Engineering Botulinum Toxins to Improve and Expand Targeting and SNARE Cleavage Activity

Botulinum neurotoxins (BoNTs) are highly successful protein therapeutics. Over 40 naturally occurring BoNTs have been described thus far and, of those, only 2 are commercially available for clinical use. Different members of the BoNT family present different biological properties but share a similar multi-domain structure at the molecular level. In nature, BoNTs are encoded by DNA in producing clostridial bacteria and, as such, are amenable to recombinant production through insertion of the coding DNA into other bacterial species. This, in turn, creates possibilities for protein engineering. Here, we review the production of BoNTs by the natural host and also recombinant production approaches utilised in the field. Applications of recombinant BoNT-production include the generation of BoNT-derived domain fragments, the creation of novel BoNTs with improved performance and enhanced therapeutic potential, as well as the advancement of BoNT vaccines. In this article, we discuss site directed mutagenesis, used to affect the biological properties of BoNTs, including approaches to alter their binding to neurons and to alter the specificity and kinetics of substrate cleavage. We also discuss the target secretion inhibitor (TSI) platform, in which the neuronal binding domain of BoNTs is substituted with an alternative cellular ligand to re-target the toxins to non-neuronal systems. Understanding and harnessing the potential of the biological diversity of natural BoNTs, together with the ability to engineer novel mutations and further changes to the protein structure, will provide the basis for increasing the scope of future BoNT-based therapeutics.

Engineered botulinum neurotoxins as new therapeutics

Botulinum neurotoxins (BoNTs) cause flaccid paralysis by inhibiting neurotransmission at cholinergic nerve terminals. Each BoNT consists of three domains that are essential for toxicity: the binding domain, the translocation domain, and the catalytic light-chain domain. BoNT modular architecture is associated with a multistep mechanism that culminates in the intracellular proteolysis of SNARE (soluble N-ethylmaleimide-sensitive-fusion-protein attachment protein receptor) proteins, which prevents synaptic vesicle exocytosis. As the most toxic proteins known, BoNTs have been extensively studied and are used as pharmaceutical agents to treat an increasing variety of disorders. This review summarizes the level of sophistication reached in BoNT engineering and highlights the diversity of approaches taken to utilize the modularity of the toxin. Improved efficiency and applicability have been achieved by direct mutagenesis and interserotype domain rearrangement. The scope of BoNT activity has been extended to nonneuronal cells and offers the basis for novel biomolecules in the treatment of secretion disorders.

Engineering botulinum neurotoxin domains for activation by toxin light chain

Targeted secretion inhibitors (TSI) are a new class of biopharmaceuticals designed from a botulinum neurotoxin protein scaffold. The backbone consists of the 50-kDa endopeptidase light chain and translocation domain (N-terminal portion of the heavy chain), lacks neuronal toxicity, but retains the ability to target cytoplasmic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. TSI are produced as single-chain proteins and then cleaved post-translationally to generate functional heterodimers. Precise proteolytic cleavage is essential to activate the protein to a dichain form. TSI are themselves highly specific proteases. We have exploited this activity to create self-activating enzymes by replacing the native proteolytic site with a substrate SNARE peptide for the TSI protease. We have also created cross-activating backbones. By replacing the proteolytic activation site in one backbone with the substrate SNARE peptide for another serotype, controlled activation is achieved. SNARE peptides encompassing the whole of the coiled-coil region enabled complete activation and assembly of the dichain backbone. These engineered TSI backbones are capable of translocating their enzymatic domains to target intracellular SNARE proteins. They are also investigative tools with which to further the understanding of endopeptidase activity of light chain in SNARE interactions.

