Role of prenylation in the interaction of the a-factor mating pheromone with phospholipid bilayers

Mechanobiology and the microcirculation: cellular, nuclear and fluid mechanics

Endothelial cells are stimulated by shear stress throughout the vasculature and respond with changes in gene expression and by morphological reorganization. Mechanical sensors of the cell are varied and include cell surface sensors that activate intracellular chemical signaling pathways. Here, possible mechanical sensors of the cell including reorganization of the cytoskeleton and the nucleus are discussed in relation to shear flow. A mutation in the nuclear structural protein lamin A, related to Hutchinson-Gilford progeria syndrome, is reviewed specifically as the mutation results in altered nuclear structure and stiffer nuclei; animal models also suggest significantly altered vascular structure. Nuclear and cellular deformation of endothelial cells in response to shear stress provides partial understanding of possible mechanical regulation in the microcirculation. Increasing sophistication of fluid flow simulations inside the vessel is also an emerging area relevant to the microcirculation as visualization in situ is difficult. This integrated approach to study--including medicine, molecular and cell biology, biophysics and engineering--provides a unique understanding of multi-scale interactions in the microcirculation.

Interaction of the C-terminal region of the Ggamma protein with model membranes

Heterotrimeric G-proteins interact with membranes. They accumulate around membrane receptors and propagate messages to effectors localized in different cellular compartments. G-protein-lipid interactions regulate G-protein cellular localization and activity. Although we recently found that the Gbetagamma dimer drives the interaction of G-proteins with nonlamellar-prone membranes, little is known about the molecular basis of this interaction. Here, we investigated the interaction of the C-terminus of the Ggamma(2) protein (P(gamma)-FN) with model membranes and those of its peptide (P(gamma)) and farnesyl (FN) moieties alone. X-ray diffraction and differential scanning calorimetry demonstrated that P(gamma)-FN, segregated into P(gamma)-FN-poor and -rich domains in phosphatidylethanolamine (PE) and phosphatidylserine (PS) membranes. In PE membranes, FN increased the nonlamellar phase propensity. Fourier transform infrared spectroscopy experiments showed that P(gamma) and P(gamma)-FN interact with the polar and interfacial regions of PE and PS bilayers. The binding of P(gamma)-FN to model membranes is due to the FN group and positively charged amino acids near this lipid. On the other hand, membrane lipids partially altered P(gamma)-FN structure, in turn increasing the fluidity of PS membranes. These data highlight the relevance of the interaction of the C-terminal region of the Ggamma protein with the cell membrane and its effect on membrane structure.

Farnesylated peptides in model membranes: a biophysical investigation

Protein prenylation plays an important role in signal transduction, protein-protein interactions, and the localization and association of proteins with membranes. Using three different techniques, this study physically characterizes the interactions between model dimyristoylphosphatidylcholine membranes and a series of farnesylated peptides. Magic angle spinning nuclear Overhauser enhancement spectroscopy and differential scanning calorimetry reveal that both charged [Ac-Asn-Lys-Asn-Cys-(farnesyl)-OMe and Ac-Asn-Lys-Asn-Cys-(farnesyl)-NH(2)] and uncharged [Ac-Cys-(farnesyl)-OMe and farnesol] species partition into dimyristoylphosphatidylcholine bilayers. Calorimetry and vesicle fluctuation analysis of giant unilamellar vesicles show that the charged peptides modestly decrease the main gel-fluid phase transition and markedly increase the bending rigidity of large unilamellar vesicles. Uncharged species, on the other hand, dramatically decrease the main phase transition and modestly decrease the bending rigidity. No difference with carboxyl methylation is detected.

