Activation-dependent carboxyl methylation of neutrophil G-protem γ subunit

The γ subunits of heterotrimeric guanine nucleotide-binding regulatory (G) proteins (G_γ) are posttranslationally processed at their C termini by prenylation, proteolysis, and carboxyl methylation. Whereas prenylation of G_γ is required for membrane association of G proteins, the role of carboxyl methylation is unknown. Here we show that human neutrophils express G_γ2 but not G_γ3 or G_γ7 and that carboxyl methylation of G_γ2 is associated with signal transduction. In a reconstituted cell-free system, neutrophil G_γ2 was labeled by the methyl donor S-[methyl-³H]adenosyl-L-methionine. Carboxyl methylation was confirmed by alkaline hydrolysis and quantitation of volatile [³H] methanol. Neutrophil G_γ2 methylation was stimulated by activation of G protein with guanosine 5′-[β,γ-thio]triphosphate. We estimate that after 1 hr of G-protein activation at least 6% of the total pool of G_γ2 was carboxyl-methylated. The inflammatory agonist fMet-Leu-Phe stimulated guanosine 5′-[β,γ-thio]triphosphate-dependent carboxyl methylation. Methylation of G_γ2 was inhibited by the carboxyl methyltransferase inhibitor N-acetyl-S-trans,fransesylcysteine at concentrations that affected signal transduction in neutrophils. These results demonstrate that activation of neutrophil G_i is associated with α-carboxyl methyl esterification of G_γ2 and suggest that carboxyl methylation of G_γ may play a role in signal transduction.