Synthesis and potency of novel uracil nucleotides and derivatives as P2Y2 and P2Y6 receptor agonists

The phosphate, uracil, and ribose moieties of uracil nucleotides were varied structurally for evaluation of agonist activity at the human P2Y(2), P2Y(4), and P2Y(6) receptors. The 2-thio modification, found previously to enhance P2Y(2) receptor potency, could be combined with other favorable modifications to produce novel molecules that exhibit high potencies and receptor selectivities. Phosphonomethylene bridges introduced for stability in analogues of UDP, UTP, and uracil dinucleotides markedly reduced potency. Truncation of dinucleotide agonists of the P2Y(2) receptor, in the form of Up(4)-sugars, indicated that a terminal uracil ring is not essential for moderate potency at this receptor and that specific SAR patterns are observed at this distal end of the molecule. Key compounds reported in this study include 9, alpha,beta-methylene-UDP, a P2Y(6) receptor agonist; 30, Up(4)-phenyl ester and 34, Up(4)-[1]glucose, selective P2Y(2) receptor agonists; dihalomethylene phosphonate analogues 16 and 41, selective P2Y(2) receptor agonists; 43, the 2-thio analogue of INS37217 (P(1)-(uridine-5')-P(4)-(2'-deoxycytidine-5')tetraphosphate), a potent and selective P2Y(2) receptor agonist.

DLC-1 suppresses non-small cell lung cancer growth and invasion by RhoGAP-dependent and independent mechanisms

Expression of the tumor suppressor deleted in liver cancer-1 (DLC-1) is lost in non-small cell lung (NSCLC) and other human carcinomas, and ectopic DLC-1 expression dramatically reduces proliferation and tumorigenicity. DLC-1 is a multi-domain protein that includes a Rho GTPase activating protein (RhoGAP) domain which has been hypothesized to be the basis of its tumor suppressive actions. To address the importance of the RhoGAP function of DLC-1 in tumor suppression, we performed biochemical and biological studies evaluating DLC-1 in NSCLC. Full-length DLC-1 exhibited strong GAP activity for RhoA as well as RhoB and RhoC, but only very limited activity for Cdc42 in vitro. In contrast, the isolated RhoGAP domain showed 5- to 20-fold enhanced activity for RhoA, RhoB, RhoC, and Cdc42. DLC-1 protein expression was absent in six of nine NSCLC cell lines. Restoration of DLC-1 expression in DLC-1-deficient NSCLC cell lines reduced RhoA activity, and experiments with a RhoA biosensor demonstrated that DLC-1 dramatically reduces RhoA activity at the leading edge of cellular protrusions. Furthermore, DLC-1 expression in NSCLC cell lines impaired both anchorage-dependent and -independent growth, as well as invasion in vitro. Surprisingly, we found that the anti-tumor activity of DLC-1 was due to both RhoGAP-dependent and -independent activities. Unlike the rat homologue p122RhoGAP, DLC-1 was not capable of activating the phospholipid hydrolysis activity of phospholipase C-delta1. Combined, these studies provide information on the mechanism of DLC-1 function and regulation, and further support the role of DLC-1 tumor suppression in NSCLC.

Activation of human phospholipase C-eta2 by Gbetagamma

Phospholipase C-eta2 (PLC-eta2) was recently identified as a novel broadly expressed phosphoinositide-hydrolyzing isozyme [Zhou, Y., et al. (2005) Biochem. J. 391, 667-676; Nakahara, M., et al. (2005) J. Biol. Chem. 280, 29128-29134]. In this study, we investigated the direct regulation of PLC-eta2 by Gbetagamma subunits of heterotrimeric G proteins. Coexpression of PLC-eta2 with Gbeta 1gamma 2, as well as with certain other Gbetagamma dimers, in COS-7 cells resulted in increases in inositol phosphate accumulation. Gbeta 1gamma 2-dependent increases in phosphoinositide hydrolysis also were observed with a truncation mutant of PLC-eta2 that lacks the long alternatively spliced carboxy-terminal domain of the isozyme. To begin to define the enzymatic properties of PLC-eta2 and its potential direct activation by Gbetagamma, a construct of PLC-eta2 encompassing the canonical domains conserved in all PLCs (PH domain through C2 domain) was purified to homogeneity after expression from a baculovirus in insect cells. Enzyme activity of purified PLC-eta2 was quantified after reconstitution with PtdIns(4,5)P 2-containing phospholipid vesicles, and values for K m (14.4 microM) and V max [12.6 micromol min (-1) (mg of protein) (-1)] were similar to activities previously observed with purified PLC-beta or PLC-epsilon isozymes. Moreover, purified Gbeta 1gamma 2 stimulated the activity of purified PLC-eta2 in a concentration-dependent manner similar to that observed with purified PLC-beta2. Activation was dependent on the presence of free Gbeta 1gamma 2 since its sequestration in the presence of Galpha i1 or GRK2-ct reversed Gbeta 1gamma 2-promoted activation. The PH domain of PLC-eta2 is not required for Gbeta 1gamma 2-mediated regulation since a purified fragment encompassing the EF-hand through C2 domains but lacking the PH domain nonetheless was activated by Gbeta 1gamma 2. Taken together, these studies illustrate that PLC-eta2 is a direct downstream effector of Gbetagamma and, therefore, of receptor-activated heterotrimeric G proteins.