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 by chemoselective ligation and biological evaluation of novel cell-permeable PKC-zeta pseudosubstrate lipopeptides

The ability of lipopeptides to passively cross the cell membrane opens new opportunities for the intracellular delivery of bioactive peptides. However, the production of large series of cell-permeable lipopeptides is not trivial due to their generally low solubility. We have evaluated the possibility of associating the fatty acid to the functional cargo using generally applicable ligation chemistries. To this end, we have designed an amphiphilic shuttle in which arginine residues served to solubilize the lipid part in aqueous media, during both the assembly of the lipopeptide and the cellular assays. Our model peptide, the pseudosubstrate sequence of protein kinase C-zeta (PKC-zeta), was associated to the pentapeptide Gly-Arg-Gly-Arg-Lys(Pam)-NH2 through thiazolidine, thioether, disulfide, or hydrazone linkages. The cytoplasm import of the resulting constructs was monitored through the quantification of the apoptosis specifically induced by PKC-zeta inhibition. Our observations suggested the interest of this noninvasive cellular import method to modulate the activity of an intracytoplasmic pharmacological target and showed the influence of a non-amide link created between the functional peptide and the lipidic vector: optimal results, in terms of both specific activity and low basal cytotoxicity, were obtained with the thiazolidine ligation product.

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.

Biophysical studies of lipopeptide-membrane interactions

There are a number of naturally occurring motifs for lipidation of peptides and proteins. In cases in which this involves adding a single hydrocarbon chain to the peptide, it is either a fatty acid or an isoprenyl group. Lipopeptides will partition between membrane and aqueous phases. When only one hydrocarbon chain is attached to the peptide, the affinity of the lipopeptide for the membrane is only marginally increased over that of the free peptide. The resulting partitioning is largely determined by the extent of the interaction of the peptide moiety with the membrane. In contrast, lipidation involving two hydrocarbon chains, either as two single chains attached at distinct locations of the peptide or a double-chain lipid anchor, firmly attaches the lipopeptide to the membrane. This can allow the placement of specific binding sites on a membrane surface. Such a strategy can be used, for example, to place specific antibodies on the surface of drug-carrying liposomes for the purpose of targeting drug delivery. In addition, lipopeptides will alter the physical properties of membranes. One of these effects is to increase the bilayer to hexagonal phase transition temperature. Substances having this property may also alter functional properties of membranes. While it is unlikely that these changes in the biophysical properties of the membranes. While it is unlikely that these changes in the biophysical properties of the membrane are responsible for specific functions of lipopeptides, such changes may be used to modulate certain properties of a membrane, such as the rate of viral fusion.

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.

Structural requirements for activity of the pheromones of Ustilago hordei

Ustilago hordei, the cause of barley-covered smut, initiates mating with pheromones. Gene sequence analysis suggested that these pheromones, Uhmfa1 and Uhmfa2, would be farnesylated peptides. Although isolation of mating-type-specific activity was rarely possible, chromatographic separations of culture supernatants yielded fractions that stimulated or inhibited mating. Based on predicted amino acid sequences and mass spectra of stimulating fractions, a series of pheromone analogs were synthesized and their activities were determined. Underivatized Uhmfa1 (PGKSGSGLGYSTC) or Uhmfa2 (EGKGEPAPYC) peptides were inactive, while peptides that were farnesylated and/or methyl esterified specifically induced conjugation tubes by cells of the opposite mating type. Uhmfa1 truncated from the amino terminus beyond the lysine lost activity, while truncated Uhmfa2 remained partially active. In mating bioassays, a pheromone concentration-dependent default mating response was observed. In competition studies, shorter Uhmfa1 peptides lacking pheromone activity inhibited activity of full-length peptides most effectively when both had the same functional groups.