Structures of engineered Clostridium botulinum neurotoxin derivatives

Targeted secretion inhibitors (TSIs) are a new class of engineered biopharmaceutical molecules derived from the botulinum neurotoxins (BoNTs). They consist of the metalloprotease light chain (LC) and translocation domain (Hn) of BoNT; they thus lack the native toxicity towards motor neurons but are able to target soluble N-ethylmaleimide-sensitive fusion protein attachment receptor (SNARE) proteins. These functional fragment (LHn) derivatives are expressed as single-chain proteins and require post-translational activation into di-chain molecules for function. A range of BoNT derivatives have been produced to demonstrate the successful use of engineered SNARE substrate peptides at the LC-Hn interface that gives these molecules self-activating capabilities. Alternatively, recognition sites for specific exoproteases can be engineered to allow controlled activation. Here, the crystal structures of three LHn derivatives are reported between 2.7 and 3.0 Å resolution. Two of these molecules are derivatives of serotype A that contain a SNARE peptide. Additionally, a third structure corresponds to LHn serotype B that includes peptide linkers at the exoprotease activation site. In all three cases the added engineered segments could not be modelled owing to disorder. However, these structures highlight the strong interactions holding the LHn fold together despite the inclusion of significant polypeptide sequences at the LC-Hn interface.

Engineering toxins for 21st century therapies

'Engineering Toxins for 21st Century Therapies' (9-10 September 2010) was part of the Royal Society International Seminar series held at the Kavli International Centre, UK. Participants were assembled from a range of disciplines (academic, industry, regulatory, public health) to discuss the future potential of toxin-based therapies. The meeting explored how the current structural and mechanistic knowledge of toxins could be used to engineer future toxin-based therapies. To date, significant progress has been made in the design of novel recombinant biologics based on domains of natural toxins, engineered to exhibit advantageous properties. The meeting concluded, firstly that future product development vitally required the appropriate combination of creativity and innovation that can come from the academic, biotechnology and pharma sectors. Second, that continued investigation into understanding the basic science of the toxins and their targets was essential in order to develop new opportunities for the existing products and to create new products with enhanced properties. Finally, it was concluded that the clinical potential for development of novel biologics based on toxin domains was evident.

Crystal structure of a catalytically active, non-toxic endopeptidase derivative of Clostridium botulinum toxin A

Botulinum neurotoxins (BoNTs) modulate cholinergic nerve terminals to result in neurotransmitter blockade. BoNTs consists of catalytic (LC), translocation (Hn) and cell-binding domains (Hc). The binding function of the Hc domain is essential for BoNTs to bind the neuronal cell membrane, therefore, removal of the Hc domain results in a product that retains the endopeptidase activity of the LC but is non-toxic. Thus, a molecule consisting of LC and Hn domains of BoNTs, termed LHn, is a suitable molecule for engineering novel therapeutics. The structure of LHA at 2.6 A reported here provides an understanding of the structural implications and challenges of engineering therapeutic molecules that combine functional properties of LHn of BoNTs with specific ligand partners to target different cell types.

Re-engineering the target specificity of Clostridial neurotoxins - a route to novel therapeutics

The ability to chemically couple proteins to LH(N)-fragments of clostridial neurotoxins and create novel molecules with selectivity for cells other than the natural target cell of the native neurotoxin is well established. Such molecules are able to inhibit exocytosis in the target cell and have the potential to be therapeutically beneficial where secretion from a particular cell plays a causative role in a disease or medical condition. To date, these molecules have been produced by chemical coupling of the LH(N)-fragment and the targeting ligand. This is, however, not a suitable basis for producing pharmaceutical agents as the products are ill defined, difficult to control and heterogeneous. Also, the molecules described to date have targeted neuroendocrine cells that are susceptible to native neurotoxins, and therefore the benefit of creating a molecule with a novel targeting domain has been limited. In this paper, the production of a fully recombinant fusion protein from a recombinant gene encoding both the LH(N)-domain of a clostridial neurotoxin and a specific targeting domain is described, together with the ability of such recombinant fusion proteins to inhibit secretion from non-neuronal target cells. Specifically, a novel protein consisting of the LH(N)-domains of botulinum neurotoxin type C and epidermal growth factor (EGF) that is able to inhibit secretion of mucus from epithelial cells is reported. Such a molecule has the potential to prevent mucus hypersecretion in asthma and chronic obstructive pulmonary disease.

