Acid phospholipid vesicles produce conformational changes on the antitumour protein alpha-sarcin

The interaction of the ribotoxin α-sarcin with complex model lipid vesicles

Fungal ribotoxins are extracellular RNases that inactivate ribosomes by cleaving a single phosphodiester bond at the universally conserved sarcin-ricin loop of the large rRNA. However, to reach the ribosomes, they need to cross the plasma membrane. It is there where these toxins show their cellular specificity, being especially active against tumoral or virus-infected cells. Previous studies have shown that fungal ribotoxins interact with negatively charged membranes, typically containing phosphatidylserine or phosphatidylglycerol. This ability is rooted on their long, non-structured, positively charged loops, and its N-terminal β-hairpin. However, its effect on complex lipid mixtures, including sphingophospholipids or cholesterol, remains poorly studied. Here, wild-type α-sarcin was used to evaluate its interaction with a variety of membranes not assayed before, which resemble much more closely mammalian cell membranes. The results confirm that α-sarcin is particularly sensitive to charge density on the vesicle surface. Its ability to induce vesicle aggregation is strongly influenced by both the lipid headgroup and the degree of saturation of the fatty acid chains. Acyl chain length is indeed particularly important for lipid mixing. Finally, cholesterol plays an important role in diluting the concentration of available negative charges and modulates the ability of α-sarcin to cross the membrane.

Electronic structure variations of polar and nonpolar ZnO lattices with nitrogen-ion bombardment using synchrotron-based in situ photoemission and X-ray absorption spectroscopy

Surface polarity with different crystal orientations has been demonstrated as a crucial parameter in determining the physical properties and device applications in many transition metal oxide and semiconductor compound systems. The influences of surface polarity on electronic structures in nitrogen-incorporated ZnO lattices have been investigated in the present work. The successful doping of nitrogen atoms in ZnO lattices is suggested by the existence of N-related chemical bonds obtained from X-ray photoelectron spectroscopy analysis where a pronounced N-Zn peak intensity has been observed in the (000\bar 1)-terminated polar ZnO compound compared with the (10\bar 10)-terminated nonpolar ZnO compound. An energy shift of the valence band maximum towards the Fermi level has been resolved for both polar and nonpolar ZnO lattices, whereas a charge redistribution of the O 2p hybridized states is only resolved for o-plane ZnO with a polar surface. Angular-dependent X-ray absorption analyses at the O K-edge reveal enhanced surface-state contributions and asymmetric O 2p orbital occupations in the (000\bar 1)-terminated o-plane ZnO compound. The results shed light on the efficient nitrogen doping in ZnO lattices with polar surfaces. The comprehensive electronic structure investigations of correlations between impurity doping and surface polarity in ZnO lattices may also offer guidance for the material design in other transition metal oxide and semiconductor systems.

Heterologous Production and Functional Characterization of Ageritin, a Novel Type of Ribotoxin Highly Expressed during Fruiting of the Edible Mushroom Agrocybe aegerita

Fungi produce various defense proteins against antagonists, including ribotoxins. These toxins cleave a single phosphodiester bond within the universally conserved sarcin-ricin loop of ribosomes and inhibit protein biosynthesis. Here, we report on the structure and function of ageritin, a previously reported ribotoxin from the edible mushroom Agrocybe aegerita The amino acid sequence of ageritin was derived from cDNA isolated from the dikaryon A. aegerita AAE-3 and lacks, according to in silico prediction, a signal peptide for classical secretion, predicting a cytoplasmic localization of the protein. The calculated molecular weight of the protein is slightly higher than the one reported for native ageritin. The A. aegerita ageritin-encoding gene, AaeAGT1, is highly induced during fruiting, and toxicity assays with AaeAGT1 heterologously expressed in Escherichia coli showed a strong toxicity against Aedes aegypti larvae yet not against nematodes. The activity of recombinant A. aegerita ageritin toward rabbit ribosomes was confirmed in vitro Mutagenesis studies revealed a correlation between in vivo and in vitro activities, indicating that entomotoxicity is mediated by ribonucleolytic cleavage. The strong larvicidal activity of ageritin makes this protein a promising candidate for novel biopesticide development.IMPORTANCE Our results suggest a pronounced organismal specificity of a protein toxin with a very conserved intracellular molecular target. The molecular details of the toxin-target interaction will provide important insight into the mechanism of action of protein toxins and the ribosome. This insight might be exploited to develop novel bioinsecticides.

