Characterization of the molecular mechanisms of the coupling between intracellular loops of prostacyclin receptor with the C-terminal domain of the Galphas protein in human coronary artery smooth muscle cells

The C-terminal domain of the Gs protein alpha subunit (Galphas Ct) and the first intracellular loop (iLP1) of prostacyclin receptor (IP) have been predicted to be involved in the receptor signaling mediated through the IP/Gs protein coupling by our previous NMR studies using synthetic peptides. To test whether the results of the peptide studies can be applied to the protein interaction between the IP receptor and the Gs protein in cells, a minigene technique was used to construct cDNAs that encoded either the amino acid residues of the Galphas or that of the individual intracellular loops of the IP receptor. The effects of the minigene-expressed protein fragments on cAMP production mediated by the IP/Gs coupling were evaluated through experiments that co-expressed peptides either through the Galphas Ct or the IP intracellular loops with the IP receptor in HEK293 cells. The first (iLP1) and third (iLP3) IP intracellular loops, as well as the Galphas Ct, which are important to the IP/Gs coupling-mediated signaling, were identified by the significant reduction of cAMP production when the corresponding peptides were expressed in the cells. Furthermore, the cAMP productions were significantly impaired in Galphas-knockout cells co-expressing the IP receptor with the Galphas C-terminal mutants (E392A, L393A and L394A), compared with the Galphas wild type. Blocking of the endogenous IP/Gs coupling by the minigene-expressed peptides of the Galphas CT, iLP1 and iLP3 was further observed in the human coronary artery smooth muscle cells (SMCs). These results indicate that the three residues (E392-L394) of the Galphas protein predicted from NMR peptide studies, and the IP iLP1 and iLP3 play important roles in the Galphas-mediated IP receptor signaling in the cells, which may be a general binding site for the corresponding regions of the other prostanoid receptors that couple to Gs protein.     

Solution structure of the first intracellular loop of prostacyclin receptor and implication of its interaction with the C-terminal segment of G alpha s protein

The amino acids (residues 39-51) responsible for the interaction between the first intracellular loop (iLP1) of the human prostacyclin receptor (IP) and G alpha s protein have been identified [Zhang, L., Huang, G., Wu, J., and Ruan, K. H. (2005) Biochemistry 44, 11389-11401]. To further characterize the structural/functional relationship of the iLP1 in coupling with the G alpha s protein, the solution structures of a constrained peptide (IP iLP1) that mimicked the iLP1 of the IP receptor in the absence and presence of a synthetic peptide, corresponding to the C-terminal 11 residues (Q384-L394 in the protein sequence) of the G alpha s protein (G alpha s-Ct), were determined by 2D 1H NMR spectroscopy. The NMR solution structural model of the iLP1 domain showed two turn structures in residues Arg41-Ala44 and Arg45-Phe49 with the conserved Arg45 at the center. The conformational change of the side chain of the Arg45 was observed upon the addition of the G alpha s-Ct peptide. On the other hand, the solution structural models of the G alpha s-Ct peptide in the absence and presence of the IP iLP1 peptide were also determined. The N-terminal domain (Q384-Q390 in the G alpha s protein) of the peptide adopted an alpha-helical conformation. However, the helical structure of the C-terminal domain (Q390-E392 in the G alpha s protein) of the peptide was destabilized upon addition of the IP iLP1 peptide. These structural studies have implied that there are direct or indirect contacts between the IP iLP1 domain and the C-terminal residues of the G alpha s protein in the receptor/G protein coupling. The possible charge and hydrophobic interactions between the two peptides were also discussed. These data prompted intriguing speculations on the IP/G alpha s coupling which mediates vasodilatation and inhibition of platelet aggregation.     

Structural and functional characterization of the first intracellular loop of human thromboxane A2 receptor

The conformation of a constrained peptide mimicking the putative first intracellular domain (iLP1) of thromboxane A(2) receptor (TP) was determined by (1)H 2D NMR spectroscopy. Through completed assignments of TOCSY, DQF-COSY, and NOESY spectra, a NMR structure of the peptide showed a beta-turn in residues 56-59 and a short helical structure in the residues 63-66. It suggests that residues 63-66 may be part of the second transmembrane domain (TM), and that Arg60, in an exposed position on the outer surface of the loop, may be involved in signaling through charge contact with Gq protein. The sequence alignment of Lys residue in the same position of other prostanoid receptors mediates different G protein couplings, suggesting that the chemical properties of Arg and Lys may also affect the receptor signaling activity. These hypotheses were supported by mutagenesis studies, in which the mutant of Arg60Leu completely lost activity in increasing intracellular calcium level through Gq coupling, and the mutant of Arg60Lys retained only about 35% signaling activity. The difference between the side chain functions of Lys and Arg in effecting the signaling was discussed.     

