S-prenylated cysteine analogues inhibit receptor-mediated G protein activation in native human granulocyte and reconstituted bovine retinal rod outer segment membranes

A model for how Gβγ couples Gα to GPCR

Representing ∼5% of the human genome, G-protein-coupled receptors (GPCRs) are a primary target for drug discovery; however, the molecular details of how they couple to heterotrimeric G protein subunits are incompletely understood. Here, I propose a hypothetical initial docking model for the encounter between GPCR and Gβγ that is defined by transient interactions between the cytosolic surface of the GPCR and the prenyl moiety and the tripeptide motif, asparagine-proline-phenylalanine (NPF), in the C-terminus of the Gγ subunit. Analysis of class A GPCRs reveals a conserved NPF binding site formed by the interaction of the TM1 and H8. Functional studies using differentially prenylated proteins and peptides further suggest that the intracellular hydrophobic core of the GPCR is a prenyl binding site. Upon binding TM1 and H8 of GPCRs, the propensity of the C-terminal region of Gγ to convert into an α helix allows it to extend into the hydrophobic core of the GPCR, facilitating the GPCR active state. Conservation of the NPF motif in Gγ isoforms and interacting residues in TM1 and H8 suggest that this is a general mechanism of GPCR-G protein signaling. Analysis of the rhodopsin dimer also suggests that Gγ-rhodopsin interactions may facilitate GPCR dimer transactivation.

N-acetyl-S-farnesyl-l-cysteine suppresses chemokine production by human dermal microvascular endothelial cells

Isoprenylcysteine (IPC) molecules modulate G-protein-coupled receptor signalling. The archetype of this class is N-acetyl-S-farnesyl-l-cysteine (AFC). Topical application of AFC locally inhibits skin inflammation and elicitation of contact hypersensitivity in vivo. However, the mechanism of these anti-inflammatory effects is not well understood. Dermal microvascular endothelial cells (ECs) are involved in inflammation, in part, by secreting cytokines that recruit inflammatory cells. We have previously shown that the sympathetic nerve cotransmitter adenosine-5'-triphosphate (ATP) and adenosine-5'-O-(3-thio) triphosphate (ATPγS), an ATP analogue that is resistant to hydrolysis, increase secretion of the chemokines CXCL8 (interleukin-8), CCL2 (monocyte chemotactic protein-1) and CXCL1 (growth-regulated oncogene α) by dermal microvascular ECs. Production of these chemokines can also be induced by the exposure to the proinflammatory cytokine TNFα. We have now demonstrated that AFC dose-dependently inhibits ATP-, ATPγS- and TNFα-induced production of CXCL1, CXCL8 and CCL2 by a human dermal microvascular EC line (HMEC-1) in vitro under conditions that do not affect cell viability. Inhibition of ATPγS- or TNFα-stimulated release of these chemokines was associated with reduced mRNA levels. N-acetyl-S-geranyl-l-cysteine, an IPC analogue that is inactive in inhibiting G-protein-coupled signalling, had greatly reduced ability to suppress stimulated chemokine production. AFC may exert its anti-inflammatory effects through the inhibition of chemokine production by stimulated ECs.

Liver prenylated methylated protein methyl esterase is the same enzyme as Sus scrofa carboxylesterase

