Differential Isoprenylation of Carboxy-Terminal Mutants of an Inhibitory G-Protein α-Subunit: Neither Farnesylation nor Geranylgeranylation Is Sufficient for Membrane Attachment

To determine the effect of protein isoprenylation with farnesyl vs geranylgeranyl groups on membrane association in vivo, COS cells were transfected with cDNAs encoding the wild-type G-protein α_il (WT) subunit, the soluble nonmyristoylated G-protein α_il glycine to alanine mutant (GA), a double mutant in which the carboxy-terminal residues CGLF of GA were mutated to CVLS (GA-CVLS), and a double mutant in which the carboxy terminus of GA was mutated to CALL (GA-CALL). As opposed to the WT and GA proteins, the GA-CVLS and GA-CALL proteins were not pertussis toxin substrates nor were they recognized by antibodies that recognize the nonmutated α_i1 carboxy terminus. Only the GA-CVLS and GA-CALL proteins incorporated [³H]mevalonate in the form of a farnesyl and a geranylgeranyl moiety, respectively. Subcellular localization, as assessed by immunoblotting and immunoprecipitation, revealed that the WT protein localizes almost exclusively to the membrane fraction, whereas the GA, GA-CVLS, and GA-CALL proteins localize predominantly to the soluble fraction. The soluble GA-CVLS and GA-CALL proteins were not carboxyl methylated, but the small amount localized to the membrane was partially carboxyl methylated. These results indicate that neither farnesylation nor geranylgeranylation is sufficient alone to lead to membrane association.