Cytotoxicity of a recombinant ricin-A-chain fusion protein containing a proteolytically-cleavable spacer sequence

Computational design of fusion proteins against ErbB2-amplified tumors inspired by ricin toxin

Although the anti-cancer activity of ricin is well-known, its non-specific targeting challenges the development of ricin-derived medicines. In the present study, novel potential ribosome-inactivating fusion proteins (RIPs) were computationally engineered by incorporation of an ErbB2-dependant penetrating peptide (KCCYSL, MARAKE, WYSWLL, MARSGL, MSRTMS, and WYAWML), a linker (either EAAAK or GGGGS) and chain A of ricin which is responsible for the ribosome inactivation. Molecular dynamics simulations assisted in making sure that the least change is made in conformation and dynamic behavior of ricin chain A in selected chimeric protein (CP). Moreover, the potential affinity of the selected CPs against the ligand-uptaking ErbB2 domain was explored by molecular docking. The results showed that two CPs (CP2 and 10) could bind the receptor with the greatest affinity.

A Proof-of-Concept, Two-Tiered Approach for Ricin Detection Using Ambient Mass Spectrometry

Ricin is a naturally occurring, highly potent toxin native to castor bean plants that has recently been used as a biological weapon in cases of bioterrorism and suicide attempts. Difficulties with direct detection arise from large heterogeneities in ricin glycosylation, which leads to markedly different bioactivity, and the fact that carefully developed and laborious sample preparation steps are required to maintain the activity of the protein during analysis. Herein, we present an alternative, two-tiered approach to identify the presence of ricin by detecting ricinoleic acid and ricinine, which are co-extracted with the protein. This direct mass spectrometric-based technique takes as little as 2 minutes, and we determined its sensitivity to be in the parts-per-trillion range. Our method is applicable to paper substrates from suspected contaminated envelopes and biofluids from at-risk patients. The fact that prior sample preparations are not needed in this procedure means that analysis can be performed in the field for emergency cases.

Ricin is a naturally occurring, highly potent toxin native to castor bean plants that has recently been used as a biological weapon in cases of bioterrorism and suicide attempts.

Plant Ribosome-Inactivating Proteins: Progesses, Challenges and Biotechnological Applications (and a Few Digressions)

Plant ribosome-inactivating protein (RIP) toxins are EC3.2.2.22 N-glycosidases, found among most plant species encoded as small gene families, distributed in several tissues being endowed with defensive functions against fungal or viral infections. The two main plant RIP classes include type I (monomeric) and type II (dimeric) as the prototype ricin holotoxin from Ricinus communis that is composed of a catalytic active A chain linked via a disulphide bridge to a B-lectin domain that mediates efficient endocytosis in eukaryotic cells. Plant RIPs can recognize a universally conserved stem-loop, known as the α-sarcin/ ricin loop or SRL structure in 23S/25S/28S rRNA. By depurinating a single adenine (A4324 in 28S rat rRNA), they can irreversibly arrest protein translation and trigger cell death in the intoxicated mammalian cell. Besides their useful application as potential weapons against infected/tumor cells, ricin was also used in bio-terroristic attacks and, as such, constitutes a major concern. In this review, we aim to summarize past studies and more recent progresses made studying plant RIPs and discuss successful approaches that might help overcoming some of the bottlenecks encountered during the development of their biomedical applications.

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.

A lyophilized formulation of RiVax, a recombinant ricin subunit vaccine, retains immunogenicity

Ricin is a CDC level B biothreat. Our recombinant ricin A chain vaccine (RiVax) contains two mutations, rendering it non-toxic at high doses. Frozen or alum formulations of RiVax protected mice against ricin administered by injection, gavage or aerosol. Without alum, RiVax was safe and immunogenic in rabbits and human volunteers. For military use, the predominant target group, it would be optimal not to require a cold chain for transport and storage. We have now developed a lyophilized formulation and demonstrated stability and efficacy for at least 1 year stored refrigerated or at room temperature administered with or without alum.

RiVax, a recombinant ricin subunit vaccine, protects mice against ricin delivered by gavage or aerosol

Ricin is a plant toxin that is a CDC level B biothreat. Our recombinant ricin A chain vaccine (RiVax), which contains mutations in both known toxic sites, has no residual toxicity at doses at least 800 times the immunogenic dose. RiVax without adjuvant given intramuscularly (i.m.) protected mice against intraperitoneally administered ricin. Furthermore the vaccine without alum was safe and immunogenic in human volunteers. Here we describe the development of gavage and aerosol ricin challenge models in mice and demonstrate that i.m. vaccination protects mice against ricin delivered by either route. Also RiVax protects against aerosol-induced lung damage as determined by histology and lung function tests.

