RNA methyltransferases involved in 5' cap biosynthesis

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

Cap-specific 2 0 -O-ribose methyltransferases A poxvirus cap1-forming enzyme (VP39 protein from vaccinia virus), was the first methyltransferase involved in the cap structure formation, for which a crystal structure was determined 35 and also the first one for which a structure of a ternary complex of an enzyme with the cofactor and RNA substrate was determined. 36 It has become one of the best studied members of a large family of methyltransferases that act on the 2 0 -OH-ribose group in RNA, which includes also enzymes such as RrmJ and fibrillarin. 37 Although they share little sequence identity with each other, these 2 0 -O-ribose methyltransferases are characterized by the presence of a conserved tertiary fold characteristic for all RFM enzymes and a conserved K-D-K catalytic triad between the methyl group donor binding site, and the cap binding site. VP39 is a single-domain protein with additional structural elements at both the N-and C-termini, which wrap around the RFM core and form a binding pocket for the cap. In the ternary complex, the m 7 G base of the cap is bound sandwiched between 2 aromatic side chains, and oriented in such a way that the Hoogsteen edge modified by addition of the methyl group on N 7 faces the protein, thus explaining the ability of VP39 to sense the methylation status of the substrate, which is the basis of its preference for substrates that already have an N 7 -methylated cap.