Gβγ and palmitate target newly synthesized Gαz to the plasma membrane

The subcellular location of a signaling protein determines its ability to transmit messages accurately and efficiently. Three different lipid modifications tether heterotrimeric G proteins to membranes: alpha subunits are myristoylated and/or palmitoylated, and gamma subunits are prenylated, In a previous study, we examined the role of lipid modifications in maintaining the membrane attachment of a G protein alpha subunit, alpha(z), which is myristoylated and palmitoylated (Morales, J., Fishburn, C. S., Wilson, P. T., and Bourne, H. R. (1998) Mel. Biol. Cell 9, 1-14), Now we extend this analysis by characterizing the mechanisms that target newly synthesized alpha(z) to the plasma membrane (PM) and analyze the role of lipid modifications in this process. In comparison with newly synthesized alpha(s), which is palmitoylated but not myristoylated, alpha(z) moves more rapidly to the membrane fraction following synthesis in the cytosol, Newly synthesized alpha(z) associates randomly with cellular membranes, but with time accumulates at the PM. Palmitoylated alpha(z) is present only in PM-enriched fractions, whereas a nonpalmitoylated mutant of alpha(z) (alpha(z) C3A) associates less stably with the PM than does wild-type alpha(z). Expression of a C-terminal fragment of the beta-adrenoreceptor kinase, which sequesters free beta gamma, impairs association of both alpha(z) and alpha(z) C3A with the PM, suggesting that the alpha subunit must bind beta gamma in order to localize at the PM. Based on these findings, we propose a model in which, following synthesis on soluble ribosomes, myristoylated a, associates randomly and reversibly with membranes; upon association with the PM, alpha(z) binds beta gamma, which promotes its palmitoylation, thus securing it in the proper place for transmitting the hormonal signal.