A baculovirus expression system was used to determine the contribution of carboxyl methylation of specific G protein γ subunits to the interaction between α and βγ subunits. βγ subunits were carboxyl methylated by a membrane bound methyltransferase in Sf9 cells, and periodate-oxidized adenosine inhibited this methylation by 90%. Carboxyl methylation of β 1 γ 2 , β 2 γ 3 , and β 2 γ 7 enhanced pertussis toxin-catalyzed ADP-ribosylation of α i 2 and α i 3 by about 2-fold. On the other hand, methylation did not enhance membrane attachment of βγ subunits. These results suggest that methylation of isoprenylated γ subunits is required for optimal G protein-mediated signal transduction, but not membrane attachment.