Combination of T cell-redirecting strategies with a bispecific antibody blocking TGF-β and PD-L1 enhances antitumor responses

T cell-based immunotherapies for solid tumors have not achieved the clinical success observed in hematological malignancies, partially due to the immunosuppressive effect promoted by the tumor microenvironment, where PD-L1 and TGF-β play a pivotal role. However, durable responses to immune checkpoint inhibitors remain limited to a minority of patients, while TGF-β inhibitors have not reached the market yet. Here, we describe a bispecific antibody for dual blockade of PD-L1 and TFG-β, termed AxF (scFv)2, under the premise that combination with T cell redirecting strategies would improve clinical benefit. The AxF (scFv)2 antibody was well expressed in mammalian and yeast cells, bound both targets and inhibited dose-dependently the corresponding signaling pathways in luminescence-based cellular reporter systems. Moreover, combined treatment with trispecific T-cell engagers (TriTE) or CAR-T cells significantly boosted T cell activation status and cytotoxic response in breast, lung and colorectal (CRC) cancer models. Importantly, the combination of an EpCAMxCD3×EGFR TriTE with the AxF (scFv)2 delayed CRC tumor growth in vivo and significantly enhanced survival compared to monotherapy with the trispecific antibody. In summary, we demonstrated the feasibility of concomitant blockade of PD-L1 and TGF-β by a single molecule, as well as its therapeutic potential in combination with different T cell redirecting agents to overcome tumor microenvironment-mediated immunosuppression.

Nanobody-Based EGFR-Targeting Immunotoxins for Colorectal Cancer Treatment

Immunotoxins (ITXs) are chimeric molecules that combine the specificity of a targeting domain, usually derived from an antibody, and the cytotoxic potency of a toxin, leading to the selective death of tumor cells. However, several issues must be addressed and optimized in order to use ITXs as therapeutic tools, such as the selection of a suitable tumor-associated antigen (TAA), high tumor penetration and retention, low kidney elimination, or low immunogenicity of foreign proteins. To this end, we produced and characterized several ITX designs, using a nanobody against EGFR (V_HH 7D12) as the targeting domain. First, we generated a nanoITX, combining V_HH 7D12 and the fungal ribotoxin α-sarcin (αS) as the toxic moiety (V_HHEGFRαS). Then, we incorporated a trimerization domain (TIE^XVIII) into the construct, obtaining a trimeric nanoITX (TriV_HHEGFRαS). Finally, we designed and characterized a bispecific ITX, combining the V_HH 7D12 and the scFv against GPA33 as targeting domains, and a deimmunized (DI) variant of α-sarcin (BsITXαSDI). The results confirm the therapeutic potential of α-sarcin-based nanoITXs. The incorporation of nanobodies as target domains improves their therapeutic use due to their lower molecular size and binding features. The enhanced avidity and toxic load in the trimeric nanoITX and the combination of two different target domains in the bispecific nanoITX allow for increased antitumor effectiveness.

A New Optimized Version of a Colorectal Cancer-Targeted Immunotoxin Based on a Non-Immunogenic Variant of the Ribotoxin α-Sarcin

Due to its incidence and mortality, cancer remains one of the main risks to human health and lifespans. In order to overcome this worldwide disease, immunotherapy and the therapeutic use of immunotoxins have arisen as promising approaches. However, the immunogenicity of foreign proteins limits the dose of immunotoxins administered, thereby leading to a decrease in its therapeutic benefit. In this study, we designed two different variants of non-immunogenic immunotoxins (IMTXA33αSDI and IMTXA33furαSDI) based on a deimmunized variant of the ribotoxin α-sarcin. The inclusion of a furin cleavage site in IMTXA33furαSDI would allow a more efficient release of the toxic domain to the cytosol. Both immunotoxins were produced and purified in the yeast Pichia pastoris and later functionally characterized (both in vitro and in vivo), and immunogenicity assays were carried out. The results showed that both immunotoxins were functionally active and less immunogenic than the wild-type immunotoxin. In addition, IMTXA33furαSDI showed a more efficient antitumor effect (both in vitro and in vivo) due to the inclusion of the furin linker. These results constituted a step forward in the optimization of immunotoxins with low immunogenicity and enhanced antitumor activity, which can lead to potential better outcomes in cancer treatment.

Antibody-Based Immunotoxins for Colorectal Cancer Therapy

Monoclonal antibodies (mAbs) are included among the treatment options for advanced colorectal cancer (CRC). However, while these mAbs effectively target cancer cells, they may have limited clinical activity. A strategy to improve their therapeutic potential is arming them with a toxic payload. Immunotoxins (ITX) combining the cell-killing ability of a toxin with the specificity of a mAb constitute a promising strategy for CRC therapy. However, several important challenges in optimizing ITX remain, including suboptimal pharmacokinetics and especially the immunogenicity of the toxin moiety. Nonetheless, ongoing research is working to solve these limitations and expand CRC patients' therapeutic armory. In this review, we provide a comprehensive overview of targets and toxins employed in the design of ITX for CRC and highlight a wide selection of ITX tested in CRC patients as well as preclinical candidates.