Clostridial neurotoxins: structure-function led design of new therapeutics

The neurotoxins produced by various species of Clostridia are the causative agents of botulism and tetanus. The ability of the toxins, specifically those of the botulinum neurotoxin family, to disrupt neurotransmission has been exploited for use in several medical indications and now represents the therapeutic option of choice in a number of cases. Clostridial neurotoxins have been discovered to have a multi-domain structure that is shared between the various proteins of the family, and it has also been determined that each domain contributes a specific role to the holotoxin. The extensive use of recombinant expression approaches, along with solution of multiple crystallographic structures of individual domains, has enabled researchers to explore structurefunction relationships of the toxin domains more closely. These advances have facilitated a greater understanding of the potential use of individual domains for a wide variety of purposes, including the development of new therapeutics.

Preparation of specifically activatable endopeptidase derivatives of Clostridium botulinum toxins type A, B, and C and their applications

Clostridium botulinum neurotoxins are potently toxic proteins of 150 kDa with specific endopeptidase activity for SNARE proteins involved in vesicle docking and release. Following treatment with trypsin, a fragment of botulinum neurotoxin serotype A that lacks the C-terminal domain responsible for neuronal cell binding, but retains full catalytic activity, can be obtained. Known as the LH(N) fragment, we report the development of a recombinant expression and purification scheme for the isolation of comparable fragments of neurotoxin serotypes B and C. Expressed as maltose-binding protein fusions, both have specific proteolytic sites present between the fusion tag and the light chain to facilitate removal of the fusion, and between the light chain endopeptidase and the H(N) translocation domains to facilitate activation of the single polypeptide. We have also used this approach to prepare a new variant of LH(N)/A with a specific activation site that avoids the need to use trypsin. All three LH(N)s are enzymatically active and are of low toxicity. The production of specifically activatable LH(N)/A, LH(N)/B, and LH(N)/C extends the opportunities for exploitation of neurotoxin fragments. The potential utility of these fragments is discussed.

Novel application of an in vitro technique to the detection and quantification of botulinum neurotoxin antibodies

Detection of Clostridium botulinum neurotoxin (BoNT) neutralising antibodies is currently achieved using the mouse lethality assay (MLA). This technique has provided the majority of the data for vaccine development and, with the increasing use of BoNT as a therapeutic agent, the MLA is the assay of choice to evaluate 'non-responder' antisera. However, the MLA is semi-quantitative and has an animal consumption rate that raises ethical concerns. The development of an alternative is therefore desirable. Here, we describe an in vitro neuronal release assay that may represent such an alternative in terms of both its sensitivity and ability to produce quantitative data. Initially recognised in the course of assessing a novel vaccine candidate, the suitability of this assay has been further explored using an International standard. The results support the conclusion that the detection of neutralising antibodies in human sera should be attempted using this method.

Retargeted clostridial endopeptidases: inhibition of nociceptive neurotransmitter release in vitro, and antinociceptive activity in in vivo models of pain

Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types. Previously reported data have demonstrated that the catalytically active LH(N) endopeptidase fragment of botulinum neurotoxin type A (termed LH(N)/A) can be retargeted to a range of cell types in vitro to lead to inhibition of secretion of a range of transmitters. Here, we report the synthesis of endopeptidase conjugates with in vitro selectivity for nociceptive afferents compared to spinal neurons. Chemical conjugates prepared between Erythrina cristagalli lectin and LH(N)/A are assessed in vitro and in in vivo models of pain. Chemical conjugates prepared between E. cristagalli lectin and either natively sourced LH(N)/A, or recombinant LH(N)/A purified from Escherichia coli are assessed, and equivalence of the recombinant material is demonstrated. The duration of action of inhibition of neurotransmitter release by the conjugate in vitro is also assessed and is comparable to that observed with Clostridium botulinum neurotoxin. Selectivity of targeting and therapeutic potential have been confirmed by in vivo electrophysiology studies. Furthermore, the analgesic properties of the conjugate have been assessed in in vivo models of pain and extended duration effects observed. These data provide proof of principle for the concept of retargeted clostridial endopeptidases as novel analgesics.

