Exocytic pathway-independent plasma membrane targeting of heterotrimeric G proteins.

Heterotrimeric G proteins are lipid-modified, peripheral membrane proteins that function at the inner surface of the plasma membrane (PM) to relay signals from cell-surface receptors to downstream effectors. Cellular trafficking pathways that direct nascent G proteins to the PM are poorly defined. In this report, we test the proposal that G proteins utilize the classical exocytic pathway for PM targeting. PM localization of the G protein heterotrimers alpha s beta 1 gamma 2 and alpha q beta 1 gamma 2 occurred independently of treatment of cells with Brefeldin A, which disrupts the Golgi, or expression of Sar1 mutants, which prevent the formation of endoplasmic reticulum to Golgi transport vesicles. Moreover, the palmitoylation of alpha q was unaffected by Brefeldin A treatment, even though the palmitoylation of SNAP25 was blocked by Brefeldin A. Non-palmitoylated mutants of alpha s and alpha q failed to stably bind to beta gamma and displayed a dispersed cytoplasmic localization when co-expressed with beta gamma. These findings support a refined model of the PM trafficking pathway of G proteins, involving assembly of the heterotrimer at the endoplasmic reticulum and transport to the PM independently of the Golgi.