[Show abstract][Hide abstract] ABSTRACT: Many eukaryotic signaling proteins are modified by the covalent attachment of long-chain lipids. These highly hydrophobic
molecules bind target proteins and facilitate interaction with cellular membranes, lipid molecules and other proteins. Generally,
there are three classes of lipids that modify target proteins: myristate, isoprenoids and palmitate (for reviews, see refs. 1, 2). Myristate, a 14-carbon saturated fatty acid, binds proteins at N-terminal glycine residues via an amide linkage.
This usually occurs cotranslationally after removal of the initiating methionine, although it also can occur after proteolytic
cleavage and exposure of an internal glycine. Long-chain isoprenoid lipids, including far-nesyl and geranylgeranyl groups,
modify proteins posttranslationally and attach via a thioether linkage to C-terminal cysteine residues. Palmitate is a 16-carbon
saturated fatty acid that modifies proteins posttranslationally through thioester incorporation at cysteines. Palmitate also
can modify proteins at additional sites and by alternative mechanisms, as in proteins palmitoylated at serine and threo-nine
residues via oxyester linkages and by amide-linked N-terminal cysteines and glycines. In addition, other fatty acid species
form thioester linkages with proteins. For these reasons, the general term for protein lipid modification by thioester attachment
is thioacylation, or S-acylation, and the term palmitoylation refers specifically to protein modification by palmitate. Proteins that undergo palmitoylation by thioester attachment at
cysteine residues, or S-palmitoylation, represent the majority of protein targets of palmitoylation. This chapter will focus on methods for identifying protein
substrates for thioester incorporation of palmitate.
[Show abstract][Hide abstract] ABSTRACT: Regulator of G Protein Signalling (RGS) proteins impede heterotrimeric G protein signalling. RGS2 decreases cAMP production and appears to interact with both adenylyl cyclase (AC) and its stimulatory G protein Gs. We showed previously that Green Fluorescent Protein-tagged RGS2 (GFP-RGS2) localizes to the nucleus in HEK 293 cells and is recruited to the plasma membrane when co-expressed with Gsalpha, or the Gs-coupled beta2-adrenergic receptor (beta2AR). Here, using confocal microscopy we show that co-expression of various AC isoforms (ACI, ACII, ACV, ACVI) also leads to GFP-RGS2 recruitment to the plasma membrane. Bioluminescence Resonance Energy Transfer (BRET) was also used to examine physical interactions between RGS2 and components of the Gs-signalling pathway. A BRET signal was detected between fusion constructs of RGS2-Renilla luciferase (energy donor) and Gsalpha-GFP (energy acceptor) co-expressed in HEK 293 cells. BRET was also observed between GFP-RGS2 and ACII or ACVI fused to Renilla luciferase. Additionally, RGS2 was found to interact with the beta2AR. Purified RGS2 selectively bound to the third intracellular loop of the beta2AR in GST pulldown assays, and a BRET signal was observed between GFP-RGS2 and beta2AR fused to Renilla luciferase when these two proteins were co-expressed together with either ACIV or ACVI. This interaction was below the limit of detection in the absence of co-expressed AC, suggesting that the effector enzyme stabilized or promoted binding between the receptor and the RGS protein inside the cell. Taken together, these results suggest the possibility that RGS2 might bind to a receptor-G protein-effector signalling complex to regulate Gs-dependent cAMP production.
[Show abstract][Hide abstract] ABSTRACT: Regulators of G-protein signaling (RGS) proteins act directly on Galpha subunits to increase the rate of GTP hydrolysis and to terminate signaling. However, the mechanisms involved in determining their specificities of action in cells remain unclear. Recent evidence has raised the possibility that RGS proteins may interact directly with G-protein-coupled receptors to modulate their activity. By using biochemical, fluorescent imaging, and functional approaches, we found that RGS2 binds directly and selectively to the third intracellular loop of the alpha1A-adrenergic receptor (AR) in vitro, and is recruited by the unstimulated alpha1A-AR to the plasma membrane in cells to inhibit receptor and Gq/11 signaling. This interaction was specific, because RGS2 did not interact with the highly homologous alpha1B- or alpha1D-ARs, and the closely related RGS16 did not interact with any alpha1-ARs. The N terminus of RGS2 was required for association with alpha1A-ARs and inhibition of signaling, and amino acids Lys219, Ser220, and Arg238 within the alpha1A-AR i3 loop were found to be essential for this interaction. These findings demonstrate that certain RGS proteins can directly interact with preferred G-protein-coupled receptors to modulate their signaling with a high degree of specificity.
Journal of Biological Chemistry 08/2005; 280(29):27289-95. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: RGS proteins serve as GTPase-activating proteins and/or effector antagonists to modulate Galpha signaling events. In live cells, members of the B/R4 subfamily of RGS proteins selectively modulate G protein signaling depending on the associated receptor (GPCR). Here we examine whether GPCRs selectively recruit RGS proteins to modulate linked G protein signaling. We report the novel finding that RGS2 binds directly to the third intracellular (i3) loop of the G(q/11)-coupled M1 muscarinic cholinergic receptor (M1 mAChR; M1i3). This interaction is selective because closely related RGS16 does not bind M1i3, and neither RGS2 nor RGS16 binds to the G(i/o)-coupled M2i3 loop. When expressed in cells, RGS2 and M1 mAChR co-localize to the plasma membrane whereas RGS16 does not. The N-terminal region of RGS2 is both necessary and sufficient for binding to M1i3, and RGS2 forms a stable heterotrimeric complex with both activated G(q)alpha and M1i3. RGS2 potently inhibits M1 mAChR-mediated phosphoinositide hydrolysis in cell membranes by acting as an effector antagonist. Deletion of the N terminus abolishes this effector antagonist activity of RGS2 but not its GTPase-activating protein activity toward G(11)alpha in membranes. These findings predict a model where the i3 loops of GPCRs selectively recruit specific RGS protein(s) via their N termini to regulate the linked G protein. Consistent with this model, we find that the i3 loops of the mAChR subtypes (M1-M5) exhibit differential profiles for binding distinct B/R4 RGS family members, indicating that this novel mechanism for GPCR modulation of RGS signaling may generally extend to other receptors and RGS proteins.
Journal of Biological Chemistry 06/2004; 279(20):21248-56. · 4.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Palmitoylation refers to the covalent attachment of a 16-carbon fatty acid to cysteine residues of proteins. This modification occurs on many intracellular signaling proteins including regulators of G protein signaling proteins (RGS). Palmitoylation mediates the interaction of proteins with membranes and other proteins and can control the biological activity of a protein. Palmitate attachment occurs through a labile thioester bond and is readily reversible in cells, thus providing a particularly important means for protein regulation. This chapter presents protocols for investigating RGS protein palmitoylation in mammalian cells. The RGS protein of interest is heterologously expressed in HEK293 cells, and cells are metabolically labeled with [3H]palmitate. The RGS protein is isolated from fractionated cells by immunoprecipitation and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and fluorography to determine if [3H] has been incorporated. To confirm that the radiolabeled fatty acid is linked to the protein through a thioester bond, labeled proteins are treated with neutral hydroxylamine. Oxyester-linked palmitate, which is occasionally found on serine and threonine residues, is insensitive to this treatment, whereas thioesters are sensitive. To verify that incorporated radiolabel is palmitate, the protein is treated with base, which also cleaves thioester bonds. The resulting lipids are extracted from the sample, then analyzed by chromatography.