Crystal structures of Erythrina cristagalli lectin with bound N-linked oligosaccharide and lactose

Erythrina cristagalli lectin (ECL) is a galactose-specific legume lectin. Although its biological function in the legume is unknown, ECL exhibits hemagglutinating activity in vitro and is mitogenic for T lymphocytes. In addition, it has been recently shown that ECL forms a novel conjugate when coupled to a catalytically active derivative of the type A neurotoxin from Clostridium botulinum, thus providing a therapeutic potential. ECL is biologically active as a dimer in which each protomer contains a functional carbohydrate-combining site. The crystal structure of native ECL was recently reported in complex with lactose and 2'-fucosyllactose. ECL protomers adopt the legume lectin fold but form non-canonical dimers via the handshake motif as was previously observed for Erythrina corallodendron lectin. Here we report the crystal structures of native and recombinant forms of the lectin in three new crystal forms, both unliganded and in complex with lactose. For the first time, the detailed structure of the glycosylated hexasaccharide for native ECL has been elucidated. The structure also shows that in the crystal lattice the glycosylation site and the carbohydrate binding site are involved in intermolecular contacts through water-mediated interactions.

Isolation of the gene and large-scale expression and purification of recombinant Erythrina cristagalli lectin

Using polymerase chain reaction, the coding sequence for Erythrina cristagalli lectin (ECL) has been cloned and expressed in Escherichia coli. The amplified DNA sequence of ECL is highly homologous to that previously reported for Erythrina corallodendron lectin (ECorL), confirming the absence of introns in the ECL gene. The polypeptide sequences of ECL and ECorL have been compared and five amino acids have been identified that differentiate the two proteins. Recombinant E. cristagalli lectin (recECL) was expressed in E. coli from a genomic clone encoding the mature E. cristagalli lectin gene. Constitutive expression localised recombinant protein in inclusion bodies, which were solubilised, and recECL, subsequently refolded and purified by lactose affinity chromatography. Significant advantages were observed for purification from inclusion bodies rather than from a clone optimised to express soluble protein. A large-scale purification scheme has been developed that can prepare functional recECL from inclusion bodies with a yield of 870 mg/l culture. By the range of characterisation methods employed in this study, it has been demonstrated that recECL is functionally equivalent to native ECL obtained from the E. cristagalli plant. In addition, characterisation of the binding of radiolabelled recECL to cultured dorsal root ganglia demonstrated that recECL binds to a single pool of receptors.

Botulinum and tetanus neurotoxins: structure, function and therapeutic utility

The toxic products of the anaerobic bacteria Clostridium botulinum, Clostridium butyricum, Clostridium barati and Clostridium tetani are the causative agents of botulism and tetanus. The ability of botulinum neurotoxins to disrupt neurotransmission, often for prolonged periods, has been exploited for use in several medical applications and the toxins, as licensed pharmaceutical products, now represent the therapeutics of choice for the treatment for several neuromuscular conditions. Research into the structures and activities of botulinum and tetanus toxins has revealed features of these proteins that might be useful in the design of improved vaccines, effective inhibitors and novel biopharmaceuticals. Here, we discuss the relationships between structure, mechanism of action and therapeutic use.

Inhibition of release of neurotransmitters from rat dorsal root ganglia by a novel conjugate of a Clostridium botulinum toxin A endopeptidase fragment and Erythrina cristagalli lectin

Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types. Here we report that a catalytically active derivative (termed LH(N)/A) of the type A neurotoxin from Clostridium botulinum has been coupled to a lectin obtained from Erythrina cristagalli to form a novel conjugate. This conjugate exhibits an in vitro selectivity for nociceptive afferents compared with the anatomically adjacent spinal neurons, as assessed using in vitro primary neuronal culture systems to measure inhibition of release of neurotransmitters. Chemical conjugates prepared between E. cristagalli lectin and either natively sourced LH(N)/A or recombinant LH(N)/A purified from Escherichia coli are assessed, and equivalence of the recombinant material are demonstrated. Furthermore, the dependence of inhibition of neurotransmitter release on the cleavage of SNAP-25 is demonstrated through the use of an endopeptidase-deficient LH(N)/A conjugate variant. The duration of action of inhibition of neurotransmitter released by the conjugate in vitro is assessed and is comparable with that observed with Clostridium botulinum neurotoxin. Finally, in vivo electrophysiology shows that these in vitro actions have biological relevance in that sensory transmission from nociceptive afferents through the spinal cord is significantly attenuated. These data demonstrate that the potent endopeptidase activity of clostridial neurotoxins can be selectively retargeted to cells of interest and that inhibition of release of neurotransmitters from a neuronal population of therapeutic relevance to the treatment of pain can be achieved.

Expression and purification of catalytically active, non-toxic endopeptidase derivatives of Clostridium botulinum toxin type A

Clostridium botulinum neurotoxin type A is a potently toxic protein of 150 kDa with specific endopeptidase activity for the SNARE protein SNAP-25. Proteolytic cleavage of BoNT/A with trypsin leads to removal of the C-terminal domain responsible for neuronal cell binding. Removal of this domain result in a catalytically active, non-cell-binding derivative termed LH(N)/A. We have developed a purification scheme to prepare LH(N)/A essentially free of contaminating BoNT/A. LH(N)/A prepared by this scheme retains full enzymatic activity, is stable in solution, and is of low toxicity as demonstrated in a mouse toxicity assay. In addition, LH(N)/A has minimal effect on release of neurotransmitter from a primary cell culture model. Both the mouse bioassay and in vitro release assay suggest BoNT/A is present at less than 1 in 10(6) molecules of LH(N)/A. This represents a significant improvement on previously reported figures for LH(N)/A, and also the light chain domain, previously purified from BoNT/A. To complement the preparation of LH(N)/A from holotoxin, DNA encoding LH(N)/A has been introduced into Escherichia coli to facilitate expression of recombinant product. Expression and purification parameters have been developed to enable isolation of soluble, stable endopeptidase with a toxicity profile enhanced on that of LH(N)/A purified from BoNT/A. The recombinant-derived material has been used to prepare antisera that neutralise a BoNT/A challenge. The production of essentially BoNT/A-free LH(N)/A by two different methods and the possibilities for exploitation are discussed.

Inhibition of vesicular secretion in both neuronal and nonneuronal cells by a retargeted endopeptidase derivative of Clostridium botulinum neurotoxin type A

Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types by a mechanism that involves cleavage of specific components of the vesicle docking/fusion complex, the SNARE complex. A derivative of the type A neurotoxin from Clostridium botulinum (termed LH(N)/A) that retains catalytic activity can be prepared by proteolysis. The LH(N)/A, however, lacks the putative native binding domain (H(C)) of the neurotoxin and is thus unable to bind to neurons and effect inhibition of neurotransmitter release. Here we report the chemical conjugation of LH(N)/A to an alternative cell-binding ligand, wheat germ agglutinin (WGA). When applied to a variety of cell lines, including those that are ordinarily resistant to the effects of neurotoxin, WGA-LH(N)/A conjugate potently inhibits secretory responses in those cells. Inhibition of release is demonstrated to be ligand mediated and dose dependent and to occur via a mechanism involving endopeptidase-dependent cleavage of the natural botulinum neurotoxin type A substrate. These data confirm that the function of the H(C) domain of C. botulinum neurotoxin type A is limited to binding to cell surface moieties. The data also demonstrate that the endopeptidase and translocation functions of the neurotoxin are effective in a range of cell types, including those of nonneuronal origin. These observations lead to the conclusion that a clostridial endopeptidase conjugate that can be used to investigate SNARE-mediated processes in a variety of cells has been successfully generated.