Der p 1-based immunotoxin as potential tool for the treatment of dust mite respiratory allergy

Immunotoxins appear as promising therapeutic molecules, alternative to allergen-specific-immunotherapy. In this work, we achieved the development of a protein chimera able to promote specific cell death on effector cells involved in the allergic reaction. Der p 1 allergen was chosen as cell-targeting domain and the powerful ribotoxin α-sarcin as the toxic moiety. The resultant construction, named proDerp1αS, was produced and purified from the yeast Pichia pastoris. Der p 1-protease activity and α-sarcin ribonucleolytic action were effectively conserved in proDerp1αS. Immunotoxin impact was assayed by using effector cells sensitized with house dust mite-allergic sera. Cell degranulation and death, triggered by proDerp1αS, was exclusively observed on Der p 1 sera sensitized-humRBL-2H3 cells, but not when treated with non-allergic sera. Most notably, equivalent IgE-binding and degranulation were observed with both proDerp1αS construct and native Der p 1 when using purified basophils from sensitized patients. However, proDerp1αS did not cause any cytotoxic effect on these cells, apparently due to its lack of internalization after their surface IgE-binding, showing the complex in vivo panorama governing allergic reactions. In conclusion, herein we present proDerp1αS as a proof of concept for a potential and alternative new designs of therapeutic tools for allergies. Development of new, and more specific, second-generation of immunotoxins following proDerp1αS, is further discussed.

A deimmunised form of the ribotoxin, α-sarcin, lacking CD4+ T cell epitopes and its use as an immunotoxin warhead

Fungal ribotoxins that block protein synthesis can be useful warheads in the context of a targeted immunotoxin. α-Sarcin is a small (17 kDa) fungal ribonuclease produced by Aspergillus giganteus that functions by catalytically cleaving a single phosphodiester bond in the sarcin–ricin loop of the large ribosomal subunit, thus making the ribosome unrecognisable to elongation factors and leading to inhibition of protein synthesis. Peptide mapping using an ex vivo human T cell assay determined that α-sarcin contained two T cell epitopes; one in the N-terminal 20 amino acids and the other in the C-terminal 20 amino acids. Various mutations were tested individually within each epitope and then in combination to isolate deimmunised α-sarcin variants that had the desired properties of silencing T cell epitopes and retention of the ability to inhibit protein synthesis (equivalent to wild-type, WT α-sarcin). A deimmunised variant (D9T/Q142T) demonstrated a complete lack of T cell activation in in vitro whole protein human T cell assays using peripheral blood mononuclear cells from donors with diverse HLA allotypes. Generation of an immunotoxin by fusion of the D9T/Q142T variant to a single-chain Fv targeting Her2 demonstrated potent cell killing equivalent to a fusion protein comprising the WT α-sarcin. These results represent the first fungal ribotoxin to be deimmunised with the potential to construct a new generation of deimmunised immunotoxin therapeutics.

Synergistic Action of Actinoporin Isoforms from the Same Sea Anemone Species Assembled into Functionally Active Heteropores

Among the toxic polypeptides secreted in the venom of sea anemones, actinoporins are the pore-forming toxins whose toxic activity relies on the formation of oligomeric pores within biological membranes. Intriguingly, actinoporins appear as multigene families that give rise to many protein isoforms in the same individual displaying high sequence identities but large functional differences. However, the evolutionary advantage of producing such similar isotoxins is not fully understood. Here, using sticholysins I and II (StnI and StnII) from the sea anemone Stichodactyla helianthus, it is shown that actinoporin isoforms can potentiate each other's activity. Through hemolysis and calcein releasing assays, it is revealed that mixtures of StnI and StnII are more lytic than equivalent preparations of the corresponding isolated isoforms. It is then proposed that this synergy is due to the assembly of heteropores because (i) StnI and StnII can be chemically cross-linked at the membrane and (ii) the affinity of sticholysin mixtures for the membrane is increased with respect to any of them acting in isolation, as revealed by isothermal titration calorimetry experiments. These results help us understand the multigene nature of actinoporins and may be extended to other families of toxins that require oligomerization to exert toxicity.

Involvement of loop 5 lysine residues and the N-terminal β-hairpin of the ribotoxin hirsutellin A on its insecticidal activity

Ribotoxins are cytotoxic members of the family of fungal extracellular ribonucleases best represented by RNase T1. They share a high degree of sequence identity and a common structural fold, including the geometric arrangement of their active sites. However, ribotoxins are larger, with a well-defined N-terminal β-hairpin, and display longer and positively charged unstructured loops. These structural differences account for their cytotoxic properties. Unexpectedly, the discovery of hirsutellin A (HtA), a ribotoxin produced by the invertebrate pathogen Hirsutella thompsonii, showed how it was possible to accommodate these features into a shorter amino acid sequence. Examination of HtA N-terminal β-hairpin reveals differences in terms of length, charge, and spatial distribution. Consequently, four different HtA mutants were prepared and characterized. One of them was the result of deleting this hairpin [Δ(8-15)] while the other three affected single Lys residues in its close spatial proximity (K115E, K118E, and K123E). The results obtained support the general conclusion that HtA active site would show a high degree of plasticity, being able to accommodate electrostatic and structural changes not suitable for the other previously known larger ribotoxins, as the variants described here only presented small differences in terms of ribonucleolytic activity and cytotoxicity against cultured insect cells.

Efficient in vivo antitumor effect of an immunotoxin based on ribotoxin α-sarcin in nude mice bearing human colorectal cancer xenografts

Tagging of RNases, such as the ribotoxin α-sarcin, with the variable domains of antibodies directed to surface antigens that are selectively expressed on tumor cells endows cellular specificity to their cytotoxic action. A recombinant single-chain immunotoxin based on the ribotoxin α-sarcin (IMTXA33αS), produced in the generally regarded as safe (GRAS) yeast Pichia pastoris, has been recently described as a promising candidate for the treatment of colorectal cancer cells expressing the glycoprotein A33 (GPA33) antigen, due to its high specific and effective cytotoxic effect on in vitro assays against targeted cells. Here we report the in vivo antitumor effectiveness of this immunotoxin on nude mice bearing GPA33-positive human colon cancer xenografts. Two sets of independent assays were performed, including three experimental groups: control (PBS) and treatment with two different doses of immunotoxin (50 or 100 μg/ injection) (n = 8). Intraperitoneal administration of IMTXA33αS resulted in significant dose-dependent tumor growth inhibition. In addition, the remaining tumors excised from immunotoxin-treated mice showed absence of the GPA33 antigen and a clear inhibition of angiogenesis and proliferative capacity. No signs of immunotoxin-induced pathological changes were observed from specimens tissues. Overall these results show efficient and selective cytotoxic action on tumor xenografts, combined with the lack of severe side effects, suggesting that IMTXA33αS is a potential therapeutic agent against colorectal cancer.