Evidence of the residues involved in ligand recognition in the second extracellular loop of the prostacyclin receptor characterized by high resolution 2D NMR techniques

In previous studies, we have determined the solution structure of the second extracellular loop (eLP(2)) of the human thromboxane A(2) receptor (TP) and identified the residues in the eLP(2) domain involved in ligand recognition, by using a combination of approaches including a constrained synthetic peptide, 2D NMR spectroscopy, and recombinant proteins. These findings led us to hypothesize that the specific ligand recognition sites may be localized in the eLP(2) for all the prostanoid receptors. To test this hypothesis, we have investigated the ligand recognition site for another prostanoid receptor, the prostacyclin receptor (IP), which mediates an opposite biological function compared to that of the TP receptor. The identification of the interaction between the IP receptor and its agonist, iloprost, was achieved with a constrained synthetic peptide mimicking the eLP(2) region of the receptor. The IP eLP(2) segment was designed and synthesized to form a constrained loop, using a homocysteine disulfide bond connecting the ends of the peptide, based on the distance predicted from the IP receptor model created by homology modeling using the crystal structure of bovine rhodopsin as a template. The evidence of the constrained IP eLP(2) interaction with iloprost was found by the identification of the conformational changes of the eLP(2) induced by iloprost using fluorescence spectroscopy, and was further confirmed by 1D and 2D 1H NMR experiments. In addition, the IP eLP(2)-induced structure of iloprost in solution was elucidated through a complete assignment of the 2D 1H NMR spectra for iloprost in the presence of the IP eLP(2) segment. In contrast, no ordered structure was observed in the 2D 1H NMR experiments for iloprost alone in solution. These studies not only identified that the eLP(2) segment of the IP receptor is involved in ligand recognition, but also solved the 3D solution structure of the bound-form of iloprost, which could be used to study the receptor-ligand interaction in structural terms.     

Assembling NMR structures for the intracellular loops of the human thromboxane A2 receptor: implication of the G protein-coupling pocket

It has been reported that the multiple intracellular loops (iLPs) of the thromboxane A₂ receptor (TP) are involved in the receptor G protein coupling. In this study, a high-resolution 2D NMR technique was used to determine the 3D structures of the first, second, and third iLPs of the TP using synthetic peptides constrained into the loop structures. 2D ¹H NMR spectra, TOCSY and NOESY were obtained for the two peptides from proton NMR experiments. The NMR data was processed and assigned through the Felix 2000 program. Standard methods were used to acquire sequence-specific assignments. Structure calculations were processed through DGII and NMR refinement programs within the Insight II program. We were able to calculate and use the NOE constraints to obtain the superimposed structure of 10 structures for each iLP peptide. The NMR-determined structures of the iLP peptides were used to refine a homology model of the TP. A 3D G-protein-binding cavity, formed by the three intracellular loops, was predicted by the docking of the C-terminal domain of the Gαq. Based on the structural model and the previous mutagenesis studies, the residues, R130, R60, C223, F138, L360, V361, E358 and Y359, which are important for interaction with the G protein, were further highlighted. These results reveal the possibly important molecular mechanisms in TP signaling and provide structural information to characterize other prostanoid receptor signalings.     

