X-ray structure and activities of an essential Mononegavirales L-protein domain

Paesen, Guido C.; Collet, Axelle; Sallamand, Corinne; Debart, Françoise; Vasseur, Jean-Jacques; Canard, Bruno; Decroly, Etienne; Grimes, Jonathan M.

T he Mononegavirales order groups five families of monopartite, negative-strand RNA viruses many of which are highly pathogenic and/or contagious; the Filoviridae (of which Ebola virus is a representative), the Bornaviridae (Borna disease virus), the Nyamiviridae (midway virus), the Rhabdoviridae (rabies, vesicular stomatitis virus (VSV)) and the Paramyxoviridae (measles virus, human metapneumovirus (hMPV)). These viruses encode a large RNA polymerase (L) (usually 42,000 amino acids) that is crucial to viral replication (Fig. 1a) . It has two distinct roles to replicate the RNA genome and to transcribe viral mRNA. As such it not only polymerizes RNA but also synthesizes fully methylated cap structures 1 . Capping involves the co-transcriptional addition of a guanosine (G) to the first nucleotide (N 1 ) of the nascent RNA chain via a 5 0 -5 0 triphosphate bridge, resulting in a GpppN 1 -structure. Typically, this is followed by methylation of nitrogen 7 (N7) of G, giving rise to m GpppN 1 -, and of the 2 0 -oxygen (2 0 O) of the N 1 ribose ( m GpppN 1m -). The cap protects mRNAs against 5 0 -exonucleases and promotes RNA transport and translation, while 2 0 Omethylation prevents detection by cellular-immunity sensors [2] [3] [4] . In Rhabdoviridae, CR-V catalyses cap addition by means of an unconventional polyribonucleotydyl-transferase (PRNTase) reaction where a conserved histidine in CR-V forms a covalent phosphoamide bond with the transcript, resulting in a CR-V-pRNA intermediate. The capped transcript is released after ligation of a Gpp to the pRNA. This mechanism differs from capping in eukaryotes and most other viruses, in which a guanylyltransferase (GTase) forms a phosphoamide bond with Gp, before transferring it to 5 0 ppRNA 2 . Paramyxoviridae also contain a PRNTase signature motif in their CR-V domains, suggesting they use the same capping strategy as Rhabdoviridae.