ACTIVATION-DEPENDENT CARBOXYL METHYLATION OF NEUTROPHIL G-PROTEIN GAMMA-SUBUNIT

The gamma subunits of heterotrimeric guanine nucleotide-binding regula tory (G) proteins (G(gamma)) are posttranslationally processed al thei r C termini by prenylation, proteolysis, and carboxyl methylation. Whe reas prenylation of G(gamma) is required for membrane association of G proteins, the role of carboxyl methylation is unknown. Here we show t hat human neutrophils express G(gamma 2) but not G(gamma 3) or G(gamma 7) and that carboxyl methylation of G(gamma 2) is associated with sig nal transduction, In a reconstituted cell-free system, neutrophil G(ga mma 2) was labeled by the methyl donor S-[methyl-H-3] adenosyl-L-methi onine. Carboxyl methylation was confirmed by alkaline hydrolysis and q uantitation of volatile [H-3]methanol. Neutrophil G(gamma 2) methylati on was stimulated by activation of G protein with guanosine 5'-[beta,g amma thio] triphosphate. We estimate that after 1 hr of G-protein acti vation at least 6% of the total pool of G(gamma 2) was carboxyl-methyl ated, The inflammatory agonist fMet-Leu-Phe stimulated guanosine 5'-[b eta,gamma-thio]triphosphate-dependent carboxyl methylation. Methylatio n of G(gamma 2) was inhibited by the carboxyl methyltransferase inhibi tor N-acetyl-S-trans, trans-farnesyl cysteine at concentrations that a ffected signal transduction in neutrophils. These results demonstrate that activation of neutrophil G(i) is associated with alpha-carboxyl m ethyl esterification of G(gamma 2) and suggest that carboxyl methylati on of G(gamma) may play a role in signal transduction.