Sticholysin, Sphingomyelin, and Cholesterol: A Closer Look at a Tripartite Interaction

Actinoporins are a group of soluble toxic proteins that bind to membranes containing sphingomyelin (SM) and oligomerize to form pores. Sticholysin II (StnII) is a member of the actinoporin family produced by Stichodactyla helianthus. Cholesterol (Chol) is known to enhance the activity of StnII. However, the molecular mechanisms behind this activation have remained obscure, although the activation is not Chol specific but rather sterol specific. To further explore how bilayer lipids affect or are affected by StnII, we have used a multiprobe approach (fluorescent analogs of both Chol and SM) in combination with a series of StnII tryptophan (Trp) mutants to study StnII/bilayer interactions. First, we compared StnII bilayer permeabilization in the presence of Chol or oleoyl-ceramide (OCer). The comparison was done because both Chol and OCer have a 1-hydroxyl, which helps to orient the molecule in the bilayer (although OCer has additional polar functional groups). Both Chol and OCer also have increased affinity for SM, which StnII may recognize. However, our results show that only Chol was able to activate StnII-induced bilayer permeabilization; OCer failed to activate it. To further examine possible Chol/StnII interactions, we measured Förster resonance energy transfer between Trp in StnII and cholestatrienol, a fluorescent analog of Chol. We could show higher Förster resonance energy transfer efficiency between cholestatrienol and Trps in position 100 and 114 of StnII when compared to three other Trp positions further away from the bilayer binding region of StnII. Taken together, our results suggest that StnII was able to attract Chol to its vicinity, maybe by showing affinity for Chol. SM interactions are known to be important for StnII binding to bilayers, and Chol is known to facilitate subsequent permeabilization of the bilayers by StnII. Our results help to better understand the role of these important membrane lipids for the bilayer properties of StnII.

Involvement of loops 2 and 3 of α-sarcin on its ribotoxic activity

Ribotoxins are a family of fungal ribosome-inactivating proteins displaying highly specific ribonucleolytic activity against the sarcin/ricin loop (SRL) of the larger rRNA, with α-sarcin as its best-characterized member. Their toxicity arises from the combination of this activity with their ability to cross cell membranes. The involvement of α-sarcin's loops 2 and 3 in SRL and ribosomal proteins recognition, as well as in the ribotoxin-lipid interactions involving cell penetration, has been suggested some time ago. In the work presented now different mutants have been prepared in order to study the role of these loops in their ribonucleolytic and lipid-interacting properties. The results obtained confirm that loop 3 residues Lys 111, 112, and 114 are key actors of the specific recognition of the SRL. In addition, it is also shown that Lys 114 and Tyr 48 conform a network of interactions which is essential for the catalysis. Lipid-interaction studies show that this Lys-rich region is indeed involved in the phospholipids recognition needed to cross cell membranes. Loop 2 is shown to be responsible for the conformational change which exposes the region establishing hydrophobic interactions with the membrane inner leaflets and eases penetration of ribotoxins target cells.

Hirsutellin A: A Paradigmatic Example of the Insecticidal Function of Fungal Ribotoxins

The fungal pathogen Hirsutella thompsonii produces an insecticidal protein named hirsutellin A (HtA), which has been described to be toxic to several species of mites, insect larvae, and cells. On the other hand, on the basis of an extensive biochemical and structural characterization, HtA has been considered to be a member of the ribotoxins family. Ribotoxins are fungal extracellular ribonucleases, which inactivate ribosomes by specifically cleaving a single phosphodiester bond located at the large rRNA. Although ribotoxins were brought to light in the 1960s as antitumor agents, their biological function has remained elusive. Thus, the consideration of hirsutellin A, an insecticidal protein, as a singular ribotoxin recalled the idea of the biological activity of these toxins as insecticidal agents. Further studies have demonstrated that the most representative member of the ribotoxin family, α-sarcin, also shows strong toxic action against insect cells. The determination of high resolution structures, the characterization of a large number of mutants, and the toxicity assays against different cell lines have been the tools used for the study of the mechanism of action of ribotoxins at the molecular level. The aim of this review is to serve as a compilation of the facts that allow identification of HtA as a paradigmatic example of the insecticidal function of fungal ribotoxins.

Production and characterization of scFvA33T1, an immunoRNase targeting colon cancer cells

Within the last 10 years, the use of different RNases as therapeutic agents for various diseases has been pursued. Furthermore, the advancements of recombinant technology have allowed the assembly of proteins with different functions. In this regard, immunoribonucleases (immunoRNases) stand out as some of the most promising therapeutic candidates given their enzymatic and non-mutagenic character. Accordingly, the work reported here describes fusing RNase T1, one of the most studied members of the microbial RNase family, to the single-chain variable fragment (scFv) of a monoclonal antibody that targets the glycoprotein A33 antigen (GPA33) from human colon cancer cells. A heterologous production system, which employs the yeast Pichia pastoris, has been optimized to produce this immunoRNase (scFvA33T1) with yields of ∼ 5-10 mg · L(-1). The purified protein appears to be correctly folded as it retains its antigen specificity and ribonucleolytic activity. Finally, it also shows specific binding to, internalization into and toxicity against GPA33-positive cell lines compared with the control, GPA33-negative cells. Overall, it can be concluded that scFvA33T1 is a promising therapeutic fusion protein with the additional advantage that presumably it can be produced and purified in large amounts using an easily scalable yeast-based system.