The C-terminal --COOH of prenylated proteins is methylated to --COOCH3. The --COOCH3 ester forms are hydrolyzed by prenylated methylated protein methyl esterase (PMPMEase) to the original acid forms. This is the only reversible step of the prenylation pathway. PMPMEase has not been purified and identified and is therefore understudied. Using a prenylated-L-cysteine methyl ester as substrate, PMPMEase was purified to apparent homogeneity from porcine liver supernatant. SDS-PAGE analysis revealed an apparent mass of 57 kDa. Proteomics analyses identified 17 peptides (242 amino acids). A Mascot database search revealed these as portions of the Sus scrofa carboxylesterase, a 62-kDa serine hydrolase with the C-terminal HAEL endoplasmic reticulum-retention signal. It is at least 71% identical to such mammalian carboxylesterases as human carboxylesterase 1 with affinities toward hydrophobic substrates and known to activate prodrugs, metabolize active drugs, as well as detoxify various substances such as cocaine and food-derived esters. The purified enzyme hydrolyzed benzoyl-Gly-farnesyl-L-cysteine methyl ester and hydrocinamoyl farnesyl-L-cysteine methyl ester with Michaelis-Menten constant (K(m)) values of 33 +/- 4 and 25 +/- 4 microM and V(max) values of 4.51 +/- 0.28 and 6.80 +/- 0.51 nmol/min/mg of protein, respectively. It was inhibited by organophosphates, chloromethyl ketones, ebelactone A and B, and phenylmethylsulfonyl fluoride.

Liver prenylated methylated protein methyl esterase is an organophosphate-sensitive enzyme

Prenylation and subsequent methylation are essential modifications on a significant proportion of eucaryotic proteins. Proteins such as the G-gamma subunits of G-protein coupled receptors, nuclear lamins, and guanine nucleotide-binding proteins such as Ras are prenylated and undergo methylation. Prenylated methylated protein methyl esterase (PMPMEase) readily hydrolyses the prenylated protein methyl esters, thus making this step reversible and possibly regulatory. Benzoyl-glycyl-farnesyl-cysteine methyl ester (BzGFCM) was developed as a specific PMPMEase substrate and characterized by electron spray ionization mass spectrometry (ESI-MS) to be of the calculated molecular mass. Rat liver and brain PMPMEase hydrolyzed BzGFCM, forming benzoyl-glycyl-farnesyl-cysteine (BzGFC) in a time- and concentration-dependent manner. Both enzymes cleaved BzGFCM with K(m) values of 4.58 +/- 0.30 and 25.57 +/- 2.36 microM and V(max) values of 2.21 +/- 0.03 and 0.17 +/- 0.003 nmol/min/mg, respectively. The liver enzyme eluted from a gel-filtration column as a single peak of apparent size, 89 kDa. The brain enzyme eluted as two main peaks of 53 and 890 kDa. Organophosphorus pesticides (OPs), which are suspected to be involved in human disorders such as parkinsonism, neuronal, and retinal degeneration, inhibited the liver enzyme with IC(50) values from 4.77 muM for parathion to 0.04 microM for paraoxon, respectively. Only about 25% of the brain enzyme was inhibited by 0.5-1 mM solutions of mipafox, while 0.1 and 1 mM paraoxon inhibited over 50% and 95% of the enzyme, respectively. Paraoxon is thus about 2,250 times less potent against the brain than the liver PMPMEase. BzGFCM was not hydrolyzed by various cholinesterases, indicating its specificity for PMPMEase. Perturbations in prenylated protein metabolism might play a role in noncholinergic OPs-induced toxicity, since prenylated proteins play such important roles in cell signaling, proliferation, differentiation, and apoptosis.

Inhibition mechanism of S-adenosylmethionine-induced movement deficits by prenylcysteine analogs

