G protein alpha subunits and beta gamma dimers are covalently modified by lipids. The emerging picture is one in which attached lipids provide more than just a nonspecific "glue" for sticking G proteins to membranes. We are only beginning to understand how different lipid modifications of different G protein subunits affect specific protein-protein interactions and localization to specific cellular sites. In addition, regulation of these modifications, particularly palmitoylation, can provide new ways to regulate signals transmitted by G proteins.

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؊ . This revealed the specific ability of bovine brain G␣ q/11 to bind to both GRK2 and GRK3 in an AlF 4 ؊ -dependent manner. In contrast, G␣ s , G␣ i , and G␣ 12/13 did not bind to GRK2 or GRK3 despite their presence in the extract. Additional studies revealed that bovine brain G␣ q/11 could also bind to an N-terminal construct of GRK2, while no binding of G␣ q/11 , G␣ s , G␣ i , or G␣ 12/13 to comparable constructs of GRK5 or GRK6 was observed. Experiments using purified G␣ q revealed significant binding of both G␣ q GDP/AlF 4 ؊ and G␣ q (GTP␥S), but not G␣ q (GDP), to GRK2. Activation-dependent binding was also observed in both COS-1 and HEK293 cells as GRK2 significantly co-immunoprecipitated constitutively active G␣ q (R183C) but not wild type G␣ q . In vitro analysis revealed that GRK2 possesses weak GAP activity toward G␣ q that is dependent on the presence of a G proteincoupled receptor. However, GRK2 effectively inhibited G␣ q -mediated activation of phospholipase C-␤ both in vitro and in cells, possibly through sequestration of activated G␣ q . These data suggest that a subfamily of the GRKs may be bifunctional regulators of G protein-coupled receptor signaling operating directly on both receptors and G proteins.

G protein-coupled receptors (GPCRs) are increasingly recognized to operate from intracellular membranes as well as the plasma membrane. The β2-adrenergic GPCR can activate Gs-linkedcyclic AMP (cAMP) signaling from endosomes. We show here that the homologous human β1-adrenergic receptor initiates an internal Gs-cAMP signal from the Golgi apparatus. By developing a chemical method to acutely squelch G protein coupling at defined membrane locations, we demonstrate that Golgi activation contributes significantly to the overall cellular cAMP response. Golgi signalling utilizes a pre-existing receptor pool rather than receptors delivered from the cell surface, requiring separate access of extracellular ligands. Epinephrine, a hydrophilic endogenous ligand, accesses the Golgi-localized receptor pool by facilitated transport requiring the organic cation transporter 3 (OCT3) whereas drugs can access the Golgi pool by passive diffusion according to hydrophobicity. We demonstrate marked differences among both agonist and antagonist drugs in Golgi-localized receptor access, and show that β-blocker drugs presently used in the clinic differ markedly in ability to antagonize the Golgi signal. We propose ’location bias’ as a new principle for achieving functional selectivity of GPCR-directed drug action.

To be activated by cell surface G protein-coupled receptors, heterotrimeric G proteins must localize at the cytoplasmic surface of plasma membranes. Moreover, some G protein subunits are able to traffic reversibly from the plasma membrane to intracellular locations upon activation. This review will highlight new insights into how nascent G protein subunits are assembled and how they arrive at plasma membranes. In addition, recent reports have increased our knowledge of activation-induced trafficking of G proteins. Understanding G protein assembly and trafficking will lead to a greater understanding of novel ways that cells regulate G protein signaling.