Structural insights into the mechanism and evolution of the vaccinia virus mRNA cap N7 methyl-transferase

The vaccinia virus mRNA capping enzyme is a multifunctional heterodimeric protein associated with the viral polymerase that both catalyses the three steps of mRNA capping and regulates gene transcription. The structure of a subcomplex comprising the C‐terminal N7‐methyl‐transferase (MT) domain of the large D1 subunit, the stimulatory D12 subunit and bound S‐adenosyl‐homocysteine (AdoHcy) has been determined at 2.7 Å resolution and reveals several novel features of the poxvirus capping enzyme. The structure shows for the first time the critical role played by the proteolytically sensitive N‐terminus of the MT domain in binding the methyl donor and in catalysis. In addition, the poxvirus enzyme has a completely unique mode of binding of the adenosine moiety of AdoHcy, a feature that could be exploited for design of specific anti‐poxviral compounds. The structure of the poxvirus‐specific D12 subunit suggests that it was originally an RNA cap 2′O‐MT that has evolved to a catalytically inactive form that has been retained for D1 stabilisation and MT activity enhancement through an allosteric mechanism.