We previously showed that S-adenosylmethionine (SAM) induces movement impairments similar to those observed in Parkinson's disease (PD) apparently by prenylated protein methylation; 5 kDa molecules being methylated and the symptoms being inhibited by prenylcysteine (PC) analogs. In the present study, we explore the biochemical mechanism of action of the PC analogs. N-acetylgeranylcysteine (AGC), N-acetylfarnesylcysteine (AFC), N-acetylgeranylgeranylcysteine (AGGC), farnesylthioacetic acid (FTA), farnesyl-2-ethanesulfonic acid (FTE) and farnesylsuccinic acid (FMS), but not farnesylthiotriazole (FTT) and farnesylthiolactic acid (FTL), inhibited the SAM-induced motor impairments. Incubation of the respective analogs with rat brain membranes containing prenylated protein methyltransferase (PPMTase) resulted in the methylation of AGC, AFC and AGGC. FTA, FTE, FMS and FTT, but not FTL, inhibited the enzyme activity. A single injection of the active analogs remained effective for at least 3 days against repeated injections of 1 micromol SAM. Amphetamine-induced hyperactivity in rats was inhibited by SAM but potentiated by FTE. During 60 min, the movement time for amphetamine-treated rats was 1477 s compared with 633 and 1664 s for amphetamine+SAM- and amphetamine+FTE-treated rats, respectively. The total distance for amphetamine+FTE-treated rats was 82% higher than for amphetamine. The horizontal activity was 30,728 (amphetamine), 15,430 (FTE), 18,526 (amphetamine+SAM), 41,736 (amphetamine+FTE) and 7004 (SAM) as compared to the PBS control (4726). The intricate relationship between the actions of SAM, which speeds up prenylated protein methylation and impairs movement, amphetamine, which increases synaptic dopamine levels and movement, and the PC analogs, which prevent the SAM-induced movement impairments, suggests a SAM-induced defect on dopamine signaling as the likely cause of the symptoms. The data reveal that interaction of PC analogs with PPMTase may not be an indicator of anti-PD-like activity.

Studies on G-protein alpha.betagamma heterotrimer formation reveal a putative S-prenyl-binding site in the alpha subunit

The alpha and betagamma subunits of heterotrimeric G-proteins contain specific lipid modifications, which are required for their biological function. However, the relevance of these modifications to the interactions within the heterotrimeric G-protein is not fully understood. In order to explore the role of the S-prenyl moiety of the isoprenylated betagamma dimer of retinal transducin, betagamma(t), in the formation of the heterotrimeric complex with the corresponding N-acylated alpha subunit, alpha(t), we employed purified fully processed subunits, which are soluble in aqueous solutions without detergents. Pertussis-toxin-mediated [(32)P]ADP-ribosylation of alpha(t) is strongly stimulated (approximately 10-fold) in the presence of betagamma(t) and can thus serve as a measure for heterotrimer formation. Using this assay, preincubation of alpha(t) with S-prenyl analogues containing farnesyl or geranylgeranyl moieties was found to inhibit heterotrimer formation in a dose-dependent manner. The inhibition was competitive and reversible, as indicated by its reversal upon increase of the betagamma(t) dimer concentration or by removal of the S-prenyl analogue using gel filtration. The competitive nature of the inhibition is supported by the marked attenuation of the inhibition when the S-prenyl analogue was added to alpha(t) together with or after betagamma(t). The inhibition does not involve interaction with the alpha(t) acyl group, since an S-prenyl analogue inhibited the [(32)P]ADP-ribosylation of an unlipidated alpha(t) mutant. These data suggest the existence of a hitherto unrecognized S-prenyl-binding site in alpha(t), which is critical for its interaction with prenylated betagamma(t).

Rhodopsin: insights from recent structural studies

The recent report of the crystal structure of rhodopsin provides insights concerning structure-activity relationships in visual pigments and related G protein-coupled receptors (GPCRs). The seven transmembrane helices of rhodopsin are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The ligand-binding pocket of rhodopsin is remarkably compact, and several chromophore-protein interactions were not predicted from mutagenesis or spectroscopic studies. The helix movement model of receptor activation, which likely applies to all GPCRs of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor includes a helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. The cytoplasmic surface appears to be approximately large enough to bind to the transducin heterotrimer in a one-to-one complex. The structural basis for several unique biophysical properties of rhodopsin, including its extremely low dark noise level and high quantum efficiency, can now be addressed using a combination of structural biology and various spectroscopic methods. Future high-resolution structural studies of rhodopsin and other GPCRs will form the basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