Antibody internalization studied using a novel IgG binding toxin fusion

Targeted therapy encompasses a wide variety of different strategies, which can be divided into direct or indirect approaches. Direct approaches target tumor-associated antigens by monoclonal antibodies (mAbs) binding to the relevant antigens or by small-molecule drugs that interfere with these proteins. Indirect approaches rely on tumor-associated antigens expressed on the cell surface with antibody-drug conjugates or antibody-based fusion proteins containing different kinds of effector molecules. To deliver a lethal cargo into tumor cells, the targeting antibodies should efficiently internalize into the cells. Similarly, to qualify as targets for such drugs newly-discovered cell-surface molecules should facilitate the internalization of antibodies that bind to them. Internalization can be studied be several biochemical and microscopy approaches. An undisputed proof of internalization can be provided by the ability of an antibody to specifically deliver a drug into the target cells and kill it. We present a novel IgG binding toxin fusion, ZZ-PE38, in which the Fc-binding ZZ domain, derived from Streptococcal protein A, is linked to a truncated Pseudomonas exotoxin A, the preparation of complexes between ZZ-PE38 and IgGs that bind tumor cells and the specific cytotoxicity of such immunocomplexes is reported. Our results suggest that ZZ-PE38 could prove to be an invaluable tool for the evaluation of the suitability potential of antibodies and their cognate cell-surface antigens to be targeted by immunotherapeutics based on armed antibodies that require internalization.

Preclinical toxicity and efficacy testing of RiVax, a recombinant protein vaccine against ricin

Ricin toxin is a plant-derived ribosome inactivating protein (RIP) of extraordinary toxicity. Vaccination using ricin toxoid or its A chain (RTA) is protective in animals but both vaccines have two potential toxicities, RIP and vascular leak syndrome (VLS). Previously we described three recombinant RTA constructs from which both toxicities were eliminated by site-specific mutations. One mutant, V76M/Y80A, RiVax, has now been further characterized for immunogenicity and toxicity in animals. We have found that RiVax is safe at doses of at least 8 mg in mice, 800-fold higher than the protective dose, and induces neutralizing antibodies in both mice and rabbits.

Influence of protein transduction domains on target-specific chimeric proteins

Direct targeting to the cytoplasm and nucleus using protein transduction domains (PTD) has been described to be efficient but non-cell-type-specific, and only has clinical relevance when the molecule is active exclusively in the diseased cell. The use of PTDs is an attractive mechanism to improve drug delivery. In this work, we designed recombinant proteins that contain epidermal growth factor as ligand to render uptake target cell-specific. We evaluated the potential of several PTDs to induce the cytosolic uptake of the catalytic domain of diphtheria toxin by measuring cytotoxicity. Although PTD-dependent membrane transfer is very low, the proteins exhibited concentration-dependent cytotoxic activity. Higher binding at 4 degrees C compared to 37 degrees C suggests that uptake by the PTDs MTS and TLM occurs via an endocytic pathway. Non-specific binding is predominantly a function of the PTD and greatly increases by substitution of a non-polar glycine with a negatively charged glutamate in the PTD HA2.

Improvement of downstream processing of recombinant proteins by means of genetic engineering methods

The rapid advancement of genetic engineering has allowed to produce an impressive number of proteins on a scale which would not have been achieved by classical biotechnology. At the beginning of this development research was focussed on elucidating the mechanisms of protein overexpression. The appearance of inclusion bodies may illustrate the success. In the meantime, genetic engineering is not only expected to achieve overexpression, but to improve the whole process of protein production. For downstream processing of recombinant proteins, the synthesis of fusion proteins is of primary importance. Fusion with certain proteins or peptides may protect the target protein from proteolytic degradation and may alter its solubility. Intracellular proteins may be translocated by means of fusions with signal peptides. Affinity tags as fusion complements may render protein separation and purification highly selective. These methods as well as similar ones for improving the downstream processing of proteins will be discussed on the basis of recent literature.

