A NOVEL N-TERMINAL MOTIF FOR PALMITOYLATION OF G-PROTEIN ALPHA-SUBUNITS

We have examined the post-translational processing of Galpha subunits expressed endogenously in rat PC12 and NG108-15 rat/mouse hybrid cells, and after transfection of cDNA expression constructs into COS cells. Thioester-linked palmitoylation Of alpha(o), alpha(s), alpha(q)/alpha11 and alpha(i2) has been detected by metabolic labelling with [H-3]palmitate and immunoprecipitation. Palmitoylation of alpha(o) occurs post-translationally in cells treated with protein-synthesis inhibitors, suggesting possible dynamic acylation. Palmitoylation of the C-terminal CAAX motif has been excluded. Site-directed mutagenesis of alpha(o) has been used to implicate the site of modification as a cysteine residue next to the N-terminal myristoylated glycine, in a novel protein-lipid modification motif Met-Gly-Cys. The non-palmitoylated alpha(o) mutant is still myristoylated but shows reduced membrane binding, suggesting that reversible palmitoylation may regulate G(alpha) localization and function.