A photoactivatable prenylated cysteine designed to study isoprenoid recognition

Protein prenylation, involving the alkylation of a specific C-terminal cysteine with a C(15) or C(20) isoprenoid unit, is an essential posttranslational modification required by most GTP-binding proteins for normal biological activity. Despite the ubiquitous nature of this modification and numerous efforts aimed at inhibiting prenylating enzymes for therapeutic purposes, the function of prenylation remains unclear. To explore the role the isoprenoid plays in mediating protein-protein recognition, we have synthesized a photoactivatable, isoprenoid-containing cysteine analogue (2) designed to act as a mimic of the C-terminus of prenylated proteins. Photolysis experiments with 2 and RhoGDI (GDI), a protein which interacts with prenylated Rho proteins, suggest that the GDI is in direct contact with the isoprenoid moiety. These results, obtained using purified GDI as well as Escherichia coli (E. coli) crude extract containing GDI, suggest that this analogue will be an effective and versatile tool for the investigation of putative isoprenoid binding sites in a variety of systems. Incorporation of this analogue into peptides or proteins should allow for even more specific interactions between the photoactivatable isoprenoid and any number of isoprenoid binding proteins.

Synthesis and biological evaluation of the geometric farnesylated analogues of the a-factor mating peptide of Saccharomyces cerevisiae

The a-factor of Saccharomyces cerevisiae is a dodecapeptide pheromone (YIIKGVFWDPAC(Farnesyl)-OCH(3), 1), in which post-translational modification with a farnesyl isoprenoid and carboxymethyl group is required for full biological activity. This peptide has been used as a model system to explore the biological function of the farnesylcysteine moiety, which is found on and required for the biological activity of many key mammalian proteins. The objective of this particular study was the determination of the biological effect of double bond isomerization of the natural E, E-farnesyl moiety on the biological activity of the a-factor. A unified, stereoselective synthetic route to the three geometric isomers of E,E-farnesol (12, 13, and 14) has been developed. The key feature of this synthesis is the ability to control the stereochemistry of triflation of the beta-ketoester 22 to give either 23 or 25. The three farnesol isomers were converted to the corresponding isomeric a-factors (9, 10 and 11) via a modified version of a previously utilized synthetic route. Biological evaluation of these peptides indicates that, surprisingly, all three possess nearly equivalent activity to the natural a-factor bearing the E,E-farnesyl moiety.

Isoprenylation/methylation and transducin function

Freshly prepared proteolyzed (deprenylated) T beta gamma and material isolated from retina are inert with respect to activating T alpha in the presence of R* in detergent and in disk membranes. In addition, proteolyzed T beta gamma is also incapable of supporting the pertussis toxin-catalyzed ADP ribosylation of T alpha-GDP. These experiments show that isoprenylation/methylation is essential for the fruitful interactions between T alpha and T beta gamma at the membrane. When tested for its ability to support GTP-for-GDP exchange catalyzed by R*, demethylated T beta gamma proved to be approximately 50% as active as methylated T beta gamma in photoreceptor disk membranes (Fig. 3) and in reconstituted liposomes containing rhodopsin. In detergent, no difference was observed between methylated and demethylated T beta gamma, suggesting no role at all for the methyl group in functional interactions between T alpha, T beta gamma, and R*. The twofold activity difference observed in membranes can be accounted for by the twofold lessened affinity of the demethylated T beta gamma, compared with its methylated counterpart, for membranes in the presence of R* and T alpha. It is interesting to note that a substantially larger difference (> 10-fold) in the relative binding of methylated versus demethylated T beta gamma to membranes is observed in the absence of R* and T alpha. However, R* has a substantial affinity for T alpha beta gamma, and the influence of R* and T alpha greatly reduces any differences resulting from the presence or absence of a methyl group on T beta gamma. The results from studies of demethylated T beta gamma demonstrate that specific lipid-receptor interactions are unlikely to play a critical role in the rhodopsin-transducin system, and further show that the effect of methylation is probably due to the increased hydrophobicity of methylated T beta gamma versus its unmethylated counterpart. These studies are, of course, relevant to heterotrimeric G proteins, and specifically to the interactions of receptor (R*) with T alpha and T beta gamma. If a hydrophobic lipid-lipid mechanism is operative, the state of methylation would be expected to have a more profound effect on the membrane-associative properties of farnesylated proteins, but not on those of geranylgeranylated proteins. The increased hydrophobicity of the C20 geranylgeranyl group relative to the C15 farnesyl group will compensate for the loss of the methyl substituent. The results obtained in the transducin-rhodopsin system can be contrasted with the effect of gamma-subunit methylation on effector enzyme activation. In the case of the geranylgeranylated beta 1 gamma 2, methylation proved to have only a small effect on PIPLC beta activation (Fig. 4B). An approximately 25% diminution in efficacy, but not potency, was observed for the demethylated geranylgeranylated beta 1 gamma 2 versus its methylated counterpart. This again shows that specific lipid-protein interactions are unimportant. The effect of methylation on membrane binding would be expected to be small, given that beta 1 gamma 2 is geranylgeranylated. It is of interest to compare these results with those found with methylated and unmethylated T beta gamma as activators of PIPLC beta. In this instance there was a large effect noted, with methylated T beta gamma being at least 10-fold more potent than its unmethylated counterpart with respect to activating either enzyme (Fig. 4A). This result is readily understandable in light of the role of methylation in selectively enhancing hydrophobicity of farnesylated proteins as opposed to geranyl-geranylated proteins. Similar results were obtained for the activation of PI3K, further strengthening the conclusion that it is lipid-lipid interactions that direct beta gamma subunit membrane association. (ABSTRACT TRUNCATED)

