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Journal of Virology, February 2004, p. 1271-1280, Vol. 78, No. 3
0022-538X/04/$08.00+0     DOI: 10.1128/JVI.78.3.1271-1280.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Critical Residues for GTP Methylation and Formation of the Covalent m7GMP-Enzyme Intermediate in the Capping Enzyme Domain of Bamboo Mosaic Virus

Yih-Leh Huang, Yu-Tsung Han, Ya-Ting Chang, Yau-Heiu Hsu, and Menghsiao Meng*

Graduate Institute of Biotechnology, National Chung Hsing University, Taichung, Taiwan 40227, Republic of China

Received 9 May 2003/ Accepted 16 October 2003

Open reading frame 1 of Bamboo mosaic virus (BaMV), a Potexvirus in the alphavirus-like superfamily, encodes a 155-kDa replicase responsible for the formation of the 5' cap structure and replication of the viral RNA genome. The N-terminal domain of the viral replicase functions as an mRNA capping enzyme, which exhibits both GTP methyltransferase and S-adenosylmethionine (AdoMet)-dependent guanylyltransferase activities. We mutated each of the four conserved amino acids among the capping enzymes of members within alphavirus-like superfamily and a dozen of other residues to gain insight into the structure-function relationship of the viral enzyme. The mutant enzymes were purified and subsequently characterized. H68A, the mutant enzyme bearing a substitution at the conserved histidine residue, has an ~10-fold increase in GTP methyltransferase activity but completely loses the ability to form the covalent m7GMP-enzyme intermediate. High-pressure liquid chromatography analysis confirmed the production of m7GTP by the GTP methyltransferase activity of H68A. Furthermore, the produced m7GTP sustained the formation of the m7GMP-enzyme intermediate for the wild-type enzyme in the presence of S-adenosylhomocysteine (AdoHcy), suggesting that the previously observed AdoMet-dependent guanylation of the enzyme using GTP results from reactions of GTP methylation and subsequently guanylation of the enzyme using m7GTP. Mutations occurred at the other three conserved residues (D122, R125, and Y213), and H66 resulted in abolition of activities for both GTP methylation and formation of the covalent m7GMP-enzyme intermediate. Mutations of amino acids such as K121, C234, D310, W312, R316, K344, W406, and K409 decreased both activities by various degrees, and the extents of mutational effects follow similar trends. The affinity to AdoMet of the various BaMV capping enzymes, except H68A, was found in good correlations with not only the magnitude of GTP methyltransferase activity but also the capability of forming the m7GMP-enzyme intermediate. Taken together with the AdoHcy dependence of guanylation of the enzyme using m7GTP, a basic working mechanism, with the contents of critical roles played by the binding of AdoMet/AdoHcy, of the BaMV capping enzyme is proposed and discussed.

* Corresponding author. Mailing address: Graduate Institute of Biotechnology, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung, Taiwan 40227, Republic of China. Phone: 886-4-22840328. Fax: 886-4-22853527. E-mail: mhmeng{at}dragon.nchu.edu.tw.

Journal of Virology, February 2004, p. 1271-1280, Vol. 78, No. 3
0022-538X/04/$08.00+0     DOI: 10.1128/JVI.78.3.1271-1280.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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