Production and characterization of a colon cancer-specific immunotoxin based on the fungal ribotoxin α-sarcin

A single-chain fusion protein that directed the cytolytic activity of α-sarcin to A33 tumor antigen expressing cells was constructed and shown to effectively kill targeted cells. Glycoprotein A33 (GPA33) is a well-known colon cancer marker and a humanized antibody against it was used to target the α-sarcin. The fungal ribotoxin α-sarcin is one of the most potent and specific toxins known. It is small, protease resistant, thermostable and highly efficient towards the inactivation of ribosomes. This work describes the production and characterization of an immunotoxin resulting from fusing the single-chain variable fragment (scFv) of the monoclonal antibody that targets GPA33 to fungal α-sarcin. This chimeric protein (scFvA33αsarcin), produced in Pichia pastoris and purified in high yield was proven to be properly folded, active, specific and stable. It showed high specific toxicity against GPA33-positive tumoral cell lines providing scientific evidence to sustain that scFvA33αsarcin is a good immunotherapeutic candidate against GPA33-positive colon carcinomas.

The insecticidal protein hirsutellin A from the mite fungal pathogen Hirsutella thompsonii is a ribotoxin

The mite fungal pathogen Hirsutella thompsonii produces a single polypeptide chain, insecticidal protein named hirsutellin A (HtA) that is composed of 130 amino acid residues. This protein has been purified from its natural source and produced as a recombinant protein in Escherichia coli. Spectroscopic analysis has determined that the two protein forms are indistinguishable. HtA specifically inactivates ribosomes and produces the alpha-fragment characteristic of ribotoxin activity on rRNA. Behaving as a cyclizing ribonuclease, HtA specifically cleaves oligonucleotides that mimick the sarcin/ricin loop of the ribosome, as well as selected polynucleotides and dinucleosides. HtA interacts with phospholipid membranes as do other ribotoxins. As a consequence of its ribonuclease activity and its ability to interact with cell membranes, HtA exhibits cytotoxic activity on human tumor cells. On the basis of these results, HtA is considered to be a member of the ribotoxin group of proteins, although it is significantly smaller (130 aa) than all known ribotoxins that are composed of 149/150 amino acids. Ribotoxins are members of a larger family of fungal ribonucleases whose members of smaller size (100/110 aa) are not cytotoxic. Thus, the characterization of the fungal ribotoxin HtA represents an important milestone in the study of the diversity and the function of fungal ribonucleases.

Fungal ribotoxins: molecular dissection of a family of natural killers

RNase T1 is the best known representative of a large family of ribonucleolytic proteins secreted by fungi, mostly Aspergillus and Penicillium species. Ribotoxins stand out among them by their cytotoxic character. They exert their toxic action by first entering the cells and then cleaving a single phosphodiester bond located within a universally conserved sequence of the large rRNA gene, known as the sarcin-ricin loop. This cleavage leads to inhibition of protein biosynthesis, followed by cellular death by apoptosis. Although no protein receptor has been found for ribotoxins, they preferentially kill cells showing altered membrane permeability, such as those that are infected with virus or transformed. Many steps of the cytotoxic process have been elucidated at the molecular level by means of a variety of methodological approaches and the construction and purification of different mutant versions of these ribotoxins. Ribotoxins have been used for the construction of immunotoxins, because of their cytotoxicity. Besides this activity, Aspf1, a ribotoxin produced by Aspergillus fumigatus, has been shown to be one of the major allergens involved in allergic aspergillosis-related pathologies. Protein engineering and peptide synthesis have been used in order to understand the basis of these pathogenic mechanisms as well as to produce hypoallergenic proteins with potential diagnostic and immunotherapeutic applications.

