Gbetagamma and palmitate target newly synthesized Galphaz 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, alphaz, 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 alphaz to the plasma membrane (PM) and analyze the role of lipid modifications in this process. In comparison with newly synthesized alphas, which is palmitoylated but not myristoylated, alphaz moves more rapidly to the membrane fraction following synthesis in the cytosol. Newly synthesized alphaz associates randomly with cellular membranes, but with time accumulates at the PM. Palmitoylated alphaz is present only in PM-enriched fractions, whereas a nonpalmitoylated mutant of alphaz (alphazC3A) associates less stably with the PM than does wild-type alphaz. Expression of a C-terminal fragment of the beta-adrenoreceptor kinase, which sequesters free betagamma, impairs association of both alphaz and alphazC3A with the PM, suggesting that the alpha subunit must bind betagamma in order to localize at the PM. Based on these findings, we propose a model in which, following synthesis on soluble ribosomes, myristoylated alphaz associates randomly and reversibly with membranes; upon association with the PM, alphaz binds betagamma, which promotes its palmitoylation, thus securing it in the proper place for transmitting the hormonal signal.
Fishburn
C S
CS
Department of Cellular and Molecular Pharmacology, University of California, San Francisco, California 94143, USA.
Herzmark
P
P
Morales
J
J
Bourne
H R
HR
D000818:Animals; D001692:Biological Transport; D020126:Brefeldin A / Q000494:pharmacology; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism*; D006224:Cricetinae; D005089:Exocytosis; D044385:GTP-Binding Protein alpha Subunits; D044387:GTP-Binding Protein beta Subunits; D044388:GTP-Binding Protein gamma Subunits; D019204:GTP-Binding Proteins / Q000096:biosynthesis / Q000378:metabolism*; D020962:Heterotrimeric GTP-Binding Proteins; D019814:Myristic Acid / Q000378:metabolism; D010168:Palmitates / Q000378:metabolism*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D015398:Signal Transduction
C118981:G-protein Beta gamma; D044385:GTP-Binding Protein alpha Subunits; D044387:GTP-Binding Protein beta Subunits; D044388:GTP-Binding Protein gamma Subunits; D010168:Palmitates; D011500:Protein Synthesis Inhibitors; D019814:Myristic Acid; D020126:Brefeldin A; D019204:GTP-Binding Proteins; D020962:Heterotrimeric GTP-Binding Proteins