Palmitoylation of a G protein alpha i subunit requires membrane localization not myristoylation

Palmitoylation is a dynamic, post-translational modification of the amino terminus of heterotrimeric G protein alpha subunits. Since myristoylation, beta gamma interactions, and membrane attachment also involve the amino terminus of the G protein alpha i1 subunit, we studied the relationships between palmitoylation and these events. Using COS cell transfection, the turnover of palmitate was slower on alpha i1 subunits co-expressed with beta and gamma subunits than on the alpha i1 subunit expressed alone. Mutation of cysteine 3 of alpha i1 prevented [2H]palmitate but not [3H]myristate incorporation and decreased the membrane localization of this protein. This nonpalmitoylated mutant could form a heterotrimer with co-expressed beta gamma subunits which restored its membrane localization. A nonmyristoylated alpha i1 mutant (glycine 2 to alanine) could incorporate [3H]palmitate when co-expressed with beta gamma subunits and localized to the membrane. The [3H]palmitate turnover of this nonmyristoylated mutant was more rapid than seen with the wild-type alpha i1 subunit. While myristoylation is not required for palmitoylation, both myristoylation and beta gamma association can slow the turnover of palmitate on alpha i1. These results suggest that palmitoylation maintains the membrane attachment of the free alpha subunit and changes in beta gamma association could modulate palmitoylation during signaling.