Applications of novel affinity cassette methods: use of peptide fusion handles for the purification of recombinant proteins

In this article, recent progress related to the use of different types of polypeptide fusion handles or 'tags' for the purification of recombinant proteins are critically discussed. In addition, novel aspects of the molecular cassette concept are elaborated, together with areas of potential application of these fundamental principles in molecular recognition. As evident from this review, the use of these concepts provides a powerful strategy for the high throughput isolation and purification of recombinant proteins and their derived domains, generated from functional genomic or zeomic studies, as part of the bioprocess technology leading to their commercial development, and in the study of molecular recognition phenomena per se. In addition, similar concepts can be exploited for high sensitivity analysis and detection, for the characterisation of protein bait/prey interactions at the molecular level, and for the immobilisation and directed orientation of proteins for use as biocatalysts/biosensors.

Cytotoxic activity of recombinant bFGF-rViscumin fusion proteins

A fusion protein (bFGF-rMLA), containing the mitogen basic fibroblast growth factor (bFGF) and the cytotoxic component of rViscumin (recombinant mistletoe lectin), the enzymatic A-chain (rMLA), was expressed in Escherichia coli, purified, and functionally characterized. bFGF-rMLA is cytotoxic for mouse B16 melanoma cells expressing the FGF receptor with an IC(50) value of approximately 1 nM. rMLA shows no significant effect on the viability of the B16 cells up to a concentration of 141 nM. Additionally, bFGF-rMLA was associated with the rViscumin B-chain (rMLB) in an in vitro folding procedure. The IC(50) value of bFGF-rMLA/rMLB to B16 cells in the presence of lactose-to block rMLB lectin activity-was 134 pM. Thus, it was possible to enhance the efficacy of a rViscumin A-chain mitotoxin through addition of rMLB. We conclude that rViscumin fusion proteins may be generally applicable for the receptor-specific inactivation of target cells and point out their potential in drug development.

The emerging role of ricin A-chain immunotoxins in leukemia and lymphoma

Since MRD is the major cause for relapses of malignant diseases, strategies utilizing ITs to target tumor cells surviving conventional treatment have attracted scientific and clinical interest. Many different ITs against various blood-borne as well as solid malignancies have demonstrated specific potent anti-tumor effects in vitro and in animal models. Some of these have already undergone clinical phase I/II-trials. The dose-limiting toxicities of RTA ITs include manifestation of VLS presenting as decreased urinary sodium excretion, hypoalbuminemia, fatigue, hypotonia, myalgia, pulmonary edema, or rhabdomyolysis. Problems encountered clinically include the development of HAMA, HARA, and HACA and the selection of antigen-deficient malignant clones. Most clinical trials performed with ITs so far were conducted in heavily pretreated patients presenting with high tumor burdens. Thus, the responses observed with ITs in these trials are very encouraging and warrant further exploration.

Design, characterization and anti-tumour cytotoxicity of a panel of recombinant, mammalian ribonuclease-based immunotoxins

Bovine seminal ribonuclease (BSRNase) is an unusual member of the ribonuclease superfamily, because of its remarkable anti-tumour and immunosuppressive properties. We describe here the construction, expression, purification and characterization of a panel of six immunotoxins based upon this enzyme and show that we can increase its anti-tumour activity by over 2 x 10(4)-fold. This is achieved by improving tumour cell targeting using a single-chain Fv (scFv) directed against the oncofetal antigen placental alkaline phosphatase. As well as the simple scFv-BSRNase fusion protein, we have constructed five other derivatives with additional peptides designed to improve folding and intracellular trafficking and delivery. We find that the molecule most cytotoxic to antigen (PLAP)-positive cells in vitro is one that contains a C-terminal 'KDEL' endoplasmic reticulum retention signal and a peptide sequence derived from diphtheria toxin. All these molecules are produced in Escherichia coli (E. coli) as insoluble inclusion bodies and require extensive in vitro processing to recover antigen binding and ribonuclease activity. Despite incomplete ribonuclease activity and quaternary assembly, these molecules are promising reagents for specific chemotherapy of cancer and are potentially less harmful and immunogenic than current immunotoxins.

