CMTR1 directs mitochondrial dynamics during T cell activation through epitranscriptomic regulation of splice isoforms

During T cell activation, mitochondrial biogenesis and cellular metabolism are altered to meet the elevated energy demands of protein synthesis, rapid proliferation, and effector T cell function. The mechanisms coupling mitochondrial dynamics to T cell status are unclear. Here, we report that RNA cap methyltransferase 1 (CMTR1) is induced in activated T cells, methylating the first nucleotide on mRNA and U2 small nuclear RNA (snRNA), a component of the spliceosome. Using transcriptomic analyses, we identify a functional splicing module regulating mitochondrial dynamics in T cells, which alters the isoforms of proteins controlling mitochondrial fission and fusion. Through epitranscriptomic control of U2 snRNA and splicing, CMTR1 directs protein isoform selection during T cell activation to promote the development of longer mitochondria with increased respiratory capacity. Thus, CMTR1 upregulation supports the energetic demands of T cell activation, survival, and immune responses.