Properties of lipoamino acids incorporated into membrane bilayers

Several lipoamino acids were synthesized in which palmitic acid was coupled with the alpha-amino group of an amino acid. These lipoamino acids were tested for their inhibitory action against Sendai virus fusion to liposomes composed of egg phosphatidylethanolamine and 5 mol% of the ganglioside GD1a. A commonly employed viral fusion assay based on the dilution of the fluorescent probe octadecylrhodamine (R18) exhibited an additional complication in the presence of Nalpha-palmitoyl tryptophan (palm-Trp). At higher mol fraction of palm-Trp it was observed that there was an increase in R18 quenching. Studies on the dependence of the emission wavelength of palm-Trp on excitation wavelength demonstrated that the presence of R18 alters the environment of the indole. The results illustrate one of the complexities of viral fusion assays using the R18 probe. Despite this complication it was possible to demonstrate that several of the lipoamino acids are effective at inhibiting the fusion of Sendai virus to liposomes as measured by the R18 assay. One of the most effective inhibitors of this process is palm-Trp which, at a concentration of 4 mol% in liposomes, markedly reduces the apparent rate of fusion. At pH 5.0 this amphiphile is also an inhibitor of Sendai virus fusion, indicating that the ionization of the carboxyl group of this amphiphile is not required for its antiviral activity. The inhibitory action of palm-Trp against Sendai virus was confirmed by demonstrating inhibition of Sendai-mediated cytopathic effects studied in tissue culture. A property associated with antiviral activity is the ability of amphiphiles to raise the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine. All of these lipoamino acids were found to possess this property, but a quantitative relationship with inhibition of viral fusion was not found.

Lipogastrins as potent inhibitors of viral fusion

The rate and extent of membrane fusion is markedly sensitive to membrane interfacial properties. Lipopeptides with hydrophilic peptide moieties will insert into membranes, leaving the peptide portion at the membrane-water interface. In this work, we have used a lipopeptide composed of the peptide [Nle15]-gastrin-(2-17)-amide covalently linked to 1,2-diacyl-3-mercaptoglycerol-N(alpha)-maleoyl-beta-alanine to give DM-gastrin or DP-gastrin having 14 or 16 carbon atom acyl chains, respectively. The fluorescence emission from the two Trp residues of these lipopeptides exhibited little or no blue shift upon addition of liposomes of egg-phosphatidylethanolamine containing 5 mol% G(D1a). Iodide quenching of DP-gastrin fluorescence was also independent of lipid. These results indicate that the peptide moiety is exposed to the aqueous environment even though the lipopeptide is firmly anchored to the membrane. Both DM and DP-gastrin markedly raise the bilayer to hexagonal phase transition temperature of dipalmitoleoyl phosphatidylethanolamine. However, DM-E5 lowers this phase transition temperature. These lipopeptides have effects on the overall fusion of Sendai virus to liposomes in accord with their opposite effects on lipid curvature. The lipogastrins are potent inhibitors of viral fusion, while DM-E5 slightly promotes this process. Truncated forms of DM-gastrin are also inhibitory to viral fusion, but are less inhibitory than the full lipopeptide. Analysis of the fusion kinetics shows that DP-gastrin causes a reduction in the final extent of fusion and a marked lowering of the fusion rate constant. Binding of Sendai virus to the ganglioside receptor-containing liposomes was not affected. Consideration of the various contributions to the mechanism of inhibition of viral fusion suggests that effects of lipogastrin on membrane intrinsic monolayer curvature is of primary importance.

On the mechanism of hormone recognition and binding by the CCK-B/gastrin receptor

Lipidation with long-chain di-fattyacyl-glycerol moieties was used to anchor gastrin in CCK peptides irreversibly to lipid bilayers lipopeptide transfer to model phospholipid bilayers is fast and quantitative, leading to a different mode of insertion of lipo-gastrin and lipo-CCK in lipid layers. Lipo-gastrin remains exposed to the bulk solvent in a predominantly random coil structure as a consequence of electrostatic repulsion, whereas lipo-CCK exhibits a pronounced tendency to form peptide domains with insertion of its C-terminus into more hydrophobic compartments of the bilayers. Thereby Ca2+ at physiological concentrations favours this aggregational phenomenon. Since both lipo-peptides were found to retain almost full receptor affinity despite their irreversible anchorage to the bilayer, a membrane-bound pathway in the receptor recognition and binding process is indeed possible. According to the data collected in this study, CCK might possibly use this pathway, whereas accumulation of gastrin on the cell membrane with prefolding of the ligand at the water/lipid interface is hardly conceivable. Nevertheless the observed receptor interaction of the deliberately membrane-anchored gastrin offers interesting constraints for computational docking experiments on a modelled CCK-B/gastrin receptor by additionally taking into account information derived from mutagenesis studies. Despite the limitations of such modelling experiments, the resulting picture of the gastrin/receptor complex allowed the visualization and rationalization of the experimental results of the extensive structure-function studies performed previously on this family of gastrointestinal hormones.

