Membrane Trafficking of Heterotrimeric G Proteins via the Endoplasmic Reticulum and Golgi

Membrane targeting of G-protein αβγ heterotrimers was investigated in live cells by use of Gα and Gγ subunits tagged with spectral mutants of green fluorescent protein. Unlike Ras proteins, Gβγ contains a single targeting signal, the CAAX motif, which directed the dimer to the endoplasmic reticulum. Endomembrane localization of farnesylated Gγ(1), but not geranylgeranylated Gγ(2), required carboxyl methylation. Targeting of the heterotrimer to the plasma membrane (PM) required coexpression of all three subunits, combining the CAAX motif of Gγ with the fatty acyl modifications of Gα. Gα associated with Gβγ on the Golgi and palmitoylation of Gα was required for translocation of the heterotrimer to the PM. Thus, two separate signals, analogous to the dual-signal targeting mechanism of Ras proteins, cooperate to target heterotrimeric G proteins to the PM via the endomembrane.