Study of interaction between agonists and asn293 in helix VI of human beta(2)-adrenergic receptor.

Previously, we demonstrated the involvement of Asn293 in helix VI of the human beta(2)-adrenergic receptor in stereoselective agonist recognition and activation. In the present study, we have further explored the role of this residue by synthesizing derivatives of isoproterenol and clenbuterol, two beta-adrenergic receptor agonists. We analyzed their efficacy and affinity on the wild-type and a mutant receptor (Asn293Leu). Each compound had similar efficacy (tau values) on both the wild-type and mutant receptor, although tau values varied considerably among the eight compounds studied. It appeared that one derivative of isoproterenol, but not of clenbuterol, showed a gain in affinity from the wild type to the mutant receptor. This derivative had a methyl substituent instead of the usual beta-OH group in the aliphatic side chain of isoproterenol, compatible with the more lipophilic nature of the leucine side chain. Such a "gain of function" approach through a combination of synthetic chemistry with molecular biology, may be useful to enhance our insight into the precise atomic events that govern ligand-receptor interactions.

Interactions of phosducin with the subunits of G-proteins. Binding to the alpha as well as the betagamma subunits.

The high affinity interactions of phosducin with G-proteins involve binding of phosducin to the G-protein betagamma subunits. Here we have investigated whether phosducin interacts also with G-protein alpha subunits. Interactions of phosducin with the individual subunits of Go were measured by retaining phosducin-G-protein subunit complexes on columns containing immobilized anti-phosducin antibodies. Both the alpha and the beta subunits of trimeric Go were specifically retained by the antibodies in the presence of phosducin. This binding was almost completely abolished for both subunits following protein kinase A-mediated phosphorylation of phosducin and was reduced, more for alpha than for beta subunits, by the stable GTP analog guanosine 5'-(3-O-thio)triphosphate. Isolated alphao was also retained on the columns in the presence of phosducin but not in the presence of protein kinase A-phosphorylated phosducin. Likewise, purified G-protein betagamma subunit complexes as well as purified alpha subunits of Go and Gt were precipitated together with His6-tagged phosducin with nickel-agarose; this co-precipitation occurred concentration-dependently, with apparent affinities for phosducin of 55 nM (Gbetagamma), 110 nM (alphao), and 200 nM (alphat). In functional experiments, the steady state GTPase activity of isolated alphao was inhibited by phosducin by approximately 60% with an IC50 value of approximately 300 nM, whereas the GTPase activity of trimeric Go was inhibited by approximately 90% with an IC50 value of approximately 10 nM. Phosducin did not inhibit the GTP-hydrolytic activity of isolated alphao as measured by single-turnover assays, but it inhibited the release of GDP from alphao; the rate constant of GDP release was decreased approximately 40% by 500 nM phosducin, and the inhibition occurred with an IC50 value for phosducin of approximately 100 nM. These data suggest that phosducin binds with high affinity to G-protein betagamma subunits and with lower affinity to G-protein alpha subunits. We propose that the alpha subunit-mediated effects of phosducin might increase both the extent and the rapidity of its inhibitory effects compared with an action via the betagamma subunit complex alone.

A small region in phosducin inhibits G-protein betagamma-subunit function.

G-protein betagamma-subunits (G(betagamma)) are active transmembrane signalling components. Their function recently has been observed to be regulated by the cytosolic protein phosducin. We show here that a small fragment (amino acids 215-232) contained in the C-terminus of phosducin is sufficient for high-affinity interactions with G(betagamma). Corresponding peptides not only disrupt G(betagamma)-G(alpha) interactions, as defined by G(betagamma)-stimulated GTPase activity of alpha(o), but also other G(betagamma)-mediated functions. The NMR structure of a peptide encompassing this region shows a loop exposing the side chains of Glu223 and Tyr224, and peptides with a substitution of either of these amino acids show a complete loss of activity towards G(o). Mutation of this Tyr224 to Ala in full-length phosducin reduced the functional activity of phosducin to that of phosducin's isolated N-terminus, indicating the importance of this residue within the short, structurally defined C-terminal segment. This small peptide derived from phosducin, may represent a model of a G(betagamma) inhibitor, and illustrates the potential of small compounds to affect G(betagamma) functions.

Identification of a C-terminal binding site for G-protein betagamma-subunits in phosducin-like protein.