Identification of residues important for ligand binding of thromboxane A2 receptor in the second extracellular loop using the NMR experiment-guided mutagenesis approach

The second extracellular loop (eLP2) of the thromboxane A(2) receptor (TP) had been proposed to be involved in ligand binding. Through two-dimensional (1)H NMR experiments, the overall three-dimensional structure of a constrained synthetic peptide mimicking the eLP2 had been determined by our group (Ruan, K.-H., So, S.-P., Wu, J., Li, D., Huang, A., and Kung, J. (2001) Biochemistry 40, 275-280). To further identify the residues involved in ligand binding, a TP receptor antagonist, SQ29,548 was used to interact with the synthetic peptide. High resolution two-dimensional (1)H NMR experiments, NOESY, and TOCSY were performed for the peptide, SQ29,548, and peptide with SQ29,548, respectively. Through completed (1)H NMR assignment and by comparing the different spectra, extra peaks were observed on the NOESY spectrum of the peptide with SQ29,548, which implied the contacts between residues of eLP2 at Val(176), Leu(185), Thr(186), and Leu(187) with SQ29,548 at position H2, H7, and H8. Site-directed mutagenesis was used to confirm the possible ligand-binding sites on native human TP receptor. Each of the four residues was mutated to the residues either in the same group, with different structure or different charged. The mutated receptors were then tested for their ligand binding activity. The receptor with V176L mutant retained binding activity to SQ29,548. All other mutations resulted in decreased or lost binding activity to SQ29,548. These mutagenesis results supported the prediction from NMR experiments in which Val(176), Leu(185), Thr(186), and Leu(187) are the possible residues involved in ligand binding. This information facilitates the understanding of the molecular mechanism of thromboxane A(2) binding to the important receptor and its signal transduction.     

Description of the low‐affinity interaction between nociceptin and the second extracellular loop of its receptor by fluorescence and NMR spectroscopies

The second extracellular loop (ECL2) of the Noc receptor has been proposed to be involved in ligand binding and selectivity. The interaction of Noc with a constrained cyclic synthetic peptide, mimicking the ECL2, has been studied using fluorescence and NMR spectroscopies. Selective binding was shown with a dissociation constant of ∼10 µM (observed with the constrained cyclic loop and not with the open chain), and residues involved in ligand binding and selectivity have been identified. This bimolecular complex is stabilized by (i) ionic interactions between the two Noc basic motives and the ECL2 acidic residues; (ii) hydrophobic contacts involving Noc FGGF N‐terminal sequence and an ECL2 tryptophane residue. Our data confirm that Noc receptor's ECL2 contributes actively to ligand binding and selectivity by providing the peptidic ligand with a low affinity‐binding site. Copyright © 2008 European Peptide Society and John Wiley & Sons, Ltd.     

Solution structure of the second extracellular loop of human thromboxane A2 receptor

Thromboxane A(2) receptor (TP receptor), a prostanoid receptor, belongs to the G protein-coupled receptor family, composed of three intracellular loops and three extracellular loops connecting seven transmembrane helices. The highly conserved extracellular domains of the prostanoid receptors were found in the second extracellular loop (eLP(2)), which was proposed to be involved in ligand recognition. The 3D structure of the eLP(2) would help to further explain the ligand binding mechanism. Analysis of the human TP receptor model generated from molecular modeling based on bacteriorhodopsin crystallographic structure indicated that about 12-14 A separates the N- and C-termini of the extra- and intracellular loops. Synthetic loop peptides whose termini are constrained to this separation are presumably more likely to mimic the native loop structure than the corresponding loop region peptide with unrestricted ends. To test this new concept, a peptide corresponding to the eLP(2) (residues 173-193) of the TP receptor has been made with the N- and C-termini connected by a homocysteine disulfide bond. Through 2D nuclear magnetic resonance (NMR) experiments, complete (1)H NMR assignments, and structural construction, the overall 3D structure of the peptide was determined. The structure shows two beta-turns at residues 180 and 185. The distance between the N- and C-termini of the peptide shown in the NMR structure is 14.2 A, which matched the distance (14.5 A) between the two transmembrane helices connecting the eLP(2) in the TP receptor model. This suggests that the approach using the constrained loop peptides greatly increases the likelihood of solving the whole 3D structures of the extra- and the intracellular domains of the TP receptor. This approach may also be useful in structural studies of the extramembrane loops of other G protein-coupled receptors.     

NMR structure of alpha-bungarotoxin free and bound to a mimotope of the nicotinic acetylcholine receptor

A combinatorial library approach was used to produce synthetic peptides mimicking the snake neurotoxin binding site of nicotinic receptors. Among the sequences, which inhibited binding of alpha-bungarotoxin to muscle and neuronal nicotinic receptors, HRYYESSLPWYPD, a 14-amino acid peptide with considerably higher toxin-binding affinity than the other synthesized peptides, was selected, and the structure of its complex with the toxin was analyzed by NMR. Comparison of the solution structure of the free toxin and its complex with this peptide indicated that complex formation induced extensive conformational rearrangements mainly at finger II and the carboxy terminus of the protein. The peptidyl residues P10 and Y4 seemed to be critical for peptide folding and complex stability, respectively. The latter residue of the peptide strongly interacted with the protein by entering a small pocket delimited by D30, C33, S34, R36, and V39 toxin side chains.     