Ricin A chain can transport unfolded dihydrofolate reductase into the cytosol

Ricin is a heterodimeric protein toxin. The ricin A chain is able to cross the membrane of intracellular compartments to reach the cytosol where it catalytically inactivates protein synthesis. It is linked via a disulfide bond to the B chain, a galactose-specific lectin, which allows ricin binding at the cell surface and endocytosis. To examine the potential of ricin A to carry proteins into the cytosol and the requirement for unfolding of the passenger protein, we connected mouse dihydrofolate reductase (DHFR) to ricin A by gene fusion via a spacer peptide. DHFR-ricin A expressed in Escherichia coli displayed the biological activities of the parent proteins and associated quantitatively with ricin B to form DHFR-ricin. The resulting toxin was highly cytotoxic to cells (4-8-fold less than recombinant ricin). DHFR-ricin cytotoxicity was inhibited by methotrexate, a DHFR inhibitor stabilizing DHFR-ricin A in a folded conformation. The DHFR moiety of DHFR ricin bound to the plasma membrane. Although methotrexate prevented this binding, it did not significantly affect DHFR-ricin endocytosis, which proceeded via ricin B chain. Intoxication kinetics data and a cell-free translocation assay demonstrated that protection of cells from DHFR-ricin cytotoxicity resulted from a selective inhibition by methotrexate of DHFR-ricin A translocation. We conclude that ricin A is a potential carrier of proteins to the cytosol, provided that the passenger protein is able to unfold for transmembrane transport.

Construction, expression and characterization of chimaeric toxins containing the ribonucleolytic toxin restrictocin: intracellular mechanism of action

Restrictocin is a ribonucleolytic toxin produced by the fungus Aspergillus restrictus. Two chimaeric toxins containing restrictocin directed at the human transferrin receptor have been constructed. Anti-TFR(scFv)-restrictocin is encoded by a gene produced by fusing the DNA encoding a single-chain antigen-combining region (scFv) of a monoclonal antibody, directed at the human transferrin receptor, at the 5' end of that encoding restrictocin. The other chimaeric toxin, restrictocin-anti-TFR(scFv), is encoded by a gene fusion containing the DNA encoding the single-chain antigen-combining region of antibody to human transferrin receptor at the 3' end of the DNA encoding restrictocin. These gene fusions were expressed in Escherichia coli, and fusion proteins purified from the inclusion bodies by simple chromatography techniques to near-homogeneity. The two chimaeric toxins were found to be equally active in inhibiting protein synthesis in a cell-free in vitro translation assay system. The chimaeric toxins were selectively toxic to the target cells in culture with potent cytotoxic activities. However, restrictocin-anti-TFR(scFv) was more active than anti-TFR(scFv)-restrictocin on all cell lines studied. By using protease and metabolic inhibitors, it can be shown that, to manifest their cytotoxic activity, the restrictocin-containing chimaeric toxins need to be proteolytically processed intracellularly and the free toxin or a fragment thereof thus generated is translocated to the target via a route involving the Golgi apparatus.

Contribution of plasminogen activator urokinase to in vitro cytotoxicity of diphtheria toxin

Nicking of diphtheria toxin (DT), i.e. proteolytic cleavage at an arginine-rich region within the first disulphide loop, is a prerequisite to the intoxication process. We show that protease(s) required in this process was synthesized and secreted by the sensitive cells and that antibodies against plasminogen activator urokinase (uPA) decreased the in vitro cytotoxicity of DT on Vero cells. Our results demonstrate that uPA secreted by Vero cells cultured in vitro is one of the cellular proteases involved in the cleavage and activation of diphtheria toxin.

Recombinant immunotoxins: protein engineering for cancer therapy

Recombinant immunotoxins for cancer therapy are composed of the variable regions of 'cancer-specific' antibodies fused to truncated toxins that are usually derived from bacteria or plants. Protein engineering has been used to modify these molecules so that the toxin moiety by itself does not bind to normal human cells, but retains all other cytotoxic functions. The antibody moiety directs the toxin selectively to cancer cells, which are killed; cells that do not carry that particular cancer antigen are not recognized and are therefore spared. Many recombinant immunotoxins show a high degree of cytotoxic activity and specificity towards cancer cells cultured in vitro and have been shown to cause the regression of human tumor xenografts grown in mice. Clinical trials that are in progress will show whether these promising pre-clinical results can be translated into successful cancer therapy.