Preparation of an engineered safer immunotoxin against colon carcinoma based on the ribotoxin hirsutellin A

Immunotoxins are chimeric proteins composed of an antibody domain that specifically directs the action of the toxic domain, resulting in the death of the targeted cells. Over recent years, immunotoxins have been widely studied and the number of different constructions has increased exponentially. Protein engineering has allowed the design of optimized versions of immunotoxins with an improved tumor binding affinity, stability or cytotoxic efficacy, although sometimes this has compromised the safety of the patient in terms of undesirable adverse secondary reactions. A triple mutant at three Trp residues (HtA3ΔW) of the ribotoxin hirsutellin A retains its specific ribonucleolytic activity, although cell internalization capacity is lacking. This toxin variant has been fused to the single chain variable fragment A33 (scFvA33). This immunoconjugate (IMTXA33HtA3ΔW) was produced in the methylotrophic yeast Pichia pastoris and purified using nickel-nitrilotriacetic acid affinity chromatography. Both target and toxic domains were characterized. The immunotoxin showed an exquisite specific binding against GPA33-positive culture cells, which results in the death of the targeted cells because of specific ribonucleolytic activity against ribosomes of the engineered hirsutellin A variant. IMTXA33HtA3ΔW represents a promising structure in the search for an improved immunotoxin without compromising the safety of patients.

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.

α-sarcin and RNase T1 based immunoconjugates: the role of intracellular trafficking in cytotoxic efficiency

Toxins have been thoroughly studied for their use as therapeutic agents in search of an improvement in toxic efficiency together with a minimization of their undesired side effects. Different studies have shown how toxins can follow different intracellular pathways which are connected with their cytotoxic action inside the cells. The work herein presented describes the different pathways followed by the ribotoxin α-sarcin and the fungal RNase T1, as toxic domains of immunoconjugates with identical binding domain, the single chain variable fragment of a monoclonal antibody raised against the glycoprotein A33. According to the results obtained both immunoconjugates enter the cells via early endosomes and, while α-sarcin can translocate directly into the cytosol to exert its deathly action, RNase T1 follows a pathway that involves lysosomes and the Golgi apparatus. These facts contribute to explaining the different cytotoxicity observed against their targeted cells, and reveal how the innate properties of the toxic domain, apart from its catalytic features, can be a key factor to be considered for immunotoxin optimization.

Efficient production of single-chain fragment variable-based N-terminal trimerbodies in Pichia pastoris

Background

Recombinant antibodies are highly successful in many different pathological conditions and currently enjoy overwhelming recognition of their potential. There are a wide variety of protein expression systems available, but almost all therapeutic antibodies are produced in mammalian cell lines, which mimic human glycosylation. The production of clinical-grade antibodies in mammalian cells is, however, extremely expensive. Compared to mammalian systems, protein production in yeast strains such as Pichia pastoris, is simpler, faster and usually results in higher yields.

Results

In this work, a trivalent single-chain fragment variable (scFv)-based N-terminal trimerbody, specific for the human carcinoembryonic antigen (CEA), was expressed in human embryonic kidney 293 cells and in Pichia pastoris. Mammalian- and yeast-produced anti-CEA trimerbody molecules display similar functional and structural properties, yet, the yield of trimerbody expressed in P. pastoris is about 20-fold higher than in human cells.

Conclusions

P. pastoris is an efficient expression system for multivalent trimerbody molecules, suitable for their commercial production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0116-1) contains supplementary material, which is available to authorized users.

A non-cytotoxic but ribonucleolytically specific ribotoxin variant: implication of tryptophan residues in the cytotoxicity of hirsutellin A

Ribotoxins are a family of toxic proteins that exert a highly specific cleavage at the universally conserved sarcin/ricin loop (SRL) of the larger rRNA molecule. Before this ribonucleolytic action, passage through the cell membrane is a necessary step for ribotoxin internalization and the limiting factor for cytotoxicity. Although extensive knowledge of their ribonucleolytic activity and substrate recognition has been accumulated, little is known about the mechanisms of cell entry of ribotoxins. Hirsutellin A (HtA) is a recently described member of this family, which accommodates the main abilities of previously characterized ribotoxins into a shorter sequence, but exhibits some differences regarding membrane interaction properties. This work investigates the contribution of tryptophan (Trp) residues 71 and 78 to both endoribonucleolytic activity and cellular toxicity of this ribotoxin. Substitution mutants W71F and W78F, as well as the double mutant W71/78F, were obtained and assayed against isolated ribosomes, synthetic SRL, and human tumor cells. The results provide evidence that cell membrane passage and internalization, as well as substrate-specific recognition, require the participation of the region involving both Trp 71 and Trp 78. Additionally, the mutant W71/78F is the first non-cytotoxic but specific ribosome-cleaving ribotoxin mutant obtained to date.

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.

