Structure-based identification of functional residues in the nucleoside-2'-O-methylase domain of Bluetongue virus VP4 capping enzyme

Stewart, Meredith E.; Roy, Polly

The importance of this residue in the catalytic tetrad of 2 0 -O MTase for virus replication was clear when the same mutations were introduced into the replicating virus. The fact that VP4 D265 mutations abolishes the 2 0 O-MTase activity and were lethal for virus replication, highlights the biological significance of this residue and the critical nature of 2 0 O-MT for BTV replication. The formation of cap1 structure through the use of viral 2 0 -O MTase has been demonstrated to be an essential activity for virus replication for a number of RNA viruses (VSV, Coronavirus, Dengue, WNV, JEV; [1, 5, 7, 16] ). Such a drastic effect on virus replication due to a single mutation within the 2 0 -O MTase catalytic domain is not always observed for all viruses, most often only a reduction in the virus titres and growth kinetics has been observed [15, 20] . There are a number of plausible explanations for the failure to recover BTV mutants in normal cells. It is currently not possible to test these hypotheses as the VP4 used to complement function is assembled into the virions. Unlike the capping enzyme for other viruses which are non-structural (Dengue, alphaviruses, coronaviruses) VP4 is a minor structural protein and therefore the WT VP4 present in the complementary cell line is assembled into the core particle and cannot be dissociated from the virus.