Structure of vesicular stomatitis virus L-P-N⁰ complex by cyro-electron microscopy

The large protein (L) of nonsegmented, negative-strand RNA viruses, a viral order that includes rabies, Ebola, Nipah, and respiratory syncytial viruses, catalyze RNA-dependent RNA polymerization on nucleocapsid-sheathed RNA templates. L coordinates two distinct processes: messenger RNA (mRNA) transcription and genome replication. During transcription, L catalyzes cotranscriptional mRNA capping, methylation, and polyadenylation. The additional activities are not required during genome replication; instead, RNA synthesis is coupled to encapsidation by soluble nucleoprotein (N⁰) recruited by the phosphoprotein (P). We determined the structure of a vesicular stomatitis virus polymerase complex comprising L-P and N⁰. P directly engages the carboxyl-terminal domain of L (L_CTD) to anchor N⁰ in a continuum of conformations including a more stable configuration with N⁰ positioned at the RNA-product exit channel, poised for encapsidation. This study reveals a function for L_CTD in encapsidation of nascent RNAs and provides the structure of a polymerase complex required for genomic RNA replication.