A profile of the residues in the second extracellular loop that are critical for ligand recognition of human prostacyclin receptor

The residues in the second extracellular loop (eLP2) of the prostanoid receptors, which are important for specific ligand recognition, were previously predicted in our earlier studies of the thromboxane A2 receptor (TP) using a combination of NMR spectroscopy and recombinant protein approaches. To further test this hypothesis, another prostanoid receptor, the prostacyclin receptor (IP), which has opposite biological characteristics to that of TP, was used as a model for these studies. A set of recombinant human IPs with site-directed mutations at the nonconserved eLP2 residues were constructed using an Ala-scanning approach, and then expressed in HEK293 and COS-7 cells. The expression levels of the recombinant receptors were six-fold higher in HEK293 cells than in COS-7 cells. The residues important for ligand recognition and binding within the N-terminal segment (G159, Q162, and C165) and the C-terminal segment (L172, R173, M174, and P179) of IP eLP2 were identified by mutagenesis analyses. The molecular mechanisms for the specific ligand recognition of IP were further demonstrated by specific site-directed mutagenesis using different amino acid residues with unique chemical properties for the key residues Q162, L172, R173, and M174. A comparison with the corresponding functional residues identified in TP eLP2 revealed that three (Q162, R173, and M174) of the four residues are nonconserved, and these are proposed to be involved in specific ligand recognition. We discuss the importance of G159 and P179 in ligand recognition through configuration of the loop conformation is discussed. These studies have further indicated that characterization of the residues in the eLP2 regions for all eight prostanoid receptors could be an effective approach for uncovering the molecular mechanisms of the ligand selectivities of the G-protein-coupled receptors.     

Structure-function relationships in the activation of platelet thrombin receptors by receptor-derived peptides.

According to present models, thrombin activates platelets by cleaving its receptors after Arg41, creating a new N terminus which acts as a tethered ligand. In support of this model, a peptide (SFLLRNPNDKYEPF or TRP42/55) corresponding to residues 42-55 has been shown to activate the receptor. In the present studies, the structural basis for thrombin receptor activation was examined using fragments of this peptide, as well as variants of the peptide with selected amino acid substitutions. The results show that the features of SFLLRNPNDKYEPF required to mimic the effects of thrombin reside within the first 6 residues, SFLLRN. A hexapeptide comprised of these residues was approximately 5 times more potent than the parent peptide in assays of platelet aggregation and, in addition, caused tyrosine phosphorylation, inhibition of cAMP formation, and an increase in cytosolic Ca2+. Omission of either the Ser residue or the Arg and Asn residues greatly diminished peptide activity, as did the substitution of Ala for Phe or Arg. Substitution of Ala for Ser or the initial Leu, on the other hand, had little adverse effect. The inactive peptides SALLRN and NPNDKYEPF had no effect on platelet activation initiated by SFLLRN, but FLLRN inhibited platelet aggregation in response to both SFLLRN and thrombin. These results suggest that within SFLLRN the Phe and Arg residues are particularly important and that Phe must be preceded by another amino acid, the identity of which is not tightly constrained. This observation and comparisons with the homologous domains of proteins whose tertiary structure is known were used to predict the conformation of the SFLLR sequence. The model which emerged suggests that the SFLLR domain may be part of an extended beta structure in the intact receptor and that cleavage by thrombin causes it to contract and assume a modified helical configuration. In this predicted conformation the side chains of Phe and Arg point in the same direction, potentially into a pocket formed by the remainder of the receptor.     

