Identification of the vaccinia virus mRNA guanyltransferase active site lysine

The vaccinia virus mRNA capping enzyme is a heterodimeric protein containing subunits of 97 and 33 kDa, the products of genes D1R and D12L, respectively. The enzyme catalyzes the first three reactions in the mRNA cap formation pathway: mRNA triphosphatase, guanyltransferase and (guanine-7-)methyltransferase. The guanyltransferase reaction proceeds by way of a covalent enzyme GMP (E-GMP) intermediate (Shuman, S. and Hurwitz, J. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 187-191) in which the GMP is linked to the large subunit through a lysine residue (Toyama, R., Mizumoto, K., Nakahara, Y., Tatsuno, T., and Kaziro, Y. (1983) Eur. J. Biochem. 2, 2195-2201; Roth, M. J., and Hurwitz, J. (1984) J. Biol Chem. 259, 13488-13494). In order to identify the map position of the guanyltransferase active site lysine residue, high specific activity [32P]E-GMP was prepared. Digestion of the E-GMP with hydroxylamine at pH 9.5 yielded a 31-kDa radioactive fragment derived from amino acids 1-273. Cleavage of E-GMP with cyanogen bromide produced a radioactive peptide of 14 kDa corresponding to amino acids 242-365. Lysine residues are found at positions 244 and 260. Staphylococcus aureus V8 protease digestion of cyanogen bromide-cleaved E-GMP yields a radioactive product of about 5 kDa in molecular mass corresponding to the peptide generated by cleavage at glutamic acid residues 253 and 297, demonstrating that lysine 260 is the site of linkage of GMP.