UDP is a competitive antagonist at the human P2Y14 receptor

G protein-coupled P2Y receptors (P2Y-R) are activated by adenine and uracil nucleotides. The P2Y(14) receptor (P2Y(14)-R) is activated by at least four naturally occurring UDP sugars, with UDP-glucose (UDP-Glc) being the most potent agonist. With the goal of identifying a competitive antagonist for the P2Y(14)-R, UDP was examined for antagonist activity in COS-7 cells transiently expressing the human P2Y(14)-R and a chimeric Galpha protein that couples Gi-coupled receptors to stimulation of phosphoinositide hydrolysis. UDP antagonized the agonist action of UDP-Glc, and Schild analysis confirmed that the antagonism was competitive (pK(B) = 7.28). Uridine 5'-O-thiodiphosphate also antagonized the human P2Y(14)-R (hP2Y(14)-R) with an apparent affinity similar to that of UDP. In contrast, no antagonist activity was observed with ADP, CDP, or GDP, and other uracil analogs also failed to exhibit antagonist activity. The antagonist activity of UDP was not observed at other hP2Y-R. In contrast to its antagonist action at the hP2Y(14)-R, UDP was a potent agonist (EC(50) = 0.35 muM) at the rat P2Y(14)-R. These results identify the first competitive antagonist of the P2Y(14)-R and demonstrate pharmacological differences between receptor orthologs.

Crystal structure of the multifunctional Gbeta5-RGS9 complex

Regulators of G-protein signaling (RGS) proteins enhance the intrinsic GTPase activity of G protein alpha (Galpha) subunits and are vital for proper signaling kinetics downstream of G protein-coupled receptors (GPCRs). R7 subfamily RGS proteins specifically and obligately dimerize with the atypical G protein beta5 (Gbeta5) subunit through an internal G protein gamma (Ggamma)-subunit-like (GGL) domain. Here we present the 1.95-A crystal structure of the Gbeta5-RGS9 complex, which is essential for normal visual and neuronal signal transduction. This structure reveals a canonical RGS domain that is functionally integrated within a molecular complex that is poised for integration of multiple steps during G-protein activation and deactivation.

P2Y1 antagonists: combining receptor-based modeling and QSAR for a quantitative prediction of the biological activity based on consensus scoring

P2Y1 is an ADP-activated G protein-coupled receptor (GPCR). Its antagonists impede platelet aggregation in vivo and are potential antithrombotic agents. Combining ligand and structure-based modeling we generated a consensus model (LIST-CM) correlating antagonist structures with their potencies. We docked 45 antagonists into our rhodopsin-based human P2Y1 homology model and calculated docking scores and free binding energies with the Linear Interaction Energy (LIE) method in continuum-solvent. The resulting alignment was also used to build QSAR based on CoMFA, CoMSIA, and molecular descriptors. To benefit from the strength of each technique and compensate for their limitations, we generated our LIST-CM with a PLS regression based on the predictions of each methodology. A test set featuring untested substituents was synthesized and assayed in inhibition of 2-MeSADP-stimulated PLC activity and in radioligand binding. LIST-CM outperformed internal and external predictivity of any individual model to predict accurately the potency of 75% of the test set.