Structure, biological activity and membrane partitioning of analogs of the isoprenylated a-factor mating peptide of Saccharomyces cerevisiae

Previous biochemical investigations on the Saccharomyces cerevisiae a-factor indicated that this lipopeptide pheromone [YIIKGVFWDPAC(farnesyl)OMe] might adopt a type II beta-turn at positions 4 and 5 of the peptide sequence. To test this hypothesis, we synthesized five analogs of a-factor, in which residues at positions 4 and 5 were replaced with: L-Pro4(I); D-Pro4(II); L-Pro4-D-Ala5(III); D-Pro4-L-Ala5(IV); or Nle4(V). Analogs were purified to > 99% homogeneity as evidenced by HPLC and TLC and were characterized by mass spectrometry and amino acid analysis. Using a growth arrest assay the conformationally restricted a-factor analogs I and III were found to be almost 50-fold more active than the diastereometric homologs II and IV and were equally active to wild-type a-factor. Replacement of Lys4 with the isosteric Nle4 almost abolished the activity of the pheromone. Thus, the incorporation of residues that promote a type II beta-turn compensated for the loss of the favorable contribution of the Lys4 side chain to pheromone activity. CD spectra on these peptides suggested that they were essentially disordered in both TFE/H2O and in the presence of DMPC vesicles. There was no correlation between CD peak shape and biological activity. Using fluorescence spectroscopy we measured the interaction of lipid vesicles with these position 4 and 5 analogs as well as with three a-factor analogs with a modified farnesyl group. The results indicated that modifications of both the peptide sequence and the lipid moiety affect partitioning into lipid, and that no correlation existed between the propensity of a pheromone to partition into the lipid and its biological activity.

Synthesis and evaluation of homofarnesoyl-substituted CAAX-peptidomimetics as farnesyltransferase inhibitors and antiproliferative agents

Several CAAX-peptidomimetics were linked to homofarnesoic acid via a beta-alanyl spacer with the intention to obtain a novel type of bisubstrate analogue farnesyltransferase inhibitors. However, the compounds were found to be only weakly active in the farnesyltransferase inhibition assay. Nevertheless, they displayed antiproliferative activity against different tumor cell lines in the low micromolar range. Replacement of the beta-alanine moiety by aspartic acid-1-methyl ester resulted in a compound which inhibited the farnesyltransferase with an IC50 of 860 nM. The corresponding free acid showed a eightfold loss in activity (IC50 = 6.9 microM).