Immunotoxins: an update

The use of immunotoxins (ITs) in the therapy of cancer, graft-vs-host disease (GvHD), autoimmune diseases, and AIDS has been ongoing for the past two decades. ITs contain a targeting moiety for delivery and a toxic moiety for cytotoxicity. Theoretically, one molecule of a toxin, routed to the appropriate cellular compartment, will be lethal to a cell. Newly developed MoAbs, toxins, and molecular biological technologies have enabled researchers to construct ITs that can effectively kill many different cell types. In fact, phase I/II clinical trials have given promising results. Although nonspecific toxicity and immunogenicity still limit the use of IT therapy, these agents hold enormous promise in an optimal setting to treat minimal disease.

Expression of functional ricin B chain using the baculovirus system

The ricin B chain (RTB) was expressed using a baculovirus expression system. The RTB coding sequence downstream of the preproricin signal sequence was inserted in the baculovirus transfer vector pM34T. After cotransfection of Spodoptera frugiperda Sf9 cells with linearized baculovirus DNA, recombinant viruses were selected, cloned and amplified. Upon infection of Sf9 cells with these recombinant baculoviruses, RTB production was revealed by immunoblotting. RTB expression using this system was optimum 72 h after infection of the cells at a multiplicity of infection of 3. RTB produced was glycosylated and had an apparent molecular mass of 34 kDa. Most of the signal sequence was removed, but the resulting recombinant RTB had a 13-residue N-terminus extension. Immunofluorescence analysis showed that this protein was located in the endoplasmic reticulum/Golgi region of the cell. RTB was not present at the plasma membrane. Secretion was enhanced by the addition of lactose to the cell-culture medium up to 50 mM. Purification was achieved from both cells and media using immobilized lactose and the lectin activity of RTB. Results obtained with the purified recombinant protein (more than 2 mg/l culture) were identical to those obtained with native RTB in all assays for biological activity; binding, internalization and reassociation with the ricin A chain to produce toxic ricin. Moreover, the RTB translocation capacity was not altered by the N-terminal peptide, showing that recombinant RTB could be used to deliver antigenic peptides to the cytosol for the induction of cell-mediated immunity.

Saporin toxins directed to basic fibroblast growth factor receptors effectively target human ovarian teratocarcinoma in an animal model

BACKGROUND

The antitumor activity of the chemical conjugate and recombinant forms of the mitotoxin basic fibroblast growth factor (bFGF) saporin (SAP) and the bFGF receptor-directed immunotoxin 11A8-SAP against human ovarian teratocarcinoma PA-1 was examined in athymic nude mice. Alternative administration schedules to prolong therapeutic efficacy were explored.

METHODS

Intravenous dosing (0.01-125 micrograms/kg) of chemical conjugate and recombinant bFGF-SAP or 11A8-SAP beginning 5 days after subcutaneous implantation of PA-1 cells was administered by i) weekly injection for 4 weeks, ii) continuous infusion for one week, or iii) daily injection five times a week for 4 weeks.

RESULTS

Weekly injections (31.25 micrograms/kg) of chemical conjugate bFGF-SAP or 11A8-SAP, the latter of which is 25% the molarity of the former, resulted in mean tumor volumes that were, respectively, 35% or 52% of controls (day 30) and 52% or 76% of controls (day 60). Chemical conjugate or recombinant bFGF-SAP administered weekly resulted in mean tumor volumes that were, respectively, 51% or 77% (0.5 microgram/kg) and 42% or 31% (50 micrograms/kg) of controls (day 30). A mean tumor volume of 35% of controls (day 30) and of 49% of controls (day 60) were observed in animals treated by constant infusion of chemical conjugate bFGF-SAP (125 micrograms/kg, total dose). Alternatively, tumors of animals receiving daily injections (125 micrograms/kg, total dose) exhibited a mean volume of 21% of controls (day 30) and prolonged growth inhibition as demonstrated by a mean tumor volume of 22% of controls (day 60).

CONCLUSIONS

These studies suggest a therapeutic potential for bFGF-receptor-directed saporin toxins in the treatment of ovarian teratocarcinoma and the importance of frequency of administration in achieving optimal tumor responses.