The antifungal protein from Aspergillus giganteus causes membrane permeabilization

We investigated the inhibitory effects of the antifungal protein (AFP) from Aspergillus giganteus on the growth of several filamentous fungi. For this purpose, the MICs of AFP were determined and ranged from 0.1 micro g/ml for Fusarium oxysporum to 200 micro g/ml for Aspergillus nidulans. The antifungal activity of AFP was diminished in the presence of cations. We were able to show that incubation of AFP-sensitive fungi with the protein resulted in membrane permeabilization using an assay based on the uptake of the fluorescent dye SYTOX Green. No permeabilization by AFP could be detected at concentrations below the species-specific MIC. Furthermore, AFP-induced permeabilization could readily be detected after 5 min of incubation. Localization experiments with fluorescein isothiocyanate-labeled AFP and immunofluorescence staining with an AFP-specific antibody supported the observation that the protein interacts with membranes. After treatment of AFP-sensitive fungi with AFP, the protein was localized at the plasma membrane, whereas it was mainly detected inside the cells of AFP-resistant fungi. We conclude from these data that the growth-inhibitory effect of AFP is caused by permeabilization of the fungal membranes.

The pH-dependent interaction of cinnamomin with lipid membranes investigated by fluorescence methods

Cinnamomin, a new type II ribosome-inactivating protein (RIP), was found to be able to induce the release of calcein loaded in lecithin small unilamellar vesicles and the fusion or aggregation of the lecithin liposomes. Such induction could be promoted severalfold by a pH 5.0 environment, a condition similar to that in endocytic vesicles. Lowering the pH from 7.5 to 5.0 evoked conformational changes of cinnamomin and unmasked its hydrophobic areas, including the exposure of 1-anilino-8-naphthalenesulfonate (1,8-ANS) binding sites of the molecule. Some tryptophan residues with affinity to acrylamide were demonstrated to participate in the lipid-protein interaction. The pH dependent fusogenicity of type II RIP might suggest its in vivo function as a fusogen to exert its cytotoxicity.

Ribotoxins are a more widespread group of proteins within the filamentous fungi than previously believed

Alpha-sarcin, restrictocin and mitogillin are the best known members of the family of fungal ribotoxins. In recent years, new members of this family have been discovered and characterised. In this work, we study the occurrence of ribotoxins among different species of fungi. The presence of ribotoxins has been identified in some new species by means of genetic studies, as well as expression and activity assays. The ribotoxin genes have been partially sequenced, and demonstrate a high degree of similarity. These studies demonstrate that these toxins are more widespread than previously considered. This is surprising, considering the ribotoxins are such specific and potent toxins, of unknown biological function. These studies confirm the hypothesis that these proteins are naturally engineered toxins derived from ribonucleases of broad substrate specificity.

Characterization of pKa values and titration shifts in the cytotoxic ribonuclease alpha-sarcin by NMR. Relationship between electrostatic interactions, structure, and catalytic function

The electrostatic behavior of titrating groups in alpha-sarcin was investigated using 1H NMR spectroscopy. A total of 209 chemical shift titration curves corresponding to different protons in the molecule were determined over the pH range of 3.0-8.5. Nonlinear least-squares fits of the data to simple relationships derived from the Henderson-Hasselbalch equation led to the unambiguous determination of pKa values for all glutamic acid and histidine residues, as well as for the C-terminal carboxylate and most of the aspartic acids in the free enzyme. The ionization constants of catalytically relevant histidines, His50 and His137, and glutamic acid, Glu96, in the alpha-sarcin-2'-GMP complex were also determined. The pKa values of 15 ionizable groups (C-carboxylate, six aspartic acids, four glutamic acids, and four histidines) were found to be close to their normal values. On the other hand, a number of side chain groups, including those in the active center, showed pKa values far from their intrinsic values. Thus, the pKa values for active site residues His50, Glu96, and His137 were 7.7, 5.2, and 5.8 in the free enzyme and 7.6, approximately 4.8, and 6.8 in the alpha-sarcin-2'-GMP complex, respectively. The pKa values and the activity profile against ApA, as a function of pH, are in agreement with the proposed enzymatic mechanism (in common with RNase T1 and the family of the microbial ribonucleases), in which Glu96 and His137 act as a general base and general acid, respectively. In almost all microbial ribonucleases, a Phe-His interaction is present, which affects the pKa of one of the His residues at the active site (His137). The absence of this interaction in alpha-sarcin would explain the lower pKa value of this His residue, and provides an explanation for the decreased RNase activity of this protein as compared to those of other microbial ribonucleases.

Oligomerization of the cytotoxin alpha-sarcin associated with phospholipid membranes

alpha-Sarcin is a cytotoxic protein that specifically inactivates ribosomes. The protein translocates across phospholipid membranes. Oligomerization of the protein occurs upon interaction with membranes. Chemically cross-linked protein oligomers have been obtained by treatment of protein-vesicle complexes with the membrane impermeant reagent bis-(sulfosuccinimidyl) suberate. These structures are only obtained in the presence of acidic lipid vesicles composed of either natural or synthetic phospholipids. Such oligomers are not produced in concentrated protein solutions in the absence of vesicles. The formation of the chemically stabilized oligomers is saturated at the same lipid to protein molar ratio as all the perturbations caused by alpha-sarcin on lipid vesicles. Results are discussed in terms of the involvement of oligomer formation on protein translocation across membranes.