G protein antagonists

Heterotrimeric G proteins couple membrane-bound heptahelical receptors to their cellular effector systems (ion channels or enzymes generating a second messenger). In current pharmacotherapy, the input to G protein-regulated signalling is typically manipulated by targeting the receptor with appropriate agonists or antagonists and, to a lesser extent, by altering second messenger levels, most notably by inhibiting phosphodiesterases that hydrolyse cyclic nucleotides. When stimulated, G proteins undergo a cycle of activation and deactivation in which the alpha-subunits and the betagamma-dimers sequentially expose binding sites for their reaction partners (receptors, guanine nucleotides and effectors, as well as regulatory proteins). These domains can be blocked by inhibitors and this produces effects that cannot be achieved by receptor antagonists. Here, the structural and mechanistic information on G protein antagonists is summarized and an outline of the arguments supporting the hypothesis that G proteins per se are also potential drug targets is provided.

Structural features of heterotrimeric G-protein-coupled receptors and their modulatory proteins

Over the past 20 years, the general mechanism for signaling through 7-transmembrane helix receptors coupled to GTP hydrolysis has been worked out. Although similar in overall organization, subtype variability and subcellular localization of components have built in considerable signaling specificity. Atomic resolution structures for many of the components have delineated the domain organization of these complex proteins and have given physical form to the idea of subtype specificity. This review describes what is known about the physical structures of the 7-transmembrane helix receptors, the heterotrimeric GTP binding coupling proteins, the adenylate cyclase and phospholipase C effector proteins, and signaling modulatory proteins, such as arrestin, phosducin, recoverin-type myristoyl switch proteins, and the pleckstrin homology domain of G-protein receptor kinase-2. These images allow experimenters to contemplate the details of the supramolecular organization of the multiprotein complexes involved in the transmission of signals across the cellular lipid bilayer.

Peptide conjugates as tools for the study of biological signal transduction

Today, many biological phenomena are being investigated and understood in molecular detail, and organic chemistry is increasingly being directed towards biological phenomena. This review is intended to highlight this interplay of organic chemistry and biology, using biological signal transduction as an example. Lipo-, glyco-, phospho- and nucleoproteins play key roles in the processes whereby chemical signals are passed across cell membranes and further to the cell nucleus. For the study of the biological phenomena associated with these protein conjugates, structurally well-defined peptides containing the characteristic linkage region of the peptide backbone with the lipid, the carbohydrate or the phosphoric acid ester can provide valuable tools. The multi-functionality and pronounced acid- and base-lability of such compounds renders their synthesis a formidable challenge to conventional organic synthesis. However, the recent development of enzymatic protecting groups, provides one of the central techniques which, when coupled with classic chemical synthesis, can provide access to these complex and sensitive biologically relevant peptide conjugates under particularly mild conditions and with high selectivity.

The G-protein betagamma complex

The vast majority of signalling pathways in mammalian cells are mediated by heterotrimeric (alpha betagamma) G proteins. Reviewed here is regulation of signal transduction by the betagamma complex at different protein interfaces: subunit-subunit, receptor-G protein and G protein-effector. The role of diverse beta and gamma subunit types in achieving specificity in signalling and potentially unidentified functions for these subunits also are discussed.

Production of a Mating Inhibitor by Ustilago hordei

ABSTRACT Ustilago hordei, the cause of barley covered smut, was found to produce a factor that inhibited its own mating. The mating inhibition factor (MIF) specifically inhibited mating of U. hordei and other Ustilago spp., but not teliospore germination or sporidial growth. MIF did prevent teliospore germination of Tilletia caries and T. contraversa. MIF was found at low levels in culture supernatants of either mating type of U. hordei grown separately, but at higher levels when both mating types were grown together, in the supernatants of MAT-1 mating type cells transformed with the MAT-1 pheromone gene mfa1 and of MAT-2 cells transformed with either mfa1 or the MAT-1 pheromone receptor gene pra1. Diploid cells produced no detectable inhibitor, nor did MAT-1 cells with a disrupted mating type locus that deleted both mfa1 and pra1. MIF production was restored when mfa1, but not pra1, was added back to the MAT-1Delta cells. MIF activity was altered by protease treatment. Highly purified MIF from MAT-1 cells contained cysteine methyl ester, farnesyl cysteine, farnesyl cysteine methyl ester, and a dodecapeptide with a mass consistent with that of MAT-1 pheromone lacking the terminal cysteine. Since smut fungi must first mate to become pathogenic, mating inhibition has the potential to be an effective method of disease control for these pathogens.

