SignificanceThe ongoing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic emphasizes the urgent need to develop efficient broad-spectrum anti-CoV drugs. The structure–function characterization of conserved CoV replicative enzymes is key to identifying the most suitable drug targets. Using a multidisciplinary comparative approach and different betacoronaviruses, we characterized the key conserved residues of the nsp14 (N7-guanine)–methyltransferase, a poorly defined subunit of the CoV messenger RNA–synthesizing machinery. Our study highlights the unique structural features of this enzyme and establishes its essential role in betacoronavirus replication, while identifying two residues that are critical for the replication of the four betacoronaviruses tested, including SARS-CoV-2.