Résumé
The L protein of mononegaviruses harbours all catalytic activities for genome replication and transcription. It contains six conserved domains (CR-I to -VI; Fig. 1a). CR-III has been linked to polymerase and polyadenylation activity, CR-V to mRNA capping and CR-VI to cap methylation. However, how these activities are choreographed is poorly understood. Here we present the 2.2-Å X-ray structure and activities of CR-VI+, a portion of human Metapneumovirus L consisting of CR-VI and the poorly conserved region at its C terminus, the +domain. The CR-VI domain has a methyltransferase fold, which besides the typical S-adenosylmethionine-binding site (SAMP) also contains a novel pocket (NSP) that can accommodate a nucleoside. CR-VI lacks an obvious cap-binding site, and the SAMP-adjoining site holding the nucleotides undergoing methylation (SUBP) is unusually narrow because of the overhanging +domain. CR-VI+ sequentially methylates caps at their 2′O and N7 positions, and also displays nucleotide triphosphatase activity.