Analogs of farnesylcysteine induce apoptosis in HL-60 cells

S-Farnesyl-thioacetic acid (FTA), a competitive inhibitor of isoprenylated protein methyltransferase, potently suppressed the growth of HL-60 cells and induced apoptosis, as evidenced by the development of increased annexin-V binding, decreased binding of DNA dyes and internucleosomal DNA degradation. FTA did not impair the membrane association of ras proteins, conversely, it brought about a decrease in the proportion of ras present in the cytosolic fraction. Farnesylated molecules which are weak inhibitors of the methyltransferase also induced DNA laddering and reduced the proportion of cytosolic ras. These findings suggest that neither inhibition of isoprenylated protein methylation nor impairment of ras membrane association are essential for apoptosis induced by farnesylcysteine analogs.

Polyisoprenyl phosphates in intracellular signalling

In response to environmental stimuli, leukocyte membrane remodelling generates biologically active lipids that can serve as both intra- and extracellular mediators. There are several classes of lipids that can mediate inflammatory reactions. We report here on a new intracellular lipid signal that regulates oxygen-radical formation in neutrophils, a key response in microbial killing, inflammation and tissue injury. Screening of neutrophil-derived extracts rich in phosphorylated, non-saponifiable lipids revealed a potent inhibitor of superoxide anion (O2-) production. Structural analysis of biologically active fractions gave four major phosphorylated lipids: most abundant was presqualene diphosphate (PSDP). Upon activation of neutrophil receptors, PSDP and its monophosphate form, presqualene monophosphate (PSMP), undergo rapid remodelling. At submicromolar concentrations, PSDP but not PSMP inhibit O2- production by human neutrophil cell-free oxidase preparations. We prepared PSDP and PSMP by total organic synthesis and matched both the physical properties and biological activity of the neutrophil-derived compounds. Our results indicate that PSDP, a recognized intermediate of cholesterol biosynthesis, is present in immune effector cells and is a potent regulator of the cellular response in host defence.

Structural aspects of heterotrimeric G-protein signaling

Recently, structures of heterotrimeric G-protein subunits have been determined in isolation, in conjunction with each other, and in complex with their regulators. Along with biochemical information, these structures suggest how G-protein subunits are oriented relative to the membrane surface and relative to seven-transmembrane helix receptors. They also suggest mechanisms for receptor-catalyzed nucleotide exchange.

G protein beta gamma subunits

Guanine nucleotide binding (G) proteins relay extracellular signals encoded in light, small molecules, peptides, and proteins to activate or inhibit intracellular enzymes and ion channels. The larger G proteins, made up of G alpha beta gamma heterotrimers, dissociate into G alpha and G beta gamma subunits that separately activate intracellular effector molecules. Only recently has the G beta gamma subunit been recognized as a signal transduction molecule in its own right; G beta gamma is now known to directly regulate as many different protein targets as the G alpha subunit. Recent X-ray crystallography of G alpha, G beta gamma, and G alpha beta gamma subunits will guide the investigation of structure-function relationships.