Implication of an Asp residue in the ribonucleolytic activity of hirsutellin A reveals new electrostatic interactions at the active site of ribotoxins

Ribotoxins are fungal extracellular ribonucleases that specifically cleave ribosomes leading to cell-death via apoptosis. α-Sarcin is the ribotoxin studied in deepest detail, and therefore constitutes the referential protein for the whole family. It has been demonstrated that ribotoxin activity depends on a very precise structural microenvironment in which electrostatic interactions among residues in the active site are of the highest importance. Hirsutellin A (HtA) has been recently described as the smallest ribotoxin known to date, encompassing all the abilities of previously characterized members of this family into a shorter sequence. Comparison of HtA and α-sarcin three-dimensional structures suggested that residues presumably forming the catalytic triad of HtA would be His 42, Glu 66, and His 113. Within this same idea, the presence of an Asp residue (Asp 40) in a position equivalent to α-sarcin Tyr 48 is highlighted as a novelty in this field. In this work, substitution mutants H42Q, E66Q and H113Q, as well as double and triple mutants in all possible combinations, are studied regarding their ribonucleolytic activity and cytotoxicity. Implication of these three residues in the ribotoxin activity of HtA is confirmed, though none of them is strictly essential for ribosomal cleavage. Studies with mutants D40N and D40N/E66Q demonstrate an important role for Asp 40 in the activity of HtA and establish a new set of electrostatic interactions different from the one described for already known ribotoxins.

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.

Expression of homeotic genes Hoxa3, Hoxb3, Hoxd3 and Hoxc4 is decreased in the lungs but not in the hearts of adriamycin-exposed mice

Exposure of rat and mouse embryos to adriamycin (doxorubicin chlorhydrate) induces esophageal atresia (EA) and VACTERL association. Sonic hedgehog (Shh) and Gli2/Gli3 pathways are involved in these conditions and knockout mice for homeotic Hox genes Hoxa3, Hoxb3, Hoxc3, Hoxc4 and Hoxa5 show phenotypes with some of the associated VACTERL features. This study aims at evaluating the possible influence of Hoxa3, Hoxb3, Hoxd3 and Hoxc4 as upstream regulators of this complex signalling. Pregnant mice were exposed either to 4 mg/kg of adriamycin (EA group) or vehicle (controls) on embryonic days 7.5 and 8.5. Embryos were recovered at four endpoints (E12.5-E15.5) and randomly assigned for immunohistochemical or molecular biology studies. Lungs and hearts were separately harvested and processed for Hoxa3, Hoxb3, Hoxd3 and Hoxc4 quantitative RT-PCR measurements. Antibodies for Hoxa3, Hoxb3 and Hoxd3 proteins were used for immunohistochemical studies. RT-PCR studies showed a drastic and statistically significant decrease of the four genes in the lungs of EA mice when compared to controls, with a slight recovery from E15.5. Hearts of both groups showed a similar expression of all the genes throughout gestation. Control embryos expressed the hox3 paralogous genes in heart, skin, foregut derivatives and their surrounding mesoderm through E12.5-E15.5 whereas adriamycin-exposed embryos showed a severe decrease in expression of these three proteins in the same tissues but not in the heart. Adriamycin drastically reduced the expression of Hoxa3, Hoxb3, Hoxd3 and Hoxc4 in mice embryonic lungs. Their expression in the heart did not seem to be influenced by adriamycin in this experimental setting.

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.

Anomalous electrophoretic behavior of a very acidic protein: ribonuclease U2

Ribonuclease U2 is a low-molecular-weight acidic protein with three disulfide bridges. This protein displays an anomalous electrophoretic behavior on standard SDS-PAGE. The electrophoretic mobility of the nonreduced protein roughly corresponds to its molecular mass while the migration of the reduced protein would be in accordance with the expected molecular mass of the protein dimer. This study reveals that the protein does not bind SDS under the SDS-PAGE conditions, its electrophoretic mobility being only determined by its electrostatic charge and hydrodynamic properties. In addition, the nonreduced protein cannot be blotted to a membrane. Unfolding of the protein upon reduction of its disulfide bridges enables electrotransference to membranes due to a restricted diffusion along the electrophoresis gel.

Production and characterization of a noncytotoxic deletion variant of the Aspergillus fumigatus allergen Aspf1 displaying reduced IgE binding

Aspergillus fumigatus is responsible for many allergic respiratory diseases, the most notable of which - due to its severity - is allergic bronchopulmonary aspergillosis. Aspf1 is a major allergen of this fungus: this 149-amino acid protein belongs to the ribotoxin family, whose best characterized member is alpha-sarcin (EC 3.1.27.10). The proteins of this group are cytotoxic ribonucleases that degrade a unique bond in ribosomal RNA impairing protein biosynthesis. Aspf1 and its deletion mutant Aspf1Delta(7-22) have been produced as recombinant proteins; the deleted region corresponds to an exposed beta-hairpin. The conformation of these two proteins has been studied by CD and fluorescence spectroscopy. Their enzymatic activity and cytotoxicity against human rhabdomyosarcoma cells was also measured and their allergenic properties have been studied by using 58 individual sera of patients sensitized to Aspergillus. Aspf1Delta(7-22) lacks cytotoxicity and shows a remarkably reduced IgE reactivity. From these studies it can be concluded that the deleted beta-hairpin is involved in ribosome recognition and is a significant allergenic region.

