脳GTP結合蛋白質の異種γサブユニットに対するカルボキシメチル基転移酵素に関する研究

概要

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The heterotrimeric GTP-binding proteins (G-proteins) consist of the α, β and γ subunits, which exist physiologically in the two functional units of α and βγ. Recent observations have revealed that the COOH-terminal cysteine residue of γ-subunit (Gγ) is posttranslationally isoprenylated with thioether-linkage and carboxyl methylated. We have previously demonstrated that the βγ-subunits of bovine brain G-protein (Gi/o) could be separated into two subspecies, βγ-I and βγ-II. Both of them possess identical β-subunits but structurally different γ-subunits, γ-I and γ-II. In the present study, we investigated the carboxyl methylation of Gγ in rat brain microsome fraction, i. e., we investigated the final step of the posttranslational modification of Gγ. Gγ-carboxyl methyltransferase was characterized and its physiological significance was studied using βγ-I and βγ-II as substrates. Both γ-I and γ-II were carboxyl methylated during incubation with rat brain microsome fraction and S-adenosyl-[?H]-L-methionine. The maximum amount of the [?H] methyl incorporated into both Gγs was about 0.1 mol/mol. A kinetic study revealed that the Km for βγ-I was five times that of βγ-II, while the Vmax for βγ-II was twice that of βγ-I. This fact indicates that βγ-II is a more appropriate substrate than βγ-I for Gγ-carboxyl methyltransferase. Radioactive Gγ was detected only in the pellet fraction after centrifugation of the reaction mixture, although the Gγ band was found in both supernatant and pellet fractions under silver staining. The carboxyl methylation of Gγ was not affected by the presence of different amounts of α-subunit. The carboxyl methyltransferase was solubilized from rat brain microsomes by the treatment with various detergents; enzyme activity was found only in the supernatant solubilized with Tween 20 among the detergents examined.
札幌医科大学の論文
1993-12-01

脳GTP結合蛋白質の異種γサブユニットに対するカルボキシメチル基転移酵素に関する研究

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札幌医科大学 | 論文

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