Structure-activity relationship of uridine 5'-diphosphoglucose analogues as agonists of the human P2Y14 receptor

UDP-glucose (UDPG) and derivatives are naturally occurring agonists of the Gi protein-coupled P2Y14 receptor, which occurs in the immune system. We synthesized and characterized pharmacologically novel analogues of UDPG modified on the nucleobase, ribose, and glucose moieties, as the basis for designing novel ligands in conjunction with modeling. The recombinant human P2Y14 receptor expressed in COS-7 cells was coupled to phospholipase C through an engineered Galpha-q/i protein. Most modifications of the uracil or ribose moieties abolished activity; this is among the least permissive P2Y receptors. However, a 2-thiouracil modification in 15 (EC50 49 +/- 2 nM) enhanced the potency of UDPG (but not UDP-glucuronic acid) by 7-fold. 4-Thio analogue 13 was equipotent to UDPG, but S-alkylation was detrimental. Compound 15 was docked in a rhodposin-based receptor homology model, which correctly predicted potent agonism of UDP-fructose, UDP-mannose, and UDP-inositol. The hexose moiety of UDPG interacts with multiple H-bonding and charged residues and provides a fertile region for agonist modification.

Molecular modeling of the human P2Y2 receptor and design of a selective agonist, 2'-amino-2'-deoxy-2-thiouridine 5'-triphosphate

A rhodopsin-based homology model of the nucleotide-activated human P2Y2 receptor, including loops, termini, and phospholipids, was optimized with the Monte Carlo multiple minimum conformational search routine. Docked uridine 5'-triphosphate (UTP) formed a nucleobase pi-pi complex with conserved Phe3.32. Selectivity-enhancing 2'-amino-2'-deoxy substitution interacted through pi-hydrogen-bonding with aromatic Phe6.51 and Tyr3.33. A "sequential ligand composition" approach for docking the flexible dinucleotide agonist Up4U demonstrated a shift of conserved cationic Arg3.29 from the UTP gamma position to the delta position of Up4U and Up4 ribose. Synthesized nucleotides were tested as agonists at human P2Y receptors expressed in 1321N1 astrocytoma cells. 2'-Amino and 2-thio modifications were synergized to enhance potency and selectivity; compound 8 (EC50 = 8 nM) was 300-fold P2Y2-selective versus P2Y4. 2'-Amine acetylation reduced potency, and trifluoroacetylation produced intermediate potency. 5-Amino nucleobase substitution did not enhance P2Y2 potency through a predicted hydrophilic interaction possibly because of destabilization of the receptor-favored Northern conformation of ribose. This detailed view of P2Y2 receptor recognition suggests mutations for model validation.

Molecular dynamics simulation of the P2Y14 receptor. Ligand docking and identification of a putative binding site of the distal hexose moiety

A rhodopsin-based homology model of the P2Y14 receptor was inserted into a phospholipid bilayer and refined by molecular dynamics (MD) simulation. The binding modes of several known agonists, namely UDP-glucose and its analogues, were proposed using automatic molecular docking combined with Monte Carlo Multiple Minimum calculations. Compared to other P2Y receptors, the P2Y14 receptor has an atypical binding mode of the nucleobase, ribose, and phosphate moieties. The diphosphate moiety interacts with only one cationic residue, namely Lys171 of EL2, while in other P2Y receptor subtypes three Arg or Lys residues interact with the phosphate chain. Two other conserved cationic residues, namely Arg253 (6.55) and Lys277 (7.35) of the P2Y14 receptor together with two anionic residues (Glu166 and Glu174, located in EL2), are likely involved in interactions with the distal hexose moiety.

Crystal structure of Rac1 bound to its effector phospholipase C-β2

Although diverse signaling cascades require the coordinated regulation of heterotrimeric G proteins and small GTPases, these connections remain poorly understood. We present the crystal structure of the GTPase Rac1 bound to phospholipase C-beta2 (PLC-beta2), a classic effector of heterotrimeric G proteins. Rac1 engages the pleckstrin-homology (PH) domain of PLC-beta2 to optimize its orientation for substrate membranes. Gbetagamma also engages the PH domain to activate PLC-beta2, and these two activation events are compatible, leading to additive stimulation of phospholipase activity. In contrast to PLC-delta, the PH domain of PLC-beta2 cannot bind phosphoinositides, eliminating this mode of regulation. The structure of the Rac1-PLC-beta2 complex reveals determinants that dictate selectivity of PLC-beta isozymes for Rac GTPases over other Rho-family GTPases, and substitutions within PLC-beta2 abrogate its stimulation by Rac1 but not by Gbetagamma, allowing for functional dissection of this integral signaling node.