Dissecting Structural and Electrostatic Interactions of Charged Groups in α-Sarcin. An NMR Study of Some Mutants Involving the Catalytic Residues

The cytotoxic ribonuclease alpha-sarcin is the best characterized member of the ribotoxin family. Ribotoxins share a common structural core, catalytic residues, and active site topology with members of the broader family of nontoxic microbial extracellular RNases. They are, however, much more specific in their biological action. To shed light on the highly specific alpha-sarcin activity, we have evaluated the structural and electrostatic interactions of its charged groups, by combining the structural and pK(a) characterization by NMR of several variants with theoretical calculations based on the Tanford-Kirkwood and Poisson-Boltzmann models. The NMR data reveal that the global conformation of wild-type alpha-sarcin is preserved in the H50Q, E96Q, H137Q, and H50/137Q variants, and that His137 is involved in an H-bond that is crucial in maintaining the active site structure and in reinforcing the stability of the enzyme. The loss of this H-bond in the H137Q and H50/137Q variants modifies the local structure of the active site. The pK(a) values of active site groups H50, E96, and H137 in the four variants have been determined by two-dimensional NMR. The catalytic dyad of E96 and H137 is not sensitive to charge replacements, since their pK(a) values vary less than +/-0.3 pH unit with respect to those of the wild type. On the contrary, the pK(a) of His50 undergoes drastic changes when compared to its value in the intact protein. These amount to an increase of 0.5 pH unit or a decrease of 1.1 pH units depending on whether a positive or negative charge is substituted at the active site. The main determinants of the pK(a) values of most of the charged groups in alpha-sarcin have been established by considering the NMR results in conjunction with those derived from theoretical pK(a) calculations. With regard to the active site residues, the H50 pK(a) is chiefly influenced by electrostatic interactions with E96 and H137, whereas the effect of the low dielectric constant and the interaction with R121 appear to be the main determinants of the altered pK(a) value of E96 and H137. Charge-charge interactions and an increased level of burial perturb the pK(a) values of the active site residues of alpha-sarcin, which can account for its reduced ribonucleolytic activity and its high specificity.

Arginine 121 is a crucial residue for the specific cytotoxic activity of the ribotoxin alpha-sarcin

Alpha-sarcin, a cyclizing ribonuclease secreted by the mould Aspergillus giganteus, is one of the best characterized members of a family of fungal ribotoxins. This protein induces apoptosis in tumour cells due to its highly specific activity on ribosomes. Fungal ribotoxins display a three-dimensional protein fold similar to those of a larger group of microbial noncytotoxic RNases, represented by RNases T1 and U2. This similarity involves the three catalytic residues and also the Arg121 residue, whose counterpart in RNase T1, Arg77, is located in the vicinity of the substrate phosphate moiety although its potential functional role is not known. In this work, Arg121 of alpha-sarcin has been replaced by Gln or Lys. These two mutations do not modify the conformation of the protein but abolish the ribosome-inactivating activity of alpha-sarcin. In addition, the loss of the positive charge at that position produces dramatic changes on the interaction of alpha-sarcin with phospholipid membranes. It is concluded that Arg121 is a crucial residue for the characteristic cytotoxicity of alpha-sarcin and presumably of the other fungal ribotoxins.

Involvement of the amino-terminal beta-hairpin of the Aspergillus ribotoxins on the interaction with membranes and nonspecific ribonuclease activity

Ribotoxins are a family of potent cytotoxic proteins from Aspergillus whose members display a high sequence identity (85% for about 150 amino acid residues). The three-dimensional structures of two of these proteins, alpha-sarcin and restrictocin, are known. They interact with phospholipid bilayers, according to their ability to enter cells, and cleave a specific phosphodiester bond in the large subunit of ribosome thus inhibiting protein biosynthesis. Two nonconservative sequence changes between these proteins are located at the amino-terminal beta-hairpin of alpha-sarcin, a characteristic structure that is absent in other nontoxic structurally related microbial RNases. These two residues of alpha-sarcin, Lys 11 and Thr 20, have been substituted with the equivalent amino acids in restrictocin. The single mutants (K11L and T20D) and the corresponding K11L/T20D double mutant have been produced in Escherichia coli and purified to homogeneity. The spectroscopic characterization of the purified proteins reveals that the overall native structure is preserved. The ribonuclease and lipid-perturbing activities of the three mutants and restrictocin have been evaluated and compared with those of alpha-sarcin. These proteins exhibit the same ability to specifically inactivate ribosomes, although they show different activity against nonspecific substrate analogs such as poly(A). The mutant variant K11L and restrictocin display a lower phospholipid-interacting ability correlated with a decreased cytotoxicity. The results obtained are interpreted in terms of the involvement of the amino-terminal beta-hairpin in the interaction with both membranes and polyadenylic acid.

Assignment of the contribution of the tryptophan residues to the spectroscopic and functional properties of the ribotoxin alpha-sarcin

alpha-Sarcin, a potent cytotoxic protein from Aspergillus giganteus, contains two tryptophan residues at positions 4 and 51. Two single, W4F and W51F, and the double mutant, W4/51F, have been produced and purified to homogeneity. These two residues are neither required for the highly specific ribonucleolytic activity of the protein on the ribosomes (production of the so called alpha-fragment) nor for its interaction with lipid membranes (aggregation and fusion of vesicles), although the mutant forms involving Trp-51 show a decreased ribonuclease activity. Proton NMR data reveal that no significant changes in the global structure of the enzyme occur upon replacement of Trp-51 by Phe. Substitution of each Trp residue results in a 4 degrees C drop in the thermal denaturation midpoint, and the double mutant's midpoint is 9 degrees C lower. Trp-51 is responsible for most of the near-UV circular dichroism of the protein and also contributes to the overall ellipticity of the protein in the peptide bond region. Trp-51 does not show fluorescence emission. The membrane-bound proteins undergo a thermal denaturation at a lower temperature than the corresponding free forms. The interaction of the protein with phospholipid bilayers promotes a large increase of the quantum yield of Trp-51 and its fluorescence emission is quenched by anthracene incorporated into the hydrophobic region of such bilayers. This indicates that the region around this residue is located in the hydrophobic core of the bilayer following protein-vesicle interaction.