Fungal lipopeptide mating pheromones: a model system for the study of protein prenylation

In a variety of fungal species, mating between haploid cells is initiated by the action of peptide pheromones. The identification and characterization of several fungal pheromones has revealed that they have common structural features classifying them as lipopeptides. In the course of biosynthesis, these pheromones undergo a series of posttranslational processing events prior to export. One common modification is the attachment of an isoprenoid group to the C terminus of the pheromone precursor. Genetic and biochemical investigations of this biosynthetic pathway have led to the elucidation of genes and enzymes which are responsible for isoprenylation of other polypeptides including the nuclear lamins, several vesicular transport proteins, and the oncogene product Ras. The alpha-factor of Saccharomyces cerevisiae serves as a model for studying the biosynthesis, export, and bioactivity of lipopeptide pheromones. In addition to being isoprenylated with a farnesyl group, the alpha-factor is secreted by a novel peptide export pathway utilizing a yeast homolog of the mammalian multidrug resistance P-glycoprotein. The identification of putative lipopeptide-encoding loci within other fungi, including the human immunodeficiency virus-associated opportunistic pathogen Cryptococcus neoformans and the plant pathogen Ustilago maydis, has stimulated much interest in understanding possible roles for pheromones in fungal proliferation and pathogenicity. Knowledge of variations within the processing, export, and receptor-mediated signal transduction pathways associated with different fungal lipopeptide pheromones will continue to provide insights into similar mechanisms which exist in higher eukaryotes.

Lipid-mediated a-factor interactions with artificial membranes

Our understanding of the cellular export of a-factor and its interaction with the receptor do not yet allow for a description of the phenomena on a molecular level. Synthesis of a-factor analogs and biophysical studies of the lipopeptides in the presence of artificial membranes provide insights which can be analyzed with respect to the biological potency of the molecules. It is through the study of the interaction of the lipopeptides with membranes at varying levels of complexity that we will be able to develop a molecular description of the biological processes.

Consequences of altered isoprenylation targets on a-factor export and bioactivity

Cysteine-containing amino acid sequences (CAAX, CC, and CXC; C is cysteine, A is any aliphatic amino acid, and X is any amino acid) are targets for the attachment of C15 (farnesyl) and C20 (geranylgeranyl) isoprenoids to peptides and proteins by specific prenyltransferases. Although much work has centered on the enzymatic mechanisms of these enzymes, the biological consequences of the differential isoprenylation they catalyze remain to be elucidated. Farnesylation of the a-factor mating pheromone of Saccharomyces cerevisiae is a known prerequisite for its biological activity and its secretion through a pathway utilizing the yeast STE6 protein, a homolog of the mammalian multidrug resistance (MDR) P-glycoprotein. We generated specific mutations in the a-factor gene to encode isoprenylation targets for geranylgeranylation [Cys-Val-Ile-Leu (CVIL) and Ser-Val-Cys-Cys (SVCC)] in place of the natural farnesylation motif [Cys-Val-Ile-Ala (CVIA)]. The a-factors containing these modified prenylation sites were successfully exported by a STE6-dependent mechanism. Furthermore, these peptides, as well as synthetic geranylgeranyl a-factor, retained bioactivity. Chromatographic comparisons of synthetic and biosynthetic pheromones suggest that, in vivo, a peptide substrate containing the geranylgeranylation target CVIL can be both farnesylated and geranylgeranylated. These results clearly demonstrate that in vivo (i) different prenyltransferases may recognize the same substrate; (ii) both farnesylated and geranylgeranylated a-factor peptides are substrates for export via STE6, a MDR-like protein; and (iii) farnesylated and geranylgeranylated pheromones are both biologically active.