G beta gamma and palmitate target newly synthesized G alpha(z) to the plasma membrane

The subcellular location of a signaling protein determines its ability to t ransmit messages accurately and efficiently. Three different lipid modifica tions tether heterotrimeric G proteins to membranes: alpha subunits are myr istoylated and/or palmitoylated, and gamma subunits are prenylated, In a pr evious study, we examined the role of lipid modifications in maintaining th e membrane attachment of a G protein alpha subunit, alpha(z), which is myri stoylated and palmitoylated (Morales, J., Fishburn, C. S., Wilson, P. T., a nd Bourne, H. R. (1998) Mel. Biol. Cell 9, 1-14), Now we extend this analys is 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 palmi toylated but not myristoylated, alpha(z) moves more rapidly to the membrane fraction following synthesis in the cytosol, Newly synthesized alpha(z) as sociates randomly with cellular membranes, but with time accumulates at the PM. Palmitoylated alpha(z) is present only in PM-enriched fractions, where as a nonpalmitoylated mutant of alpha(z) (alpha(z) C3A) associates less sta bly with the PM than does wild-type alpha(z). Expression of a C-terminal fr agment of the beta-adrenoreceptor kinase, which sequesters free beta gamma, impairs association of both alpha(z) and alpha(z) C3A with the PM, suggest ing 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 synthe sis on soluble ribosomes, myristoylated a, associates randomly and reversib ly with membranes; upon association with the PM, alpha(z) binds beta gamma, which promotes its palmitoylation, thus securing it in the proper place fo r transmitting the hormonal signal.