THE 1.85 ANGSTROM STRUCTURE OF VACCINIA PROTEIN VP39 - A BIFUNCTIONALENZYME THAT PARTICIPATES IN THE MODIFICATION OF BOTH MESSENGER-RNA ENDS

VP39 is a bifunctional vaccinia virus protein that acts as both an mRN A cap-specific RNA 2'-O-methyltransferase and a poly(A) polymerase pro cessivity factor. Here, we report the 1.85 Angstrom crystal structure of a VP39 variant complexed with its AdoMet cofactor. VP39 comprises a single core domain with structural similarity to the catalytic domain s of other methyltransferases. Surface features and mutagenesis data s uggest two possible RNA-binding sites with novel underlying architectu re, one of which forms a cleft spanning the region adjacent to the met hyltransferase active site. This report provides a prototypic structur e for an RNA methyltransferase, a protein that interacts with the mRNA 5' cap, and an intact poxvirus protein.