Role of beta gamma subunits of G proteins in targeting the beta-adrenergic receptor kinase to membrane-bound receptors

Farnesylcysteine analogues inhibit chemotactic peptide receptor-mediated G-protein activation in human HL-60 granulocyte membranes

Analogues of S-prenylated cysteine like N-acetyl-S-trans,trans-farnesyl-L-cysteine (AFC) have previously been shown to inhibit the carboxyl methylation of proteins carrying a C-terminal S-prenylated cysteine residue and to block the endotoxin-activated serum-elicited chemotactic response of mouse macrophages. Here, we show that AFC inhibits both basal and formyl peptide receptor-stimulated binding of guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) to and hydrolysis of GTP by membranes of myeloid differentiated HL-60 granulocytes. Receptor-stimulated GTP[S] binding and GTP hydrolysis are more sensitive to AFC inhibition than basal G-protein functions. Inhibition of formyl peptide receptor-mediated G-protein activation is also observed for S-trans,trans-farnesyl-3-thiopropionic acid, but not for N-acetyl-S-trans-geranyl-L-cysteine, N-acetyl-L-cysteine, or the methyl ester of AFC, suggesting that the farnesyl moiety and the carboxyl group, but not the peptide bond of AFC are required for inhibition. The observations that exogeneous S-adenosyl-L-methionine is apparently not required for and S-adenosyl-L-homocysteine does not attenuate the inhibitory action of AFC raise the distinct possibility that AFC inhibits receptor-mediated G-protein interaction by a mechanism other than inhibition of protein carboxyl methylation.

Phosphorylation of muscarinic receptors: regulation by G proteins

Effects of G proteins on the phosphorylation of muscarinic receptors (mAChRs) have been examined. Cerebral but not atrial mAChRs were phosphorylated by any one of three types of protein kinase C and 4-6 mol of phosphate were incorporated per mol of mAChR, mostly in the 12-14 kDa from the carboxyterminus. Atrial mAChRs were better substrates of cAMP-dependent protein kinase than cerebral mAChRs. Phosphorylation of mAChRs by protein kinase C or cAMP-dependent protein kinase was not dependent on the presence of agonists and G proteins except that a slight inhibition by G proteins was observed probably because G proteins were also substrates of the two kinases. Agonist-dependent phosphorylation of atrial mAChRs or recombinant human mAChRs (m2 subtype) by a kinase (mAChR kinase), which is the same or very similar to beta adrenergic receptor kinase (beta ARK), was found to be regulated by the G proteins in a dual manner; stimulation by G protein beta gamma subunits and inhibition by G protein alpha beta gamma trimer. The inhibition by the G protein trimer is restored by addition of guanine nucleotides and is considered to be due to the formation of a ternary complex of agonist, mAChR and guanine nucleotide free G proteins. The stimulation by G protein beta gamma subunits was also observed for the light- or agonist-dependent phosphorylation of rhodopsin and beta AR by the mAChR kinase but not for the light-dependent phosphorylation of rhodopsin by rhodopsin kinase. The phosphorylation by beta ARK 1 was also found to be stimulated by G protein beta gamma subunits. The beta gamma subunit is considered to interact with the extra 130 amino acid residue carboxyterminal tail of beta ARK, which does not exist in rhodopsin kinase, and the interaction results in the activation of the kinase. We may assume that the G protein coupled receptor kinase is an effector of G protein beta gamma subunits and that one of the functions of beta gamma subunits is to stimulate the phosphorylation of G protein coupled receptors thereby facilitating their desensitization.

Expression, characterization and purification of soluble G-protein beta gamma dimers composed of defined subunits in baculovirus-infected insect cells

Recombinant beta 1 gamma 2 dimers of signal-transducing guanine nucleotide-binding proteins (G-proteins) carrying a mutation known to block isoprenylation of the gamma 2 subunit were expressed as a soluble protein in baculovirus-infected insect cells. The soluble beta gamma dimer was analyzed by sucrose density gradient centrifugation and purified to near homogeneity in the absence of detergents. The sedimentation velocity studies gave an S20,w value of 4.1 +/- 0.4 S. The two subunits segregated as a dimer upon sucrose density gradient centrifugation and purification by sequential ion exchange and hydroxylapatite chromatography. The results show that baculovirus-infected insect cells can be employed for high level production of pure G-protein beta gamma dimers suitable for functional and structural characterization.