Determination of the membrane contact residues and solution structure of the helix F/G loop of prostaglandin I2 synthase

From our topological arrangement model of prostaglandin I(2) synthase (PGIS) created by homology modeling and topology studies, we hypothesized that the helix F/G loop of PGIS contains a membrane contact region distinct from the N-terminal membrane anchor domain. To provide direct experimental data we have explored the relationship between the endoplasmic reticulum (ER) membrane and the PGIS F/G loop using a constrained synthetic peptide to mimic PGIS residues 208-230 cyclized on both ends through a disulfide bond with added Cys residues. The solution structure and the residues important for membrane contact of the constrained PGIS F/G loop peptide were investigated by high-resolution 1H two-dimensional nuclear magnetic resonance (2D NMR) experiments and a spin label incorporation technique. Through the combination of 2D NMR experiments in the presence of dodecylphosphocholine (DPC) micelles used to mimic the membrane environment, complete 1H NMR assignments of the F/G loop segment have been obtained and the solution structure of the peptide has been determined. The PGIS F/G loop segment shows a defined helix turn helix conformation, which is similar to the three-dimensional crystallography structure of P450BM3 in the corresponding region. The orientation and the residues contacted with the membrane of the PGIS F/G loop were evaluated from the effect of incorporation of a spin-labeled 12-doxylstearate into the DPC micelles with the peptide. Three residues in the peptide corresponding to the PGIS residues L217 (L11), L222 (L16), and V224 (V18) have been demonstrated to contact the DPC micelles, which implies that the residues are involved in contact with the ER membrane in the native membrane-bound PGIS. These results provided the first experimental evidence to localize the membrane contact residues in the F/G loop region of microsomal P450 and are valuable to further define and understand the membrane topology of PGIS and those of other microsomal P450s in the native membrane environment.     

A peptide from the extension of Lys-tRNA synthetase binds to transfer RNA and DNA

Eukaryotic aminoacyl-tRNA synthetases have dispensable extensions appended at the amino- or carboxyl-terminus as compared to their bacterial counterparts. While a synthetic peptide corresponding to the basic amino-terminal extension in yeast Asp-tRNA synthetase binds to DNA, the extension in the intact protein evidently binds to tRNA and enhances the tRNA specificity of Asp-tRNA synthetase. On the other hand, the amino-terminal extension in human Asp-tRNA synthetase, both within the intact protein and as a synthetic peptide, binds to tRNA. Here, the tRNA binding of a synthetic peptide, hKRS(Arg(25)-Glu(42)), corresponding to the amino-terminal extension of human Lys-tRNA synthetase (hKRS) was analyzed. This basic peptide bound to tRNA(Phe) and the apparent-binding constant increased with increasing concentrations of Mg(2+). The hKRS peptide also bound to DNA and polyphosphate; however, the apparent DNA-binding constants decreased at increasing concentrations of Mg(2+). The ability of the hKRS peptide to adopt alpha-helical conformation was demonstrated by NMR and circular dichroism. A Lys-rich peptide derived from the elongation factor 1alpha was also examined and bound to DNA but not to tRNA.     

Prostanoid Receptors Involved in Regulation of the Beating Rate of Neonatal Rat Cardiomyocytes

Although prostanoids are known to be involved in regulation of the spontaneous beating rate of cultured neonatal rat cardiomyocytes, the various subtypes of prostanoid receptors have not been investigated in detail. In our experiments, prostaglandin (PG)F_2α and prostanoid FP receptor agonists (fluprostenol, latanoprost and cloprostenol) produced a decrease in the beating rate. Two prostanoid IP receptor agonists (iloprost and beraprost) induced first a marked drop in the beating rate and then definitive abrogation of beating. In contrast, the prostanoid DP receptor agonists (PGD₂ and BW245C) and TP receptor agonists (U-46619) produced increases in the beating rate. Sulprostone (a prostanoid EP₁ and EP₃ receptor agonist) induced marked increases in the beating rate, which were suppressed by SC-19220 (a selective prostanoid EP₁ antagonist). Butaprost (a selective prostanoid EP₂ receptor agonist), misoprostol (a prostanoid EP₂ and EP₃ receptor agonist), 11-deoxy-PGE₁ (a prostanoid EP₂, EP₃ and EP₄ receptor agonist) did not alter the beating rate. Our results strongly suggest that prostanoid EP₁ receptors are involved in positive regulation of the beating rate. Prostanoid EP₁ receptor expression was confirmed by western blotting with a selective antibody. Hence, neonatal rat cardiomyocytes express both prostanoid IP and FP receptors (which negatively regulate the spontaneous beating rate) and prostanoid TP, DP₁ and EP₁ receptors (which positively regulate the spontaneous beating rate).     

