Virus-specific mRNA capping enzyme encoded by hepatitis E virus

Hepatitis E virus (HEV), a positive strand RNA virus, is an important causa tive agent of waterborne hepatitis. Expression of cDNA (encoding amino acid s 1 to 979 of HEV nonstructural open reading frame 1) in insect cells resul ted in synthesis of a 110-kDa protein (P110), a fraction of which was prote olytically processed to an 80-kDa protein. P110 was tightly bound to cytopl asmic membranes, from,which it could be released by detergents. Immunopurif ied P110 catalyzed transfer of a methyl group from S-adenosylmethionine (Ad oMet) to GTP and GDP to yield m(7)GTP or m(7)GDP. GMP, GpppG, and GpppA wer e poor substrates for the P110 methyltransferase. There was no evidence for further methylation of m7GTP when it was used as a substrate for the methy ltransferase. P110 was also a guanylyltransferase, which formed a covalent complex, P110-m(7)GMP, in the presence of AdoMet and GTP, because radioacti vity from both [alpha-P-32]GTP and [H-3-methyl] AdoMet was found in the cov alent guanylate complex. Since both methyltransferase and guanylyltransfera se reactions are strictly virus specific, they should offer optimal targets for development of antiviral drugs. Cap analogs such as m(7)GTP, m(7)GDP, et(2)m(7)GMP, and m(2)et(7)GMP inhibited the methyltransferase reaction. HE V P110 capping enzyme has similar properties to the methyltransferase and g uanylyltransferase of alphavirus nsP1, tobacco mosaic virus P126, brome mos aic virus replicase protein la, and bamboo mosaic virus (a potexvirus) nons tructural protein, indicating there is a common evolutionary origin of thes e distantly related plant and animal virus families.