Nucleic Acids Research Advance Access published online on October 5, 2006
Nucleic Acids Research, doi:10.1093/nar/gkl573
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Nucleic Acid Enzymes |
Functional characterization of a 48 kDa Trypanosoma brucei cap 2 RNA methyltransferase
Department of Biological Sciences, State University of New York at Buffalo Buffalo, NY 14260, USA
*To whom the correspondence should be addressed. Tel: +1 716 645 2363; Fax: +1 716 645 2975; Email: kiongho{at}buffalo.edu
Received June 14, 2006. Revised July 20, 2006. Accepted July 20, 2006.
Kinetoplastid mRNAs possess a unique hypermethylated cap 4 structure derived from the standard m7GpppN cap structure, with 2'-O methylations on the first four ribose sugars and additional base methylations on the first adenine and the fourth uracil. While the enzymes responsible for m7GpppN cap 0 formations has been characterized in Trypanosoma brucei, the mechanism of cap 4 methylation and the role of the hypermethylated structure remain unclear. Here, we describe the characterization of a 48 kDa T.brucei 2'-O nucleoside methyltransferase (TbCom1). Recombinant TbCom1 transfers the methyl group from S-adenosylmethionine (AdoMet) to the 2'-OH of the second nucleoside of m7GpppNpNp-RNA to form m7GpppNpNmp-RNA. TbCom1 is also capable of converting cap 1 RNA to cap 2 RNA. The methyl transfer reaction is dependent on the m7GpppN cap, as the enzyme does not form a stable interaction with GpppN-terminated RNA. Mutational analysis establishes that the TbCom1 and vaccinia virus VP39 methyltransferases share mechanistic similarities in AdoMet- and cap-recognition. Two aromatic residues, Tyr18 and Tyr187, may participate in base-stacking interactions with the guanine ring of the cap, as the removal of each of these aromatic side-chains abolishes cap-specific RNA-binding.
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