Identification of the residues in the helix F/G loop important to catalytic function of membrane-bound prostacyclin synthase

A topological model of prostaglandin I(2) synthase (PGIS) was created by homology modeling. This model, along with site-specific antibodies and other topology studies, has suggested that the residue(s) within helix F/G loop of PGIS may be involved in forming the substrate access channel and located in a position that influences the membrane-bound PGIS catalytic function (1). To test this hypothesis, we have explored an approach to identify the residues of the helix F/G loop important to enzyme activity of the membrane-bound PGIS by a combination of 2-D NMR experiment and mutagenesis methods. Using the distance measured from the model as a guide, the helix F/G loop was mimicked in a synthetic peptide by introducing a spacer to maintain a distance of about 7 A between the N- and the C-termini (PGIS residues 208 and 230). The peptide was used to interact with the enzyme substrate analogue, U46619. High-resolution 2-D NMR experiments were performed to determine the contacts between the peptide and U46619. The interaction between the constrained F/G loop peptide and U46619 was confirmed by the observation of the conformational changes of the peptide and U46619 using the comparison of the cross-peaks between the NOESY spectra of U46619 with the peptide, without the peptide, and the peptide alone. Through the combination of the 2-D NMR experiments, completed (1)H NMR assignments of the F/G loop segment in the presence and absence of U46619 were obtained, and these data were used to predict the contact residues (Leu214 and Pro215) of the F/G loop with PGIS substrate. The predicted influence of residues on enzyme catalytic activity in membrane-bound environments was confirmed by the mutagenesis of the F/G loop residues of human PGIS. These observations support that the F/G loop is involved in forming the substrate access channel for membrane-bound PGIS and suggests that the NMR experiment-based mutagenesis approach may be applied to study structure and function relationships for other proteins.     

Role of the membrane interface on the conformation of the caveolin scaffolding domain: a CD and NMR study

Circular dichroism (CD) and NMR spectroscopy were used to study the conformational properties of two synthetic peptides, D82-R101 and D82-I109, encompassing the caveolin scaffolding domain (D82-R101), in the presence of dodecylphosphocholine (DPC) micelles. Our data show that a stable helical conformation of the caveolin scaffolding domain in a membrane mimicking system is only obtained for the peptide including the L102-I109 hydrophobic stretch, a part of the caveolin intra-membrane domain. Through chemical shift variations, an ensemble of six residues of the D82-L109 peptide, mainly located in the V(94)TKYWFYR(101) motif were found to detect the presence of phosphatidylserine solubilized in DPC micelles. Our results constitute a first step for elucidating at a residue level the conformational properties of the central region of the caveolin-1 protein.     

Interaction between amyloid beta peptide and an aggregation blocker peptide mimicking islet amyloid polypeptide

Assembly of amyloid-beta peptide (Aβ) into cytotoxic oligomeric and fibrillar aggregates is believed to be a major pathologic event in Alzheimer's disease (AD) and interfering with Aβ aggregation is an important strategy in the development of novel therapeutic approaches. Prior studies have shown that the double N-methylated analogue of islet amyloid polypeptide (IAPP) IAPP-GI, which is a conformationally constrained IAPP analogue mimicking a non-amyloidogenic IAPP conformation, is capable of blocking cytotoxic self-assembly of Aβ. Here we investigate the interaction of IAPP-GI with Aβ40 and Aβ42 using NMR spectroscopy. The most pronounced NMR chemical shift changes were observed for residues 13-20, while residues 7-9, 15-16 as well as the C-terminal half of Aβ--that is both regions of the Aβ sequence that are converted into β-strands in amyloid fibrils--were less accessible to solvent in the presence of IAPP-GI. At the same time, interaction of IAPP-GI with Aβ resulted in a concentration-dependent co-aggregation of Aβ and IAPP-GI that was enhanced for the more aggregation prone Aβ42 peptide. On the basis of the reduced toxicity of the Aβ peptide in the presence of IAPP-GI, our data are consistent with the suggestion that IAPP-GI redirects Aβ into nontoxic "off-pathway" aggregates.     

