All known eukaryotic and some viral mRNA capping enzymes (CEs) transfer a GMP moiety of GTP to the 5'-diphosphate end of the acceptor RNA via a covalent enzyme-GMP intermediate to generate the cap structure. In striking contrast, the putative CE of vesicular stomatitis virus (VSV), a prototype of nonsegmented negative-strand (NNS) RNA viruses including rabies, measles, and Ebola, incorporates the GDP moiety of GTP into the cap structure of transcribing mRNAs. Here, we report that the RNA-dependent RNA polymerase L protein of VSV catalyzes the capping reaction by an RNA:GDP polyribonucleotidyltransferase activity, in which a 5'-monophosphorylated viral mRNA-start sequence is transferred to GDP generated from GTP via a covalent enzyme-RNA intermediate. Thus, the L proteins of VSV and, by extension, other NNS RNA viruses represent a new class of viral CEs, which have evolved independently from known eukaryotic CEs.

Enteropathogenic Escherichia coli (EPEC) secretes several Esp proteins via the type III secretion system (secreton). EspA, EspB, and EspD are required for translocation of the effector proteins into host cells, in which EspB and EspD are thought to form a pore in the host membrane. Recent study has shown that EspA forms a filamentous structure that assembles as a physical bridge between bacteria and host cell surfaces, which then functions as a conduit for the translocation of bacterial effectors into host cells. To investigate the supermolecular structure of the type III secreton in EPEC, we partially purified it from the bacteria membrane and observed it via transmission electron microscopy. The EPEC type III secreton was composed of a basal body and a needle part and was similar to those of Salmonella and Shigella, except for a sheath-like structure at the tip of the needle. The length of sheath-like structures varied; it extended more than 600 nm and was 10 times longer than the Shigella needle part. The putative major needle component, EscF, was required for both secretion of Esp proteins and needle complex formation. Interestingly, elongation of the sheath-like structure was observed under constitutive expression of EspA but not of EscF. Furthermore, the transmission electron microscopy view with immunogold labeled anti-EspA antibodies clearly showed that EspA is a component of the sheath-like structure. This study revealed, to our knowledge for the first time, the supermolecular structure of the EPEC type III secreton and its direct association with the EspA-sheath-like structure.

The RNA-dependent RNA polymerase L protein of vesicular stomatitis virus, a prototype of nonsegmented negative-strand (NNS) RNA viruses, forms a covalent complex with a 5′-phosphorylated viral mRNA-start sequence (L-pRNA), a putative intermediate in the unconventional mRNA capping reaction catalyzed by the RNA:GDP polyribonucleotidyltransferase (PRNTase) activity. Here, we directly demonstrate that the purified L-pRNA complex transfers pRNA to GDP to produce the capped RNA (Gpp-pRNA), indicating that the complex is a bona fide intermediate in the RNA transfer reaction. To locate the active site of the PRNTase domain in the L protein, the covalent RNA attachment site was mapped. We found that the 5′-monophosphate end of the RNA is linked to the histidine residue at position 1,227 (H1227) of the L protein through a phosphoamide bond. Interestingly, H1227 is part of the histidine-arginine (HR) motif, which is conserved within the L proteins of the NNS RNA viruses including rabies, measles, Ebola, and Borna disease viruses. Mutagenesis analyses revealed that the HR motif is required for the PRNTase activity at the step of the enzyme-pRNA intermediate formation. Thus, our findings suggest that an ancient NNS RNA viral polymerase has acquired the PRNTase domain independently of the eukaryotic mRNA capping enzyme during evolution and PRNTase becomes a rational target for designing antiviral agents.nonsegmented negative-strand RNA virus | polyribonucleotidyltransferase | RNA-dependent RNA polymerase | L protein | mRNA modification A structural hallmark of eukaryotic mRNA is the presence of the 5′-terminal cap structure ½m 7 Gð5 0 Þpppð5 0 ÞN-, in which 7-methylguanosine (m 7