RNA methyltransferases involved in 5' cap biosynthesis

Byszewska, Magdalena; Smietanski, Miroslaw; Purta, Elzbieta; Bujnicki, Janusz M

In bluetongue virus, another member of the reoviruses, the structure of the VP4 protein revealed a multi-domain protein with an N-terminal guanylyltransferase domain and 2 RFM domains, of which one was inserted into another. The inserted RFM domain exhibited clear similarities to the cap1 methyltransferases and in 3 crystal forms had GpppG, m 7 GDP, or GTP bound in the position of the cap-binding site, while the other RFM domain exhibited low but significant similarity to known cap0 methyltransferase structures. 51 In the non-segmented, negative-sense single-stranded RNA viruses [order Mononegavirales (MNV)] that include pathogens such as respiratory syncytial virus, measles, mumps, rabies, parainfluenza, vesicular stomatitis virus (VSV), and Marburg and Ebola viruses, one of the common components of the viral ribonucleoprotein core is the large (L) protein, which encodes multiple functions such as the RNA-dependent RNA polymerase and activities responsible for mRNA capping, cap0 and cap1 methylation, poly(A) polymerase and protein kinase. 52, 53 Using bioinformatics methods, we and others predicted that the C-terminal region of that protein (conserved region VI) encodes a domain homologous to 2 0 -O-ribose methyltransferases and is likely to function as a cap1 methyltransferase. 54, 55 Later it was found that, in VSV, this region is essential not only for cap1, but also for cap0 methyltransferase activity and that the same SAM-binding site and part of the K-D-K triad is used for both reactions. 15, 56 The structural basis of this phenomenon remains to be determined.