The influence of basic residues on the substrate specificity of protein kinase C

The substrate specificity of protein kinase C has been examined using a series of synthetic peptide analogs of glycogen synthase, ribosomal protein S6, and the epidermal growth factor receptor. The glycogen synthase analog peptide Pro1-Leu-Ser-Arg-Thr-Leu-Ser-Val-Ala-Ala10 was phosphorylated at Ser7 with a Km of 40.3 microM. Peptide phosphorylation was strongly dependent on Arg4. When lysine was substituted for Arg4 the Km was increased approximately 20-fold. Addition of basic residues on either the NH2-terminal or COOH-terminal side of the phosphorylation site of the glycogen synthase peptide improved the kinetics of peptide phosphorylation. The analog Pro-Leu-Ser-Arg-Thr-Leu-Ser-Val-Ala-Ala-Lys-Lys was phosphorylated with a Km of 4.1 microM. Substitution of Ser7 with threonine increased the apparent Km to 151 microM. The truncated peptide Pro1-Leu-Ser-Arg-Thr-Leu-Ser-Val8 was phosphorylated with similar kinetic constants to the parent peptide, however, deletion of Val8 increased the apparent Km to 761 microM. The ribosomal peptide S6-(229-239) was phosphorylated with a Km of approximately 0.5 microM predominantly on Ser236 and is one of the most potent synthetic peptide substrates reported for a protein kinase. The apparent Km for S6 peptide phosphorylation was increased by either deletion of the NH2-terminal 3 residues Ala229-Arg-231 or by substitution of Arg238 on the COOH-terminal side of the phosphorylation site with alanine. This analog peptide, [Ala238]S6-(229-239) was phosphorylated with an approximate 6-fold reduction in Vmax and a switch in the preferred site of phosphorylation from Ser236 to Ser235. These results support the concept that basic residues on both sides of the phosphorylation site can have an important influence on the kinetics of phosphorylation and site specificity of protein kinase C.     

Solution structure of the third extracellular loop of human thromboxane A2 receptor

The extracellular domains of the thromboxane A2 receptor (TP receptor) were found to be involved in the specific ligand recognition. Determination of the three-dimensional (3D) structure of the extracellular loops would help to explain the mechanism of the ligand binding to its receptor with regard to the tertiary structure. Based on our previous studies on the extracellular loop of the human TP receptor, the synthetic loop peptides, whose termini are constrained to 10 to 14-A separations, are more likely to mimic the native structure of the extracellular loops. In this study, a peptide with the sequence of the third extracellular loop (eLP3, residues 271-289) of the TP receptor was synthesized, and its termini were constrained by the formation of a disulfide bond between the additional homocysteines located at both ends. Fluorescence spectroscopic studies showed that the fluorescence intensity of this constrained loop peptide could be increased by the addition of SQ29,548, a TP receptor antagonist, which indicated the interaction between the peptide and the ligand. The structure of this peptide was then studied by two-dimensional 1H nuclear magnetic resonance (NMR) spectroscopy. 1H NMR assignments of the peptide were obtained and structure constraints were derived from nuclear Overhauser effects and J-coupling constants. The solution structure of the peptide was then calculated based on these constraints. The overall structure shows a beta turn from residues 278 to 281. It also shows a distance of 9.45A between the ends of the N and C termini of the peptide, which agrees with the distance between the two residues at the ends of the transmembrane helices connecting the eLP3 on the TP receptor working model generated using molecular modeling, based on the crystal structure of bovine rhodopsin. These results provide valuable information for the characterization of the complete 3D structure of the extracellular domains of the human TP receptor.     

Solution structure of GAP SH3 domain by 1H NMR and spatial arrangement of essential Ras signaling-involved sequence.

Src homology 3 (SH3) domains are found in numerous cytoplasmic proteins involved in intracellular signal transduction. We used 2-D 1H NMR to determine the structure of the SH3 domain of the guanosine triphosphatase-activating protein (GAP), an essential component of the Ras signaling pathway. The structure of the GAP SH3 domain (275-350) was found to be a compact beta-barrel made of six antiparallel beta-strands arranged in two roughly perpendicular beta-sheets with the acidic residues located at the surface of the protein. The Trp317, Trp319, Thr321 and Leu323 residues belonging to the sequence (317-326), which was shown to be essential for Ras signaling, formed two nearby lipophilic bulges followed by a hydrophilic domain (Arg324-Asp326). These structural data could be used to characterize the still unidentified downstream components of GAP, which are involved in Ras signaling, and to rationally design inhibitors of this pathway.