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 A 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.