Phosducin-like protein (PhLP) has recently been identified as a ubiquitous inhibitor of G-protein betagamma-subunit (G betagamma)-mediated signaling, with an affinity about 5-fold lower than that of phosducin. The G betagamma binding site of phosducin has been suggested to be contained in its N-terminus. A region corresponding to this N-terminus is lacking in PhLP, suggesting that PhLP must utilize a different mode of G betagamma binding. To map the G betagamma binding site in PhLP, a series of deletion mutants were constructed, expressed in E. coli as glutathione S-transferase (GST) fusion proteins, and the purified fusion proteins were examined for their ability to attenuate G(o) GTPase activity. Progressive N-terminal truncations of PhLP caused only minor reductions in potency, whereas the complementary N-terminal PhLP fragments turned out to be inactive. We further identified a short C-terminal segment comprising residues 168 to 195 that inhibited G0 GTPase activity similar in efficacy and potency to full-length PhLP. This C-terminal fragment was also capable of antagonizing a second G betagamma-mediated function, the enhancement of rhodopsin phosphorylation by the beta-adrenergic receptor kinase. Taken together, these data indicate that PhLP interacts with G betagamma via a short C-terminal binding site which is distinct from that identified previously in phosducin.

Muscarinic acetylcholine receptors: structural basis of ligand binding and G protein coupling.

Muscarinic acetylcholine receptors (m1-m5) were studied by a combined molecular genetic/pharmacologic approach to elucidate the molecular characteristics of the ligand binding site and of the receptor domains involved in G protein coupling. Site-directed mutagenesis studies of the rat m3 muscarinic receptor suggest that the acetylcholine binding domain is formed by a series of hydrophilic amino acids located in the "upper" half of transmembrane domains (TM) III, V, VI, and VII. Moreover, we showed that mutational modification of a TM VI Asn residue (Asn507 in the rat m3 receptor sequence) which is characteristic for the muscarinic receptor family has little effect on high-affinity acetylcholine binding and receptor activation, but results in dramatic reductions in binding affinities for certain subclasses of muscarinic antagonists. The N-terminal portion of the third intracellular loop (i3) of muscarinic and other G protein-coupled receptors has been shown to play a central role in determining the G protein coupling profile of a given receptor subtype. Insertion mutagenesis studies with the rat m3 muscarinic receptor suggest that this region forms an amphiphilic alpha-helix and that the hydrophobic side of this helix represents an important G protein recognition surface. Further mutational analysis of this receptor segment showed that Tyr254 located at the N-terminus of the i3 loop of the m3 muscarinic receptor plays a key role in muscarinic receptor-induced Gq activation. The studies described here, complemented by biochemical and biophysical approaches, should eventually lead to a detailed structural model of the ligand-receptor-G protein complex.

Insertion mutagenesis as a tool to predict the secondary structure of a muscarinic receptor domain determining specificity of G-protein coupling.

The N-terminal segment of the third intracellular loop (i3) of muscarinic acetylcholine receptors and other G protein-coupled receptors has been shown to largely determine the G-protein coupling selectivity displayed by a given receptor subtype. Based on secondary-structure prediction algorithms, we have tested the hypothesis that this region adopts an alpha-helical secondary structure. Using the rat m3 muscarinic receptor as a model system, a series of five mutant receptors, m3(+1A) to m3(+5A) were created in which one to five additional alanine residues were inserted between the end of the fifth transmembrane domain and the beginning of i3. We speculated that this manipulation should lead to a rotation of the N-terminal segment of the i3 domain (if it is in fact alpha-helically arranged), thus producing pronounced effects on receptor/G protein coupling. Pharmacological analysis of the various mutant receptors expressed in COS-7 cells showed that m3(+1A), m3(+3A), and m3(+4A) retained strong functional activity, whereas m3(+2A) and m3(+5A) proved to be virtually inactive. Helical wheel models show that this pattern is fully consistent with the notion that the N-terminal portion of i3 forms an amphiphilic alpha-helix and that the hydrophobic side of this helix represents the G-protein recognition surface.

Functional role in ligand binding and receptor activation of an asparagine residue present in the sixth transmembrane domain of all muscarinic acetylcholine receptors.