Structure-activity relationships of uridine 5'-diphosphate analogues at the human P2Y6 receptor

The structure-activity relationships and molecular modeling of the uracil nucleotide activated P2Y6 receptor have been studied. Uridine 5'-diphosphate (UDP) analogues bearing substitutions of the ribose moiety, the uracil ring, and the diphosphate group were synthesized and assayed for activity at the human P2Y6 receptor. The uracil ring was modified at the 4 position, with the synthesis of 4-substituted-thiouridine 5'-diphosphate analogues, as well as at positions 2, 3, and 5. The effect of modifications at the level of the phosphate chain was studied by preparing a cyclic 3',5'-diphosphate analogue, a 3'-diphosphate analogue, and several dinucleotide diphosphates. 5-Iodo-UDP 32 (EC50 = 0.15 microM) was equipotent to UDP, while substitutions of the 2'-hydroxyl (amino, azido) greatly reduce potency. The 2- and 4-thio analogues, 20 and 21, respectively, were also relatively potent in comparison to UDP. However, most other modifications greatly reduced potency. Molecular modeling indicates that the beta-phosphate of 5'-UDP and analogues is essential for the establishment of electrostatic interactions with two of the three conserved cationic residues of the receptor. Among 4-thioether derivatives, a 4-ethylthio analogue 23 displayed an EC50 of 0.28 microM, indicative of favorable interactions predicted for a small 4-alkylthio moiety with the aromatic ring of Y33 in TM1. The activity of analogue 19 in which the ribose was substituted with a 2-oxabicyclohexane ring in a rigid (S)-conformation (P = 126 degrees , 1'-exo) was consistent with molecular modeling. These results provide a better understanding of molecular recognition at the P2Y6 receptor and will be helpful in designing selective and potent P2Y6 receptor ligands.

[32P]2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate ([32P]MRS2500), a novel radioligand for quantification of native P2Y1 receptors

Analysis of the P2Y family of nucleotide-activated G-protein-coupled receptors has been compromised by the lack of selective high-affinity, high-specific-radioactivity radioligands. We have pursued quantification of the P2Y1 receptor through the development of a series of selective P2Y1 receptor antagonists. Recently, we synthesized 2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2500), a selective, competitive antagonist that exhibits a Ki of 0.8 nM in competition-binding assays with [3H]MRS2279. A 3'-monophosphate precursor molecule, MRS2608, was radiolabeled at the 5' position with 32P using polynucleotide kinase and [gamma32P]ATP to yield [32P]MRS2500. [32P]MRS2500 bound selectively to Sf9 insect cell membranes expressing the human P2Y1 receptor (Sf9-P2Y1), but did not detectably bind membranes expressing other P2Y receptors. P2Y1 receptor binding to [32P]MRS2500 was saturable with a KD of 1.2 nM. Agonists and antagonists of the P2Y1 receptor inhibited [32P]MRS2500 binding in Sf9-P2Y1 membranes with values in agreement with those observed in functional assays of the P2Y1 receptor. A high-affinity binding site for [32P]MRS2500 (KD=0.33 nM) was identified in rat brain, which exhibited the pharmacological selectivity of the P2Y1 receptor. Distribution of this binding site varied among rat tissues, with the highest amount of binding appearing in lung, liver, and brain. Among brain regions, distribution of the [32P]MRS2500 binding site varied by six-fold, with the highest and lowest amounts of sites detected in cerebellum and cortex, respectively. Taken together, these data illustrate the synthesis and characterization of a novel P2Y1 receptor radioligand and its utility for examining P2Y1 receptor expression in native mammalian tissues.

Regulation of phospholipase C isozymes by ras superfamily GTPases

The physiological effects of many extracellular stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid-signaling pathways. These signaling responses include the classically described conversion of PtdIns(4,5)P(2) to the Ca(2+)-mobilizing second messenger Ins(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. Here we discuss how the family of PLCs elaborates a minimal catalytic core typified by PLC-delta to confer multiple modes of regulation on their phospholipase activities. Although PLC-dependent signaling is prominently regulated by direct interactions with heterotrimeric G proteins or tyrosine kinases, the existence of at least 13 divergent PLC isozymes promises a diverse repertoire of regulatory mechanisms for this class of important signaling proteins. We focus here on the recently realized and extensive regulation of inositol lipid signaling by Ras superfamily GTPases directly acting on PLC isozymes and conclude by considering the biological and pharmacological ramifications of this regulation.