T cell-targeted immunofusion proteins from Escherichia coli

Fusion proteins between cell-targeting domains and cytotoxic proteins should be particularly effective therapeutic reagents. We constructed a family of immunofusion proteins linking humanized Fab, F(ab')2, or single chain antibody forms of the H65 antibody (which recognizes the CD5 antigen on the surface of human T cells) with the plant ribosome-inactivating protein gelonin. We reasoned that such an immunofusion would kill human target cells as efficiently as the previously described chemical conjugates of H65 and gelonin (Better M., Bernhard, S. L., Fishwild, D. M., Nolan, P. A., Bauer, R. J., Kung, A. H. C., and Carroll, S. F. (1994) J. Biol. Chem. 269, 9644-9650) if both the recognition and catalytic domains remained active, and a proper linkage between domains could be found. Immunofusion proteins were produced in Escherichia coli as secreted proteins and were recovered directly from the bacterial culture supernatant in an active form. All of the immunofusion proteins were purified by a common process and were tested for cytotoxicity toward antigen-positive human cells. A 20-60-fold range of cytotoxic activity was seen among the fusion family members, and several fusion proteins were identified which are approximately as active as effective chemical conjugates. Based on these constructs, immunofusion avidity and potency can be controlled by appropriate selection of antibody domains and ribosome-inactivating protein.

Recent developments in immunotoxin therapy

Immunotoxin (IT) research has been ongoing for 15 years. During the past 2 years, work has focused on several areas: on improvements and developments in first- and second-generation ITs; the preparation of new immunotoxin constructs with anti-tumor activity; novel animal models for preclinical evaluation of immunotoxins; and clinical trials, which are now entering Phase II or III in humans.

Immunotoxins in the therapy of cancer: from bench to clinic

This review presents only those contributions that have progressed from the bench to the clinic using murine monoclonal antibodies coupled chemically to toxins, their subunits or ribosome-inactivating proteins. The rationale and progress in the development, characterization, preclinical testing and clinical trials are discussed.

Proteolytic cleavage at arginine residues within the hydrophilic disulphide loop of the Escherichia coli Shiga-like toxin I A subunit is not essential for cytotoxicity

Escherichia coli Shiga-like toxin I is a type II ribosome-inactivating protein composed of an A subunit with RNA-specific N-glycosidase activity, non-covalently associated with a pentamer of B subunits possessing affinity for galabiose-containing glycolipids. The A subunit contains a single intrachain disulphide bond encompassing a hydrophilic sequence containing two trypsin-sensitive arginine residues. By analogy with other bacterial toxins it has been proposed that proteolytic nicking, deemed essential for a cytotoxic effect, occurs within this disulphide-bonded loop to generate the A1 and A2 fragments. Reduced A1 is then believed to translocate an internal membrane to inactivate protein synthesis in the cytosol. In this report, the disulphide-loop arginines of the SLT I A subunit were mutated to block the specific proteolysis presumed to occur. However, the mutant generated remained an effective toxin having similar catalytic activity to wild-type toxin and only a marginally reduced cytotoxicity towards cultured cells. We conclude that the disulphide-loop arginine residues are not the unique and essential processing sites previously assumed, but that processing may occur at alternative accessible sites to compensate for loss of target sites within the loop.

Antibody engineering using Escherichia coli as host

The expression of immunoglobulin fragments with antigen binding activities in E. coli is now routinely possible. Using such expression systems, Fv, Fab, and scFv fragments and single VH domains can be produced as secreted proteins in yields of the order of milligrams per liter. Moreover, expression systems are being rapidly developed for the production of antibody scFv or Fab fragments by repertoire cloning followed by selection. Diverse repertoires of genes encoding VH and VL domains can be isolated by the PCR and cloned for expression using these systems, which allow the selection of recombinants that produce fragments with the desired antigen binding specificities. This technology is rapidly evolving and, coupled with the development of systems for the random mutagenesis and selection of higher-affinity antibody fragments, could, in the longer term, provide an alternative rapid route to hybridoma technology.

Cytotoxic proteins

Cytotoxic proteins, which enter eukaryotic cells and catalytically inactivate protein synthesis, are being increasingly studied using a combination of molecular biology, cell biology and structural approaches. The creation of genetically engineered fusions with alternative cell-binding ligands paves the way for tailor-made, cell-type-specific killing agents.

Recombinant toxins for cancer treatment

Recombinant toxins target cell surface receptors and antigens on tumor cells. They kill by mechanisms different from conventional chemotherapy, so that cross resistance to conventional chemotherapeutic agents should not be a problem. Furthermore, they are not mutagens and should not induce secondary malignancies or accelerate progression of benign malignancies. They can be mass-produced cheaply in bacteria as homogeneous proteins. Either growth factor-toxin fusions or antibody-toxin fusions can be chosen, depending on the cellular target.