The molecular mechanisms through which muscarinic receptors are activated upon binding of the neurotransmitter acetylcholine (ACh) are still poorly understood. Classical structure-function relationship studies have previously established that the ACh ester moiety plays a key role in muscarinic receptor recognition and activation. Consistent with this notion, all recently proposed three-dimensional muscarinic receptor models predict that an asparagine residue present in transmembrane domain VI of all muscarinic receptors is critically involved in the binding of the ACh ester moiety by means of hydrogen bonding. To test the correctness of this hypothesis, we created several mutant m3 muscarinic receptors in which this residue (Asn507) was replaced with alanine, serine, or aspartic acid. Radioligand binding studies with transfected COS-7 cells showed that, in contrast to the predictions made based on molecular modeling studies, all three mutant receptors were able to bind ACh and the structurally related muscarinic agonist, carbachol, with high affinities which differed from the corresponding wild type values by less than 5-fold. However, all three mutations led to dramatic reductions (235-28,300-fold) in binding affinities for certain subclasses of muscarinic antagonists including atropine-like agents and pirenzepine. The m3(Asn507-->Ala) and m3(Asn507-->Asp) mutant receptors were able to mediate carbachol-induced phosphatidylinositol hydrolysis in a fashion similar to that of the wild type receptor. Interestingly, the m3(Asn507-->Ser) mutant receptor displayed about 2-fold increased basal inositol phosphate levels, raising the possibility that it is constitutively active. In conclusion, our data suggest that the asparagine residue present in transmembrane domain VI of all muscarinic receptors is not critical for ACh binding and agonist-induced receptor activation, but plays a key role in the binding of certain subclasses of muscarinic antagonists.

Functional role of a cytoplasmic aromatic amino acid in muscarinic receptor-mediated activation of phospholipase C.

The N-terminal portion of the third intracellular loop (i3) of muscarinic acetylcholine and other G protein-coupled receptors has been shown to largely determine the G protein coupling profile of a given receptor subtype. Using the rat m3 muscarinic receptor as a model system, we have recently demonstrated that a tyrosine residue (Tyr-254), located at the beginning of the i3 domain, is critically involved in muscarinic receptor-mediated stimulation of phosphatidylinositol (PI) hydrolysis (Blüml, K., Mutschler, E., and Wess, J. (1994) J. Biol. Chem. 269, 402-405). This study was designed to investigate the functional role of this amino acid in further molecular detail. Replacement of Tyr-254 (rat m3 receptor) with alanine or exchange of its position with Ile-253 virtually abolished receptor-mediated stimulation of PI hydrolysis studied in transfected COS-7 cells. In contrast, substitution of Tyr-254 by other aromatic residues such as phenylalanine or tryptophan resulted in mutant receptors that behaved functionally similar to the wild type m3 receptor. Introduction of Tyr-254 into the corresponding position (Ser-210) of the m2 muscarinic receptor (which is only poorly coupled to PI turnover) did not result in an enhanced PI response. However, "reinsertion" of Tyr-254 into a functionally inactive chimeric m3/m2 muscarinic receptor (containing m2 receptor sequence at the N terminus of the i3 loop) yielded a mutant receptor that was able to stimulate PI hydrolysis to a similar maximum extent as the wild type m3 receptor. Taken together, our data provide strong evidence that muscarinic receptor-mediated stimulation of PI metabolism is critically dependent on the presence and proper positioning of an aromatic residue at the beginning of the i3 loop.

Identification of an intracellular tyrosine residue critical for muscarinic receptor-mediated stimulation of phosphatidylinositol hydrolysis.

Several lines of evidence suggest that the N-terminal portion of the third cytoplasmic loop (i3) of muscarinic and other G protein-coupled receptors is of pivotal importance for G protein recognition and activation. The present study was designed to identify specific amino acids within this domain required for muscarinic receptor-induced activation of G proteins mediating stimulation of phosphatidylinositol (PI) hydrolysis. Among the five mammalian muscarinic receptors (m1-m5), only the m1, m3, and m5 receptors are efficiently coupled to this second messenger pathway. Initially, we created a series of rat m3 receptor mutants in which short segments in the N terminus of the i3 loop were replaced with the corresponding m2 receptor sequences. The effect of these substitutions on m3 receptor-mediated stimulation of PI hydrolysis was studied in transiently transfected COS-7 cells. We found that a stretch of 4 amino acids (Arg252-Ile253-Tyr254-Lys255) located at the beginning of the i3 domain of the m3 muscarinic receptor is critically involved in receptor-mediated stimulation of PI hydrolysis. Further mutational analysis of this 4-amino acid segment by single amino acid substitutions demonstrated that only Tyr254 is essential for efficient activation of the PI pathway. This tyrosine residue is conserved among all PI-coupled muscarinic receptors as well as in many other biogenic amine and glycoprotein hormone receptors, suggesting that it may also play an important functional role in other G protein-coupled receptors.