A peptide of nine amino acid residues from alpha-sarcin cytotoxin is a membrane-perturbing structure

A water-soluble synthetic peptide with only nine amino acid residues, comprising the 131-139 sequence region of the cytotoxic protein alpha-sarcin (secreted by the mold Aspergillus giganteus), interacts with large unilamellar vesicles composed of acid phospholipids. It promotes lipid mixing between bilayers and leakage of vesicle aqueous contents, and it also abolishes the phospholipid phase transition. Other larger peptides containing such an amino acid sequence also produce these effects. These peptides acquire alpha-helical conformation in the presence of trifluoroethanol, but display beta-strand conformation in the presence of sodium dodecyl sulfate. The interaction of these peptides with the lipid vesicles also results in beta-structure. The obtained data are discussed in terms of the involvement of the 131-139 stretch of alpha-sarcin in its interaction with lipid membranes.

Insights into specificity of cleavage and mechanism of cell entry from the crystal structure of the highly specific Aspergillus ribotoxin, restrictocin

BACKGROUND

Restriction, a highly specific ribotoxin made by the fungus Aspergillus restrictus, cleaves a single phosphodiester bond in the 28S RNA of eukaryotic ribosomes, inhibiting protein synthesis. The sequence around this cleavage site is a binding site for elongation factors, and is conserved in all cytoplasmic ribosomes. The catalytic mechanism of restrictocin and the reasons for its high substrate specificity are unknown. No structure has been determined for any other member of the Aspergillus ribotoxin family.

RESULTS

The crystal structure of restrictocin was determined at 2.1 A resolution by single isomorphous replacement and anomalous scattering techniques, and refined to 1.7 A resolution using synchrotron Laue data. The structural core of the protein, in which a three-turn alpha helix is packed against a five-stranded antiparallel beta sheet, can be well aligned with that of ribonuclease T1. Large positively charged peripheral loops near the active site construct a platform with a concave surface for RNA binding.

CONCLUSIONS

Restriction appears to combine the catalytic components of T1 ribonucleases with the base recognition components of Sa ribonucleases. Modeling studies using an NMR structure of an RNA substrate analog suggest that the tertiary structure of the substrate RNA is important in protein-RNA recognition, fitting closely into the concavity of the presumed binding site. We speculate that the large 39-residue loop L3, which has similarities to loops found in lectin sugar-binding domains, may be responsible for restrictocin's ability to cross cell membranes.

Thermal unfolding of the cytotoxin alpha-sarcin: phospholipid binding induces destabilization of the protein structure

The effect of membrane binding on the structure and stability of the cytotoxin alpha-sarcin has been studied by differential scanning calorimetry, Fourier-transform infrared and fluorescence spectroscopic techniques. The thermal unfolding of alpha-sarcin in aqueous solution fits into a two-state transition characterized by a transition temperature (Tm) of 52.6 degrees C and a calorimetric enthalpy (delta Hcal) of 136 kcal/mol. Upon interaction with phosphatidylglycerol vesicles, alpha-sarcin undergoes conformational changes, as deduced from the FTIR and fluorescence emission spectra. These changes result in a decreased Tm and delta Hcal values for the thermal unfolding of phospholipid-bound alpha-sarcin. The lower Tm value for lipid-bound alpha-sarcin is also observed at the level of secondary and tertiary structures, based on analyses of both the amide I' infrared spectrum and the tryptophan emission of the protein as a function of temperature, respectively. The results obtained indicate a protein destabilization promoted by the phospholipid interaction.

Spectroscopic characterization of the alkylated alpha-sarcin cytotoxin: analysis of the structural requirements for the protein-lipid bilayer hydrophobic interaction

alpha-Sarcin is a ribosome-inactivating protein that translocates across lipid bilayers, these two abilities explaining its cytotoxic character. This protein is composed of a single polypeptide chain with two disulfide bridges. Reduction and carboxyamidomethylation of alpha-sarcin results in protein unfolding, based on the results of the spectroscopic characterization of the chemically modified protein. The absorption and fluorescence emission bands of the tryptophan residues of the modified protein appear blue- and red-shifted, respectively. Far-UV circular dichroism analysis reveals the presence of residual secondary structure (beta-strands and turns) in the alkylated protein. This retains its ability to interact with lipid bilayers. It promotes vesicle aggregation, lipid-mixing between bilayers and leakage of the intravesicular aqueous contents. The modified protein tends to abolish the phase transition of acid phospholipids as detected by differential scanning calorimetry and depolarization measurements of fluorescence-labelled vesicles. The protein gain access to vesicle-entrapped trypsin. The fluorescence emission of the tryptophan residues is blue-shifted upon interaction of the protein with the bilayers, and anthracene incorporated into the hydrophobic core of the membranes quenches the tryptophan fluorescence emission of the protein. The secondary structure of the alkylated protein interacting with lipid vesicles has been studied by infrared spectroscopy. An increase in the alpha-helix and turn contents and a concomitant decrease in the beta-structure content are observed upon interaction with the bilayers. The results obtained are discussed in terms of the structural requirements for the interaction of alpha-sarcin with lipid membranes.

