title: Cap-snatching mechanism in yeast L-A double-stranded RNA virus.
abstract: The 5' cap structure (m(7)GpppX-) is an essential feature of eukaryotic mRNA required for mRNA stability and efficient translation. Influenza virus furnishes its mRNA with this structure by a cap-snatching mechanism, in which the viral polymerase cleaves host mRNA endonucleolytically 10-13 nucleotides from the 5' end and utilizes the capped fragment as a primer to synthesize viral transcripts. Here we report a unique cap-snatching mechanism by which the yeast double-stranded RNA totivirus L-A furnishes its transcript with a cap structure derived from mRNA. Unlike influenza virus, L-A transfers only m(7)Gp from the cap donor to the 5' end of the viral transcript, thus preserving the 5' α- and β-phosphates of the transcript in the triphosphate linkage of the final product. This in vitro capping reaction requires His154 of the coat protein Gag, a residue essential for decapping of host mRNA and known to form m(7)Gp-His adduct. Furthermore, the synthesis of capped viral transcripts in vivo and their expression were greatly compromised by the Arg154 mutation, indicating the involvement of Gag in the cap-snatching reaction. The overall reaction and the structure around the catalytic site in Gag resemble those of guanylyltransferase, a key enzyme of cellular mRNA capping, suggesting convergent evolution. Given that Pol of L-A is confined inside the virion and unable to access host mRNA in the cytoplasm, the structural protein Gag rather than Pol catalyzing this unique cap-snatching reaction exemplifies the versatility as well as the adaptability of eukaryotic RNA viruses.
authors: Instituto de Biología Funcional y Genómica, Consejo Superior de Investigaciones Científicas/Universidad de Salamanca, Edificio Departamental, Avenida del Campo Charro, Salamanca 37007, Spain. tfujimura@usal.es
authors: Tsutomu
authors: T
authors: Fujimura
authors: Rosa
authors: R
authors: Esteban
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D003461:Crystallography; D017931:DNA Primers / Q000235:genetics; D015683:Gene Products, gag / Q000235:genetics / Q000378:metabolism*; D006639:Histidine / Q000378:metabolism; D047468:Immunoprecipitation; D008957:Models, Genetic; D008969:Molecular Sequence Data; D012315:RNA Caps / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism / Q000821:virology*; D017422:Sequence Analysis, DNA; D019197:Totivirus / Q000235:genetics / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D015683:Gene Products, gag; D012315:RNA Caps; D006639:Histidine