
Reaction mechanism of mRNA guanylyltransferase from rat liver: isolation and characterization of a guanylyl-enzyme intermediate.
Author(s) -
Kiyohisa Mizumoto,
Yoshito Kaziro,
Fritz Lipmann
Publication year - 1982
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.79.6.1693
Subject(s) - gtp' , biochemistry , enzyme , nucleotide , biology , stereochemistry , sepharose , chemistry , gene
Rat liver RNA guanylyltransferase catalyzes a GTP-PPi exchange reaction in the absence of acceptor RNA [Mizumoto, K. & Lipmann, F. (1979) Proc. Natl. Acad. Sci. USA 76, 4961-4965] suggesting that the reaction proceeds through the formation of a covalent guanylylated intermediate. We now present more direct evidence for the existence of the enzyme-GMP intermediate: (i) the enzyme-[32P]GMP intermediate was formed on incubation of rat liver guanylyltransferase with [alpha-32P]GTP and migrated as a single radioactive band with Mr 69,000 on NaDodSO4/polyacrylamide gel electrophoresis, and (ii) the intermediate isolated on gel filtration can transfer its GMP moiety to ppGpCpC-poly(A2,U2,G) to form the capped RNA molecule or it can react with PPi to regenerate GTP. The formation of the intermediate was dependent on Mg2+ and was strongly inhibited by PPi. The addition of pyrophosphatase markedly increased the amount of the intermediate complex. On blue dextran-Sepharose affinity column chromatography, the activity of guanylyltransferase to form an enzyme-[32P]GMP intermediate comigrated with activities of cap formation and GTP-PPi exchange. A phosphoamide type linkage between GMP and enzyme is suggested by its acidlabile and alkali-stable nature and also by the susceptibility to acidic hydroxylamine. These results indicate that the reaction catalyzed by rat liver guanylyltransferase occurs through the following two partial steps: (i) E + GTP in equilibrium E-pG + PPi; and (ii) E-pG + ppN leads to GpppN + E.