Substitution of histidine-137 by glutamine abolishes the catalytic activity of the ribosome-inactivating protein alpha-sarcin

The alpha-sarcin cytotoxin is an extracellular fungal protein that inhibits protein biosynthesis by specifically cleaving one phosphodiester bond of the 28 S rRNA. The His137 residue of alpha-sarcin is suggested to be involved in the catalytic activity of this protein, based on the observed sequence similarity with some fungal ribonucleases. Replacement of this residue by Gln (H137Q mutant variant of alpha-sarcin) abolishes the ribonuclease activity of the protein. This has been demonstrated for an homogeneous preparation of the H137Q alpha-sarcin by measuring its effect against both intact rabbit ribosomes and the homopolymer poly(A). The conformation of H137Q alpha-sarcin is highly similar to that of the wild-type protein, which has been analysed by CD and fluorescence spectroscopy. Both H137Q and wild-type alpha-sarcin exhibit identical CD spectra in the peptide-bond region, indicating that no changes at the level of the secondary structure are produced upon mutation. Only minor differences are observed in both near-UV CD and fluorescence emission spectra in comparison to those of the wild-type protein. Moreover, H137Q alpha-sarcin interacts with phospholipid vesicles, promoting the same effects as the native cytotoxin. Therefore, we propose that His137 is part of the ribonucleolytic active site of the cytotoxin alpha-sarcin.

Conformational flexibility of pulmonary surfactant proteins SP-B and SP-C, studied in aqueous organic solvents

The structure of hydrophobic pulmonary surfactant-associated proteins SP-B and SP-C have been studied in different acetonitrile (ACN)/water and trifluorethanol (TFE)/water mixtures by circular dichroism and fluorescence spectroscopy to analyze the conformational flexibility of these proteins in response to changes in solvent composition. SP-B presented a very stable conformation in all the assayed ACN/water mixtures and in TFE/water mixtures containing until 70% TFE, showing around 40% alpha-helix. When SP-B was transferred to mixtures containing more than 70% TFE, the percent of alpha-helix in SP-B increased up to 60%. The fluorescence emission spectra of SP-B in the different solvents showed that tryptophan residues are more sensitive to solvent changes than those of tyrosine, reflecting differential effects on different protein microenvironments. The effect of solvent changes on the two tryptophan populations detected by fluorescence spectra was also different. A model for the folding of SP-B dimers, dominated by intra- and intermolecular disulphide bonds, is proposed. Surfactant protein SP-C revealed a secondary structure much more sensitive to solvent composition than SP-B. It had a main alpha-helical conformation in ACN/water solvents which was up to 63% in mixtures containing more than 60% ACN. When the protein was transferred to solvents containing less than 60% ACN, its secondary structure possessed less percent of alpha-helix and an increased percent of beta-structure. On the other hand, SP-C had a main beta-sheet secondary structure in all the assayed TFE/water mixtures, with 30-40% alpha-helix and around 50% beta-structure. The strong dependence of SP-C conformation on the nature of the solvent is interpreted to arise from its high hydrophobicity and the possible occurrence of protein-protein interactions.

Kinetic study of the aggregation and lipid mixing produced by alpha-sarcin on phosphatidylglycerol and phosphatidylserine vesicles: stopped-flow light scattering and fluorescence energy transfer measurements