A conjugate composed of nerve growth factor coupled to a non-toxic derivative of Clostridium botulinum neurotoxin type A can inhibit neurotransmitter release in vitro

Nerve growth factor (NGF) receptor binding, internalisation and transportation of NGF has been identified as a potential route of delivery for other molecules. A derivative of Clostridium botulinum neurotoxin type A (LHN) that retains catalytic activity but has significantly reduced cell-binding capability has been prepared and chemically coupled to NGF. Intact clostridial neurotoxins potently inhibit neurotransmitter release at the neuromuscular junction by proteolysis of specific components of the vesicle docking/fusion complex. Here we report that the NGF-LHN/A conjugate, when applied to PC12 cells, significantly inhibited neurotransmitter release and cleaved the type A toxin substrate. This work represents the successful use of NGF as a targeting moiety for the delivery of a neurotoxin fragment.

Restoration of lectin activity to a non-glycosylated ricin B chain mutant by the introduction of a novel N-glycosylation site

Ricin B chain (RTB) is an N-glycosylated, galactose-specific lectin. Removal of the two native N-glycosylation sites at Asn95 and Asn135 by site-directed mutagenesis generated a recombinant protein devoid of lectin activity. Two novel N-glycosylation sites were introduced into RTB at Asn42 and Asn123, either singly or in combination. Microinjection of pre-RTB transcripts into Xenopus oocytes showed that these novel sites became glycosylated in vivo. The single oligosaccharide site chain at Asn42 restored lectin activity to RTB, whereas glycosylation at Asn123 or simultaneous glycosylation at Asn42 and Asn123 failed to do so.

Ricin A chain fused to a chloroplast-targeting signal is unfolded on the chloroplast surface prior to import across the envelope membranes

The initial stages of chloroplast protein import involve the binding of precursor proteins to surface-bound receptors prior to translocation across the envelope membranes in a partially folded conformation. We have analyzed the unfolding process by examining the conformation of a construct, comprising the presequence of a chloroplast protein linked to ricin A chain, before and after binding to the chloroplast surface. We show that the presequence is highly susceptible to proteolysis in solution, probably reflecting a lack of tertiary structure, whereas the A chain passenger protein is resistant to extremely high concentrations of protease, unless deliberately unfolded using denaturant. The A chain moiety is furthermore active, indicating that the presence of the presequence does not prevent formation of a tightly folded, native state. In contrast, receptor-bound p33KRA (fusion protein comprising the 33-kDa presequence plus 22 residues of mature protein, linked to the A chain of ricin) is quantitatively digested by protease concentrations that have little effect on the A chain in solution. We conclude that protein unfolding can take place on the chloroplast surface in the absence of translocation and without the aid of soluble factors.

Major structural differences between pokeweed antiviral protein and ricin A-chain do not account for their differing ribosome specificity

Pokeweed antiviral protein (PAP) and the A-chain of ricin (RTA) are two members of a family of ribosome-inactivating proteins (RIPS) that are characterised by their ability to catalytically depurinate eukaryotic ribosomes, a modification that makes the ribosomes incapable of protein synthesis. In contrast to RTA, PAP can also inactivate prokaryotic ribosomes. In order to investigate the reason for this differing ribosome specificity, a series of PAP/RTA hybrid proteins was prepared to test for their ability to depurinate prokaryotic and eukaryotic ribosomes. Information from the X-ray structures of RTA and PAP was used to design gross polypeptide switches and specific peptide insertions. Initial gross polypeptide swaps created hybrids that had altered ribosome inactivation properties. Preliminary results suggest that the carboxy-terminus of the RIPs (PAP 219-262) does not contribute to ribosome recognition, whereas polypeptide swaps in the amino-terminal half of the proteins did affect ribosome inactivation. Structural examination identified three loop regions that were different in both structure and composition within the amino-terminal region. Directed substitution of RTA sequences into PAP at these sites, however, had little effect on the ribosome inactivation characteristics of the mutant PAPs, suggesting that the loops were not crucial for prokaryotic ribosome recognition. On the basis of these results we have identified regions of RIP primary sequence that may be important in ribosome recognition. The implications of this work are discussed.