Activation of signaling pathways in HL60 cells and human neutrophils by farnesylthiosalicylate

Effects of the farnesylcysteine mimetic, farnesylthiosalicylate on the activation of myeloid cells were studied. In dimethyl-sulfoxide-differentiated HL60 cells and in human neutrophils farnesylthiosalicylate (< or = 20 microM) dose-dependently elevated cytosolic Ca2+ concentrations, suggesting phospholipase-C-mediated release of the ion from intracellular stores. In human neutrophils, in addition to the production of inositol trisphosphate, farnesylthiosalicylate induced activation of the NADPH oxidase and translocation of the cytosolic oxidase components p47-phox and p67-phox to the membrane. The calcium signal, inositol-trisphosphate production and superoxide generation elicited by farnesylthiosalicylate were partially blocked by treatment of the cells with pertussis toxin, consistent with participation of pertussis-toxin-sensitive and pertussis-toxin-resistant elements. In HL60 cells, farnesylthiosalicylate (< or = 20 microM) did not activate NADPH oxidase but dose-dependently augmented PMA-elicited activity of the enzyme. This effect was resistant to pertussis-toxin treatment. In vitro augmentation of PKC-mediated phosphorylation of histone and cytosolic p47-phox by farnesylthiosalicylate and the finding that downregulation of PKC abrogated potentiation of NADPH oxidase activity by farnesylthiosalicylate were compatible with the involvement of PKC in the response of HL60 cells to farnesylthiosalicylate. It is suggested that the effects of farnesylthiosalicylate on myeloid cells reflect interaction of the analog with prenylcysteine-docking sites on cellular signaling elements.

Heterotrimeric G proteins

Over the past year, the thrust of work in the field of heterotrimeric G proteins has been primarily in the following areas: first, resolution of their three-dimensional structures by X-ray crystallography; second, elucidation of the effect of lipid modifications on the Galpha and Ggamma subunits; third, understanding the role of the Gbetagamma dimer in stimulation of a variety of effectors following receptor activation; and fourth, identification of the points of contact among the Galpha, Gbeta, and Ggamma subunits, and between these subunits and their cognate receptor or effector molecules.

Chemical biology of protein isoprenylation/methylation

Isoprenylation/methylation is an important dual hydrophobic post-translational modification which occurs at or near a carboxyl terminal cysteine residue. All known G proteins are modified in this way, making the pathway of central interest for an understanding of signal transduction. In this review, aspects of the molecular enzymology of isoprenylation/methylation are reviewed. The functional significance of these modifications is discussed, with special reference to the signal transducing G proteins. Of further interest is the possible regulatory role of methylation, since this step is the only reversible one in the pathway. The biochemical and functional consequences of isoprenylation/methylation are of especial interest. Isoprenylation/methylation is generally assumed to enhance the abilities of modified proteins to associate with membranes. This can be due either to hydrophobic lipid-lipid or lipid-protein interactions. Available evidence, taken largely from studies on visual signal transduction and ras signalling pathways, strongly points to enhanced membrane binding being a consequence of hydrophobic lipid-lipid interactions. An exciting possibility that also emerges is concerned with whether isoprenylation may also have additional roles, in addition to enhancing the membrane partitioning ability of the modified protein. In a simple mechanism of this type, the isoprenylated/methylated cysteine residue would be specifically recognized by another protein. While no compelling case can yet be made for an effector role for the isoprenylated/methylated cysteine moiety mediating protein-protein interactions, recent studies on the pharmacology of isoprenylated cysteine analogs suggests the possibility of such a role.

Efficient interaction with a receptor requires a specific type of prenyl group on the G protein gamma subunit

Post-translational prenylation of the carboxyl-terminal cysteine is a characteristic feature of the guanine nucleotide-binding protein (G protein) gamma subunits. Recent findings show that the farnesylated COOH-terminal tail of the gamma 1 subunit is a specific determinant of rhodopsin-transducin coupling. We show here that when synthetic peptides specific to the COOH-terminal tail of gamma 1 are chemically modified with geranyl, farnesyl, or geranylgeranyl groups and tested for their ability to interact with light activated rhodopsin, the farnesylated peptide is significantly more effective. These results show that an appropriate isoprenoid on the G protein gamma subunit serves not only a membrane anchoring function but in combination with the COOH-terminal domain specifies receptor-G protein coupling.