The highly refined solution structure of the cytotoxic ribonuclease alpha-sarcin reveals the structural requirements for substrate recognition and ribonucleolytic activity

alpha-Sarcin selectively cleaves a single phosphodiester bond in a universally conserved sequence of the major rRNA, that inactivates the ribosome. The elucidation of the three-dimensional solution structure of this 150 residue enzyme is a crucial step towards understanding alpha-sarcin's conformational stability, ribonucleolytic activity, and its exceptionally high level of specificity. Here, the solution structure has been determined on the basis of 2658 conformationally relevant distances restraints (including stereoespecific assignments) and 119 torsional angular restraints, by nuclear magnetic resonance spectroscopy methods. A total of 60 converged structures have been computed using the program DYANA. The 47 best DYANA structures, following restrained energy minimization by GROMOS, represent the solution structure of alpha-sarcin. The resulting average pairwise root-mean-square-deviation is 0.86 A for backbone atoms and 1.47 A for all heavy atoms. When the more variable regions are excluded from the analysis, the pairwise root-mean-square deviation drops to 0.50 A and 1.00 A, for backbone and heavy atoms, respectively. The alpha-sarcin structure is similar to that reported for restrictocin, although some differences are clearly evident, especially in the loop regions. The average rmsd between the structurally aligned backbones of the 47 final alpha-sarcin structures and the crystal structure of restrictocin is 1.46 A. On the basis of a docking model constructed with alpha-sarcin solution structure and the crystal structure of a 29-nt RNA containing the sarcin/ricin domain, the regions in the protein that could interact specifically with the substrate have been identified. The structural elements that account for the specificity of RNA recognition are located in two separate regions of the protein. One is composed by residues 51 to 55 and loop 5, and the other region, located more than 11 A away in the structure, is the positively charged segment formed by residues 110 to 114.

The solubility of the ribotoxin alpha-sarcin, produced as a recombinant protein in Escherichia coli, is increased in the presence of thioredoxin

The yield of purified recombinant alpha-sarcin increases approximately three- to fourfold when this toxin is co-expressed in Escherichia coli with thioredoxin. This increased production is attributed to the existence, in the presence of thioredoxin, of a reducing environment which allows rearrangement of incorrect disulphide bonds to produce the soluble native conformation. The protein thus produced retains the structural, spectroscopic and enzymatic features of the natural fungal alpha-sarcin.

Overproduction in Escherichia coli and purification of the hemolytic protein sticholysin II from the sea anemone Stichodactyla helianthus

The cDNA coding for the cytolytic toxins sticholysin I and sticholysin II from the sea anemone Stichodactyla helianthus has been isolated, cloned in pUC18, and sequenced. A 6His-tagged version of sticholysin II has been overproduced in Escherichia coli and purified to homogeneity in milligram amounts. Conformational and functional analyses of recombinant sticholysin II do not reveal any significant difference when compared to the natural cytolysin.

Role of histidine-50, glutamic acid-96, and histidine-137 in the ribonucleolytic mechanism of the ribotoxin alpha-sarcin

alpha-Sarcin is a ribotoxin secreted by the mold Aspergillus giganteus that degrades the ribosomal RNA by acting as a cyclizing ribonuclease. Three residues potentially involved in the mechanism of catalysis--histidine-50, glutamic acid-96, and histidine-137--were changed to glutamine. Three different single mutation variants (H50Q, E96Q, H137Q) as well as a double variant (H50/137Q) and a triple variant (H50/137Q/E96Q) were prepared and isolated to homogeneity. These variants were spectroscopically (circular dichroism, fluorescence emission, and proton nuclear magnetic resonance) characterized. According to these results, the three-dimensional structure of these variants of alpha-sarcin was preserved; only very minor local changes were detected. All the variants were inactive when assayed against either intact ribosomes or poly(A). The effect of pH on the ribonucleolytic activity of alpha-sarcin was evaluated against the ApA dinucleotide. This assay revealed that only the H50Q variant still retained its ability to cleave a phosphodiester bond, but it did so to a lesser extent than did wild-type alpha-sarcin. The results obtained are interpreted in terms of His137 and Glu96 as essential residues for the catalytic activity of alpha-sarcin (His137 as the general acid and Glu96 as the general base) and His50 stabilizing the transition state of the reaction catalyzed by alpha-sarcin.

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.

Secretion of recombinant pro- and mature fungal alpha-sarcin ribotoxin by the methylotrophic yeast Pichia pastoris: the Lys-Arg motif is required for maturation

alpha-Sarcin is a ribosome-inactivating protein from the mold Aspergillus giganteus. The methylotrophic yeast Pichia pastoris has been transformed with two plasmids (pHILD2prealphaS and pHILS1prealphaS), which contain the complete alpha-sarcin cDNA, including its original fungal leader peptide, under the control of yeast alcohol oxidase promoter. The second one is indeed fused to the signal sequence of P. pastoris acid phosphatase. The transformed yeasts secreted both mature and pro-alpha-sarcin. The presence of this pro-alpha-sarcin in the yeast extracellular medium is due to an inefficient recognition of the pro-sequence by a putative Kex2p-like endopeptidase. A third plasmid accounting for a single mutation of the alpha-sarcin leader peptide was designed to produce a more efficient Kex2p recognition motif. This approach resulted in the extracellular production of only the mature protein, suggesting the existence of a two-step mechanism for processing its leader peptide. This recombinant alpha-sarcin is identical to the original fungal protein, according to activity and spectroscopic criteria. In addition, pro-alpha-sarcin, which has been characterized for the first time, also exhibits ribonucleolytic activity as the mature protein does. Therefore, protection of the producing cells against this kind of ribotoxins may depend on an efficient recognition of the signal sequence followed by translocation of the nascent polypeptide to the endoplasmic reticulum.