alpha-Sarcin is a fungal cytotoxic protein that inactivates the eukaryotic ribosomes. A kinetic study of the aggregation and lipid mixing promoted by this protein on phosphatidylglycerol (PG) and phosphatidylserine (PS) vesicles has been performed. Egg yolk PG, bovine brain PS, dimyristoyl-PG (DMPG) and dimyristoyl-PS (DMPS) vesicles have been considered. The initial rates of the vesicle aggregation induced by the protein have been measured by stopped-flow 90 degrees light scattering. The formation of a vesicle dimer as the initial step of this process was deduced from the second-order dependence of the initial rates on phospholipid concentration. The highest alpha-sarcin concentration studied did not inhibit the vesicle aggregation, indicating that many protein molecules are involved in the vesicle cross-linking. These are common characteristics of the initial steps of the aggregation produced by alpha-sarcin in the four types of phospholipid vesicles considered. However, the kinetics of the scattering values revealed that more complex changes occurred in the later steps of the aggregation process of egg PG and brain PS vesicles than in those of their synthetic counterparts. alpha-Sarcin produced lipid mixing in vesicles composed of DMPG or DMPS, which was measured by fluorescence resonance energy transfer assays. A delay in the onset of the process, dependent on the protein concentration, was observed. Measurement of the rates of lipid mixing revealed that the process is first order on phospholipid concentration. Egg PG and brain PS vesicles did not show lipid mixing, although they avidly aggregated. However, alpha-sarcin was able to promote lipid mixing in heterogeneous systems composed of egg PG+DMPG or brain PS+DMPS vesicles. The dilution of the fluorescence probes was faster when these were incorporated into the bilayers made of synthetic phospholipids than were present in those made of natural phospholipids. The bilayer destabilization produced by the protein in the vesices composed of the dimyristoyl-phospholipids should be transmitted to the more stable ones made of natural phospholipids. The obtained results are interpreted in terms of lipid mixing occurring within vesicle aggregates larger than dimer.

Reconstitution of hepatitis B surface antigen proteins into phospholipid vesicles

Hepatitis B surface antigen (HBsAg), devoid of 75% of its total lipids has been reconstituted with several phospholipids by the detergent dialysis method, using the non-ionic detergent beta-D-octyl glucoside. Upon reconstitution with both neutral and acidic phospholipids, HBsAg particles had the same morphology and, as indicated by trypsin hydrolysis, the topology of the surface proteins was maintained. However, only negatively charged phospholipids were able to completely revert the conformational changes which had been induced by removal of the lipids. The helical content, as indicated by CD techniques, and the antigenic activity, as measured by binding to polyclonal antibodies, of HBsAg reconstituted with acidic phospholipids were practically identical to those of the native antigen. Cholesterol had no effect on the antigenic activity recovered by reconstitution with any of the phospholipids.

Overproduction and purification of biologically active native fungal alpha-sarcin in Escherichia coli

An efficient system was developed to produce, in Escherichia coli, large amounts of native alpha-sarcin (alpha Sar), a cytotoxin from the mold Aspergillus giganteus. The protein has been purified to homogeneity with a yield of 1.5 micrograms/ml of original culture. The constructed expression vector (pINPG alpha S) is based on the synthesis of a fusion protein between alpha Sar and a modified version of the OmpA signal peptide. This peptide seems to favour the postranslational processing of the fusion protein. The purified recombinant alpha-sarcin (re-alpha Sar) is structurally identical to the mature fungal protein according to the following criteria: N-terminal amino acid (aa) sequence, aa composition, electrophoretic mobility, chromatographic behaviour, immunoreactivity and spectroscopic features. Indeed, the recombinant protein recovered is completely functional, since it cleaves, in vitro, eukaryotic rRNA and it is able to interact with phospholipid vesicles with the same specificity as the native fungal alpha Sar.

Tryptophan fluorescence study on the interaction of pulmonary surfactant protein A with phospholipid vesicles

The fluorescence characteristics of surfactant protein A (SP-A) from porcine and human bronchoalveolar lavage were determined in the presence and absence of lipids. After excitation at either 275 or 295 nm, the fluorescence emission spectrum of both proteins was characterized by two maxima at about 326 and 337 nm, indicating heterogeneity in the emission of the two tryptophan residues of SP-A, and also revealing a partially buried character for these fluorophores. Interaction of both human and porcine SP-A with various phospholipid vesicles resulted in an increase in the fluorescence emission of tryptophan without any shift in the emission wavelength maxima. This change in intrinsic fluorescence was found to be more pronounced in the presence of dipalmitoyl phosphatidylcholine (DPPC) than with dipalmitoyl phosphatidylglycerol (DPPG), DPPC/DPPG (7:3, w/w) and 1-palmitoyl-sn-glycerol-3-phosphocholine (LPC). Intrinsic fluorescence of SP-A was almost completely unaffected in the presence of egg phosphatidylcholine (egg-PC). In addition, we demonstrated a shielding of the tryptophan fluorescence from quenching by acrylamide on interaction of porcine SP-A with DPPC, DPPG or LPC. This shielding was most pronounced in the presence of DPPC. In the case of human SP-A, shielding was only observed on interaction with DPPC. From the intrinsic fluorescence measurements as well as from the quenching experiments, we concluded that the interaction of some phospholipid vesicles with SP-A produces a conformational change on the protein molecule and that the interaction of SP-A with DPPC is stronger than with other phospholipids. This interaction appeared to be independent of Ca2+ ions. Physiological ionic strength was found to be required for the interaction of SP-A with negatively charged vesicles of either DPPG or DPPC/DPPG (7:3, w/w). Intrinsic fluorescence of SP-A was sensitive to the physical state of the DPPC vesicles. The increase in intrinsic fluorescence of SP-A in the presence of DPPC vesicles was much stronger when the vesicles were in the gel state than when they were in the liquid-crystalline state. The effect produced by SP-A on the lipid vesicles was also dependent on temperature. The aggregation of DPPC, DPPC/DPPG (7:3, w/w) or dimyristoyl phosphatidylglycerol (DMPG) was many times higher below the phase-transition temperature of the corresponding phospholipids. These results strongly indicate that the interaction of SP-A with phospholipid vesicles requires the lipids to be in the gel phase.