A hydrophobic region of ricin A chain which may have a role in membrane translocation can function as an efficient noncleaved signal peptide

Ricin A chain is a polypeptide of 267 amino acids containing a hydrophobic region near its carboxyl-terminus (residues 245-256) which has been implicated in the membrane translocation step necessary for this catalytically active toxin to reach its intracellular substrate. DNA fusions were constructed that encoded hybrid proteins consisting of carboxyl-terminal residues 233-267 or residues 238-267 of ricin A chain preceding mouse dihydrofolate reductase. When in vitro transcripts prepared from these constructs were translated in cell-free systems, the ricin A chain-derived sequences functioned as efficient signal peptides which directed dihydrofolate reductase into microsomes or into proteoliposomes containing microsomal membrane components.

Pokeweed antiviral protein (PAP) mutations which permit E.coli growth do not eliminate catalytic activity towards prokaryotic ribosomes

Pokeweed antiviral protein (PAP) has N-glycosidase activity towards both eukaryotic and prokaryotic ribosomes. This is in marked contrast with the A chains of type 2 ribosome inactivating proteins (RIPs) such as ricin and abrin, which inactivate only eukaryotic ribosomes. A recent report described spontaneous mutations in PAP that implicated specific amino acids to be involved in determining the activity of PAP towards prokaryotic ribosomes. As part of an ongoing study into RIP--ribosome interactions these mutations were specifically recreated in a PAP clone encoding the mature 262 amino acid PAP sequence. Mutants were tested for their N-glycosidase activity by analysing the integrity of eukaryotic and prokaryotic ribosomes after mutant protein expression. Mutations of F196Y and K211R, either individually or within the same clone, were active toward both classes of ribosome, indicating that these amino acid positions are not involved in differentiating ribosomal substrates. Mutation R68G led to a protein that appeared to be inactive towards prokaryotic ribosomes, but also very poorly active towards eukaryotic ribosomes. This mutation is currently under further investigation.

Mutagenesis and kinetic analysis of the active site Glu177 of ricin A-chain

Ricin A-chain (RTA) is an N-glycosidase which removes a specific adenine residue from the large rRNA of eukaryotic ribosomes. As a consequence, the ribosome is inactivated and protein synthesis is inhibited leading to cell death. This report describes the effects on enzyme activity of specific mutations of the conserved active site Glu177. The activity of mutant proteins was initially screened using an in vitro translation system. It was found that mutagenesis of Glu177 to Lys led to an apparent total inactivation of the enzyme, Glu177 to Ala had a small effect on activity, whereas the conservative Glu177 to Asp mutation had a significant effect. The properties of Glu177 to Asp were investigated more closely. Mutant protein was purified from an Escherichia coli expression system and kinetic analysis of the depurination activity assessed using salt-washed yeast ribosomes. It was shown that the Km of the mutant protein was unchanged when compared to data of wild type RTA; however, the kcat was significantly decreased (49-fold compared to wild type RTA). This suggests that Glu177 plays a predominant role in the rate-limiting step of the enzymatic mechanism and not in substrate binding. These data are discussed in relation to other reports of ricin Glu177 substitutions.

Addition of an ER retention signal to the ricin A chain increases the cytotoxicity of the holotoxin

With the exception of diphtheria toxin, which translocates from acidified endosomes, the intracellular organelle from which the catalytic moieties of several plant and bacterial toxins enter the target cell during endocytic uptake has not been identified. We have recently proposed that some toxins may travel the entire secretory pathway in reverse, moving from the cell surface to the lumen of the ER, before entering the cytosol. Several bacterial toxins have the ER retention sequence KDEL or a related analogue at their carboxyl termini, suggesting that the KDEL receptor may play a role in delivering these toxins to the ER. Here we provide further support for this possibility since the cytotoxicity of ricin, which lacks a KDEL sequence, can be significantly increased by adding KDEL to the C-terminus of its A chain.