The cytotoxin alpha-sarcin behaves as a cyclizing ribonuclease

The hydrolysis of adenylyl(3'-->5')adenosine (ApA) and guanylyl(3'--> 5')adenosine (GpA) dinucleotides by the cytotoxic protein alpha-sarcin has been studied. Quantitative analysis of the reaction has been performed through reverse-phase chromatographic (HPLC) separation of the resulting products. The hydrolysis of the 3'-5' phosphodiester bond of these substrates yields the 2'-3' cyclic mononucleotide; this intermediate is converted into the corresponding 3'-monophosphate derivative as the final product of the reaction. The values of the apparent Michaelis constant (KM), kcat and kcat/KM have also been calculated. The obtained results fit into a two-step mechanism for the enzymatic activity of alpha-sarcin and allow to consider this protein as a cyclizing RNase.

Sequence determination and molecular characterization of gigantin, a cytotoxic protein produced by the mould Aspergillus giganteus IFO 5818

Gigantin is a 17-kDa ribonuclease secreted by Aspergillus giganteus IFO 5818. The sequence of the genomic DNA coding for this protein is reported. The deduced amino acid sequence reveals nine amino acid variations with respect to alpha-sarcin, a well-characterized ribosome-inactivating protein from A. giganteus MDH 18894. The peptides obtained after tryptic digestion of reduced and carboxyamidomethylated gigantin have been chromatographically separated. The analysis of these peptides in comparison to those originating from alpha-sarcin corroborates the above sequence differences. These do not sensibly modify the conformation of the protein, based on the coincidence of the circular dichroism and fluorescence emission spectra of the two proteins. The obtained results are discussed in terms of the involvement of the distinctive residues in the immunological and catalytic properties that distinguish gigantin from alpha-sarcin.

Characterization of a natural larger form of the antifungal protein (AFP) from Aspergillus giganteus

Two major proteins, alpha-sarcin and an antifungal polypeptide (AFP), are secreted by the mould Aspergillus giganteus MDH 18894 when it is cultured for 70-80 h. A third major protein is also found in the extracellular medium at 48-60 h, but it disappears as the culture proceeds. This protein has been isolated and characterized in terms of apparent molecular mass, electrophoretic and chromatographic behaviour, NH2-terminal primary structure, amino acid content, spectroscopical features, reactivity against anti-AFP antibodies, and antifungal activity. Based on the obtained results it would be an extracellular inactive precursor form of AFP, designated as the large form of AFP (lf-AFP). Its amino acid composition is identical to that of AFP but containing six extra residues. NH2-terminal sequence analysis of the first eight amino acid residues of this polypeptide revealed that the extra residues can be perfectly accommodated within the DNA-deduced sequence of the precursor form of AFP. Its alignment with precursor sequences of different proteins, secreted by a variety of Aspergillus spp., reveals the existence of a common tetrapeptide at the carboxy-terminal end of their leader peptides. This sequence would be Ile/Leu-Xaa-Yaa-Arg, being mostly Xaa and Yaa an acid residue (Asp/Glu) and alanine, respectively. The presence of lf-AFP as an extracellular protein would be in perfect agreement with the existence of this tetrapeptide motif, that can be involved in the protein secretion mechanisms of filamentous fungi.

Structural basis for the catalytic mechanism and substrate specificity of the ribonuclease alpha-sarcin

alpha-Sarcin is a ribosome-inactivating protein which selectively cleaves a single phosphodiester bond in a universally conserved sequence of the major rRNA. The solution structure of a-sarcin has been determined on the basis of 1898 distance and angular experimental constraints from NMR spectroscopy. It reveals a catalytic mechanism analogous to that of the T1 family of ribonucleases while its exquisite specificity resides in the contacts provided by its distinctive loops.

1H and 15N nuclear magnetic resonance assignment and secondary structure of the cytotoxic ribonuclease alpha-Sarcin

The ribosome-inactivating protein alpha-Sarcin (alpha S) is a 150-residue fungal ribonuclease that, after entering sensitive cells, selectively cleaves a single phosphodiester bond in an universally conserved sequence of the major rRNA to inactivate the ribosome and thus exert its cytotoxic action. As a first step toward establishing the structure-dynamics-function relationships in this system, we have carried out the assignment of the 1H and 15N NMR spectrum of alpha S on the basis of homonuclear (1H-1H) and heteronuclear (1H-15N) two-dimensional correlation spectra of a uniformly 15N-labeled sample, and two selectively 15N-labeled (Tyr and Phe) samples, as well as a single three-dimensional experiment. The secondary structure of alpha S, as derived from the characteristic patterns of dipolar connectivities between backbone protons, conformational chemical shifts, and the protection of backbone amide protons against exchange, consists of a long N-terminal beta-hairpin, a short alpha-helical segment, and a C-terminal beta-sheet of five short strands arranged in a + 1, + 1, + 1, + 1 topology, connected by long loops in which the 13 Pro residues are located.

