in either the Cap1 or Cap2 form, depending on the presence of 2'-O-methylation on the first transcribed nucleotide or both the first and second transcribed nucleotides, respectively^1,2. However, the identity of Cap2-containing mRNAs and the function of Cap2 are unclear. Here we describe CLAM-Cap-seq, a method for transcriptome-wide mapping and quantification of Cap2. We find that unlike other epitranscriptomic modifications, Cap2 can occur on all mRNAs. Cap2 is formed through a slow continuous conversion of mRNAs from Cap1 to Cap2 as mRNAs age in the cytosol. As a result, Cap2 is enriched on long-lived mRNAs. Large increases in the abundance of Cap1 leads to activation of RIG-I, especially in conditions in which expression of RIG-I is increased. The methylation of Cap1 to Cap2 markedly reduces the ability of RNAs to bind to and activate RIG-I. The slow methylation rate of Cap2 allows Cap2 to accumulate on host mRNAs, yet ensures that low levels of Cap2 occur on newly expressed viral RNAs. Overall, these results reveal an immunostimulatory role for Cap1, and that Cap2 functions to reduce activation of the innate immune response.