(N)-methanocarba-2MeSADP (MRS2365) is a subtype-specific agonist that induces rapid desensitization of the P2Y1 receptor of human platelets

Adenosine diphosphate (ADP) initiates and maintains sustained aggregation of platelets through simultaneous activation of both the Gq-coupled P2Y1 receptor and the Gi-coupled P2Y12 receptor. We recently described the synthesis and P2Y1 receptor-specific agonist activity of (N)-methanocarba-2MeSADP (MRS2365). Consequences of selective activation of the P2Y1 receptor by MRS2365 have been further examined in human platelets. Whereas MRS2365 alone only induced shape change, addition of MRS2365 following epinephrine treatment, which activates the Gi/z-linked, alpha2A-adrenergic receptor, resulted in sustained aggregation that was indistinguishable from that observed with ADP. Conversely, the platelet shape change promoted by ADP in the presence of the GPIIb/IIIa antagonist eptifibatide was similar to that promoted by MRS2365. Preaddition of the high affinity P2Y1 receptor antagonist MRS2500 inhibited the effect of MRS2365, whereas addition of MRS2500 subsequent to MRS2365 reversed the MRS2365-induced shape change. Preactivation of the P2Y1 receptor with MRS2365 for 2 min resulted in marked loss of capacity of ADP to induce aggregation as evidenced by a greater than 20-fold rightward shift in the concentration effect curve of ADP. This inhibitory effect of P2Y1 receptor activation was dependent on the concentration of MRS2365 (EC50 = 34 nm). The inhibitory effect of preincubation with MRS2365 was circumvented by activation of the Gq-coupled 5-HT2A receptor suggesting that MRS2365 induces loss of the ADP response as a consequence of desensitization of the Gq-coupled P2Y1 receptor. The time course of MRS2365-induced loss of aggregation response to epinephrine was similar to that observed with ADP. These results further demonstrate the P2Y1 receptor selectivity of MRS2365 and illustrate the occurrence of agonist-induced desensitization of the P2Y1 receptor of human platelets studied in the absence of P2Y12 receptor activation .

Gα12/13- and Rho-Dependent Activation of Phospholipase C-ϵ by Lysophosphatidic Acid and Thrombin Receptors

Because phospholipase C epsilon (PLC-epsilon) is activated by Galpha(12/13) and Rho family GTPases, we investigated whether these G proteins contribute to the increased inositol lipid hydrolysis observed in COS-7 cells after activation of certain G protein-coupled receptors. Stimulation of inositol lipid hydrolysis by endogenous lysophosphatidic acid (LPA) or thrombin receptors was markedly enhanced by the expression of PLC-epsilon. Expression of the LPA(1) or PAR1 receptor increased inositol phosphate production in response to LPA or SFLLRN, respectively, and these agonist-stimulated responses were markedly enhanced by coexpression of PLC-epsilon. Both LPA(1) and PAR1 receptor-mediated activation of PLC-epsilon was inhibited by coexpression of the regulator of G protein signaling (RGS) domain of p115RhoGEF, a GTPase-activating protein for Galpha(12/13) but not by expression of the RGS domain of GRK2, which inhibits Galpha(q) signaling. In contrast, activation of the G(q)-coupled M1 muscarinic or P2Y(2) purinergic receptor was neither enhanced by coexpression with PLC-epsilon nor inhibited by the RGS domain of p115RhoGEF but was blocked by expression of the RGS domain of GRK2. Expression of the Rho inhibitor C3 botulinum toxin did not affect LPA- or SFLLRN-stimulated inositol lipid hydrolysis in the absence of PLC-epsilon but completely prevented the PLC-epsilon-dependent increase in inositol phosphate accumulation. Likewise, C3 toxin blocked the PLC-epsilon-dependent stimulatory effects of the LPA(1), LPA(2), LPA(3), or PAR1 receptor but had no effect on the agonist-promoted inositol phosphate response of the M1 or P2Y(2) receptor. Moreover, PLC-epsilon-dependent stimulation of inositol phosphate accumulation by activation of the epidermal growth factor receptor, which involves Ras- but not Rho-mediated activation of the phospholipase, was unaffected by C3 toxin. These studies illustrate that specific LPA and thrombin receptors promote inositol lipid signaling via activation of Galpha(12/13) and Rho.