Characterization of the antifungal protein secreted by the mould Aspergillus giganteus

An antifungal polypeptide (AFP) of 51 amino acid residues, secreted by the mould Aspergillus giganteus, has been purified to homogeneity and characterized. The inhibitory effect of this protein on the growth of different microorganisms has been studied. Whereas the growth of many of the filamentous fungi assayed is inhibited, no effect has been observed against yeasts or bacteria. The minimal concentration for total inhibition of the growth is in the range 6 to 25 microM. The antifungal polypeptide does not produce any effect on the growth of the producing mould. The polypeptide promotes aggregation of acidic phospholipid vesicles. A remarkable resistance to proteolysis and a low hydrogen x deuterium exchange have been observed for this protein. The protein does not show any thermal transition up to 80 degrees C when studied by differential scanning calorimetry and infrared spectroscopy. The uv absorbance, fluorescence emission, and circular dichroism (CD) characteristics of this protein have been studied. The protein exhibits a strong positive band at 230 nm as a prominent feature of the CD spectrum in the far uv region. All the spectroscopical properties of the antifungal protein are highly influenced by the abundance of tyrosine residues. These can be grouped in two different populations, buried and exposed, based on the results of pH-titration experiments. Fourier-transform infrared spectroscopy reveals a high content of beta-structure in AFP. Reduction and carboxy-amidomethylation produces a rather unstructured polypeptide as deduced from its spectroscopical properties.

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.

Escherichia coli JA221 can suppress the UAG stop signal

The Escherichia coli strain JA221 can suppress the UAG stop codon, although the existence of an amber suppressor tRNA has not previously been described for this strain. When using a plasmid to express alpha-sarcin, which has TAG as its stop signal, two proteins were obtained: a smaller protein corresponding in size to that of the expected protein, and a larger protein, which could be accounted for by the presence of a second stop codon (TGA) 18 base pairs downstream of the original. This feature of strain JA221 must therefore be considered when using this strain as a host for the production of recombinant proteins.

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.

Membrane interaction of a beta-structure-forming synthetic peptide comprising the 116-139th sequence region of the cytotoxic protein alpha-sarcin

alpha-Sarcin is a cytotoxic protein that strongly interacts with acid phospholipid vesicles. This interaction exhibits a hydrophobic component although alpha-sarcin is a highly polar protein. A peptide comprising the amino acid sequence corresponding to the 116-139th segment of the alpha-sarcin cytotoxin has been synthesized by a standard fluoren-9-yl-methoxycarbonyl-based solid phase method. Its primary structure is: (116)-NPGPARVIYTYPNKVFCGIIAHTK-(139). Two beta-strands have been predicted in this region of alpha-sarcin, where the less polar stretches of the protein are found. The synthetic peptide interacts with negatively charged large unilamellar vesicles of either natural or synthetic phospholipids. An apparent fragmentation of the vesicles is produced by the peptide based on electron microscopy studies. The peptide promotes leakage of the intravesicular aqueous contents and lipid mixing of bilayers. The packing of the phospholipid molecules is greatly perturbed by the peptide, as deduced from the drastic changes induced by the peptide in cooperative properties associated with the phase transition of the bilayers. At saturating peptide/phospholipid ratios, the phase transition of dimyristoylphosphatidylglycerol vesicles is abolished. All of these effects are saturated at about 0.3 peptide/lipid molar ratio. The peptide adopts a mostly random structure in aqueous solution. A conformation composed of a high proportion of antiparallel beta-sheet is induced as a consequence of the interaction with the phospholipid vesicles in opposition to trifluoroethanol that promotes alpha-helical peptide structures, as deduced from circular dichroism measurements. The obtained results are discussed in terms of the potential involvement of the region comprising residues 116-139 of alpha-sarcin in the hydrophobic interactions of this cytotoxic protein with membranes.

NMR solution structure of the antifungal protein from Aspergillus giganteus: evidence for cysteine pairing isomerism

The solution structure of the antifungal protein (AFP, 51 residues, 4 disulfide bridges) from Aspergillus giganteus has been determined by using experimentally derived interproton distance constraints from nuclear magnetic resonance (NMR) spectroscopy. Complete sequence-specific proton assignments were obtained at pH 5.0 and 35 degrees C. A set of 834 upper limit distance constraints from nuclear Overhauser effect measurements was used as input for the calculation of structures with the program DIANA. An initial family of 40 structures calculated with no disulfide constraints was used to obtain information about the disulfide connectivities, which could not be determined by standard biochemical methods. Three possible disulfide patterns were selected and the corresponding disulfide constraints applied to generate a family of 20 DIANA conformers for each pattern. Following energy minimization, the average pairwise RMSD of the 20 conformers of each family is 1.01, 0.89, and 1.01 A for backbone atoms and 1.82, 1.74, and 1.81 A for all heavy atoms. One of these three families contains the disulfide bridge arrangement actually present in the solution structure of AFP. Although the three families fulfill the NMR constraints, one of the disulfide patterns considered (cysteine pairs 7-33, 14-40, 26-49, 28-51) is favored among the others on the basis of previous chemical studies. It thus probably corresponds to the actual pattern of disulfide bridges present in the protein, and the corresponding family represents the solution structure of AFP. The folding of AFP consists of five antiparallel beta strands connected in a -1, -1, +3, +1 topology and highly twisted, defining a small and compact beta barrel stabilized by four internal disulfide bridges. A cationic site formed by up to three lysine side chains adjacent to a hydrophobic stretch, both at the protein surface, may constitute a potential binding site for phospholipids which would be the basis of its biological function. On the other hand, a second, minor form of AFP has been detected. NMR data, together with results from mass spectrometry, chemical analysis, and sedimentation equilibrium, suggest that this species differs from the major form in the pairs of cysteines involved in the four disulfide bridges.