RNA methyltransferases involved in 5' cap biosynthesis

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

In humans, cap1 formation is catalyzed by the CMTr1 enzyme. 38 It is composed of several domains, including the Nterminal catalytic RMF domain with a conserved K-D-K triad characteristic for 2 0 -O-ribose methyltransferases and a guanylyltransferase-like domain that lacks catalytic residues. 39 The N-terminal domain of CMTr1 shares a global architecture with the VP39 protein and is sufficient for cap1 activity in vitro. Interestingly, while the cofactor-binding sites, active sites, and the sites of binding of the nascent RNA chain exhibits similarities with the VP39 and CMTr1 enzymes (and likewise the conformations of the respective ligands), their cap-binding sites exhibit large differences in the shape of the m 7 G-binding pocket. As a result, CMTr1 binds m 7 G in a different way, in which the sugar edge of the cap guanosine faces the protein, and the methyl group on N7 faces the solvent (Fig. 3) . These structural differences explain why CMTr1 is relatively insensitive to the absence of cap0 methylation and therefore is able to act, at least in vitro, on substrates with unmethylated guanosine. Proteins with cap1 methyltransferase activities were also characterized in the alfalfa looper moth Autographa californica nucleopolyhedrovirus (orf69 40 ) and in T. brucei (TbMTr1 41, 42 ). Both of these enzymes are relatively closely related to the human CMTr1 enzyme.