Structure activity and molecular modeling analyses of ribose- and base-modified uridine 5'-triphosphate analogues at the human P2Y2 and P2Y4 receptors

With the long-term goal of developing receptor subtype-selective high affinity agonists for the uracil nucleotide-activated P2Y receptors we have carried out a series of structure activity and molecular modeling studies of the human P2Y2 and P2Y4 receptors. UTP analogues with substitutions in the 2'-position of the ribose moiety retained capacity to activate both P2Y2 and P2Y4 receptors. Certain of these analogues were equieffective for activation of both receptors whereas 2'-amino-2'-deoxy-UTP exhibited higher potency for the P2Y2 receptor and 2'-azido-UTP exhibited higher potency for the P2Y4 receptor. 4-Thio substitution of the uracil base resulted in a UTP analogue with increased potency relative to UTP for activation of both the P2Y2 and P2Y4 receptors. In contrast, 2-thio substitution and halo- or alkyl substitution in the 5-position of the uracil base resulted in molecules that were 3-30-fold more potent at the P2Y2 receptor than P2Y4 receptor. 6-Aza-UTP was a P2Y2 receptor agonist that exhibited no activity at the P2Y4 receptor. Stereoisomers of UTPalphaS and 2'-deoxy-UTPalphaS were more potent at the P2Y2 than P2Y4 receptor, and the R-configuration was favored at both receptors. Molecular docking studies revealed that the binding mode of UTP is similar for both the P2Y2 and P2Y4 receptor binding pockets with the most prominent dissimilarities of the two receptors located in the second transmembrane domain (V90 in the P2Y2 receptor and I92 in the P2Y4 receptor) and the second extracellular loop (T182 in the P2Y2 receptor and L184 in the P2Y4 receptor). In summary, this work reveals substitutions in UTP that differentially affect agonist activity at P2Y2 versus P2Y4 receptors and in combination with molecular modeling studies should lead to chemical synthesis of new receptor subtype-selective drugs.

Human P2Y(6) receptor: molecular modeling leads to the rational design of a novel agonist based on a unique conformational preference

Combining molecular dynamics (MD) in a hydrated phospholipid (DOPC) bilayer, a Monte Carlo search, and synthesis of locked nucleotide analogues, we discovered that the Southern conformation of the ribose is preferred for ligand recognition by the P2Y(6) receptor. 2'-Deoxy-(S)-methanocarbaUDP was found to be a full agonist of the receptor and displayed a 10-fold higher potency than that for the corresponding flexible 2'-deoxyUDP. MD results also suggested a conformational change of the second extracellular loop consequent to agonist binding.

Regulation of PLCβ Isoforms by Rac

Small GTPases function as molecular switches, which transduce cellular signals from upstream regulators to downstream effectors in a guanine nucleotide-dependent manner. Direct binding partners of small GTPases fall into four classes of both regulators and effectors that can be differentiated on the basis of the state of nucleotide required for binding. Here we describe a procedure for the rapid screening and quantitative assessment of direct interactions of the Rho family of small GTPases with effector molecules of the phospholipase Cbeta class of enzymes using surface plasmon resonance technology. The experimental format described is also readily adaptable toward characterizing guanine nucleotide-dependent binding events of both small and heterotrimeric G proteins with various classes of GTPase regulatory proteins.

Direct activation of purified phospholipase C epsilon by RhoA studied in reconstituted phospholipid vesicles

Phospholipase C-epsilon (PLC-epsilon) was shown recently to be a downstream effector of Rho GTPases, and we have used an in vitro phospholipid vesicle reconstitution system with purified proteins to show this regulation to be direct. This chapter describes high-level expression of a hexahistidine-tagged fragment of PLC-epsilon encompassing the catalytic core of the enzyme through the tandem RA domains by use of a recombinant baculovirus and High Five insect cells. The recombinant protein is purified to homogeneity using metal chelate affinity and size exclusion chromatography. The small GTPase RhoA also is expressed to high levels in a lipidated form after baculovirus expression in High Five cells and is purified to near homogeneity after detergent extraction and metal chelate affinity chromatography. The capacity of GTPgammaS-bound RhoA to stimulate the phospholipase activity of PLC-epsilon is assessed by reconstitution of the RhoA in mixed-detergent phospholipid micelles containing PtdIns(4,5)P2 substrate.