Separate molecules of West Nile virus methyltransferase can independently catalyze the N7 and 2′-O methylations of viral RNA cap

West Nile virus methyltransferase catalyzes N7 and 2'-O methylations of the viral RNA cap (GpppA-RNA -> m(7)GpppAm-RNA). The two methylation events are independent, as evidenced by efficient N7 methylation of GpppA-RNA -> m(7)GpppA-RNA and GpppAm-RNA -> m(7)GpppAm-RNA, and by the 2'-O methylation of GpppA-RNA -> GpppAm-RNA and m(7)GpppA-RNA -> m(7)GpppAm-RNA. However, the 2'-O methylation activity prefers substrate m(7)GpppA-RNA to GpppA-RNA, thereby determining the dominant methylation pathway as GpppARNA -> m(7)GpppA-RNA -> m(7)GpppAm-RNA. Mutant enzymes with different methylation defects can trans complement one another in vitro. Furthermore, sequential treatment of GpppA-RNA with distinct methyltransferase mutants generates fully methylated m(7)GpppAm-RNA, demonstrating that separate molecules of the enzyme can independently catalyze the two cap methylations in vitro. (c) 2008 Elsevier Inc. All rights reserved.