Translocation of alpha-sarcin across the lipid bilayer of asolectin vesicles

alpha-Sarcin is a cytotoxic protein produced by the mould Aspergillus giganteus. Insertion of alpha-sarcin into asolectin membranes has been demonstrated by protein labelling with photoreactive phospholipids. alpha-Sarcin added externally to tRNA-containing asolectin liposomes degrades the entrapped tRNA. Trypsin-containing asolectin liposomes were also prepared. Encapsulated trypsin degrades alpha-sarcin, even in the presence of a large excess of external hen egg-white trypsin inhibitor to prevent any alpha-sarin degradation outside the vesicles. These processes occur only with acidic phospholipids and were not observed when phosphatidylcholine vesicles were used. These results indicate that alpha-sarcin penetrates the lipid bilayer and becomes exposed to the lumen of negatively charged liposomes.

Solubility of hydrophobic surfactant proteins in organic solvent/water mixtures. Structural studies on SP-B and SP-C in aqueous organic solvents and lipids

The solubility of hydrophobic pulmonary surfactant proteins in different organic solvents and organic solvent/water combinations has been analyzed. Three organic solvents have been selected: methanol (MetOH), acetonitrile (ACN) and trifluoroethanol (TFE). Porcine SP-B showed very similar calculated secondary structure when dissolved in methanol, 60% ACN or 70% TFE and reconstituted in lysophosphatidylcholine (LPC) micelles or dipalmitoylphosphatidylcholine (DPPC) vesicles, as deduced from circular dichroism studies. SP-B was calculated to possess around 45% of alpha-helix in all these systems. The fluorescence emission spectrum of SP-B has been also characterized in aqueous solvents and lipids. It always showed a splitting of the tryptophan contribution into two components with different emission maxima. SP-C had a very different structure in 80% ACN or 70% TFE. While alpha-helix was the main secondary structure of SP-C in ACN/water mixtures--around 50%--, it had almost exclusively beta-structure when dissolved in 70% TFE. The CD spectrum of SP-C in TFE showed dependence on the protein concentration, suggesting that protein-protein interactions could be important in this beta-conformation. SP-C reconstituted in LPC micelles or DPPC vesicles had a CD spectrum qualitatively similar to that one in aqueous ACN, with a dominant alpha-helical structure. The alpha-helical content of SP-C in micelles of LPC and vesicles of DPPC, 60 and 70%, respectively, was calculated to be higher than the alpha-helical content of the protein dissolved in any aqueous organic solvent.

Kinetic study of the cytotoxic effect of alpha-sarcin, a ribosome inactivating protein from Aspergillus giganteus, on tumour cell lines: protein biosynthesis inhibition and cell binding

alpha-Sarcin is a ribosome inactivating protein produced by the mould Aspergillus giganteus. The effect of this protein on eight different tumour cell lines has been studied in the absence of any agent affecting membrane permeability. The protein is cytotoxic for all the tumour cell lines considered. alpha-Sarcin modifies the cell proliferation pattern by inhibiting the protein biosynthesis of the cultured cells. No membrane damage produced by alpha-sarcin has been observed by measuring lactic dehydrogenase leakage. Alteration on the cell mitochondrial activity has not been detected upon treatment with alpha-sarcin. Differences on the extent of the protein binding to the cells have been observed by flow cytometric measurements. The kinetic analysis of the protein biosynthesis inhibition produced by alpha-sarcin reveals an alpha-sarcin concentration-dependent lag phase followed by a first order decrease of the protein synthesis rate. This parameter is dependent on the external alpha-sarcin concentration. A saturable component for the action of alpha-sarcin is also deduced from these experiments. Results are discussed in terms of the protein passage across the cell membrane as the potential rate-limiting step for the action of alpha-sarcin.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.