Gβχ and palmitate target newly synthesized Gα(z) to the plasma membrane

Abstract

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