Catalytic Domain Of Diphtheria Toxin Research Articles

Genome wide CRISPR screen for Pasteurella multocida toxin (PMT) binding proteins reveals LDL Receptor Related Protein 1 (LRP1) as crucial cellular receptor.

PMT is a protein toxin produced by Pasteurella multocida serotypes A and D. As causative agent of atrophic rhinitis in swine, it leads to rapid degradation of the nasal turbinate bone. The toxin acts as a deamidase to modify a crucial glutamine in heterotrimeric G proteins, which results in constitutive activation of the G proteins and permanent stimulation of numerous downstream signaling pathways. Using a lentiviral based genome wide CRISPR knockout screen in combination with a lethal toxin chimera, consisting of full length inactive PMT and the catalytic domain of diphtheria toxin, we identified the LRP1 gene encoding the Low-Density Lipoprotein Receptor-related protein 1 as a critical host factor for PMT function. Loss of LRP1 reduced PMT binding and abolished the cellular response and deamidation of heterotrimeric G proteins, confirming LRP1 to be crucial for PMT uptake. Expression of LRP1 or cluster 4 of LRP1 restored intoxication of the knockout cells. In summary our data demonstrate LRP1 as crucial host entry factor for PMT intoxication by acting as its primary cell surface receptor.

Intein-mediated cytoplasmic reconstitution of a split toxin enables selective cell ablation in mixed populations and tumor xenografts

The application of proteinaceous toxins for cell ablation is limited by their high on- and off-target toxicity, severe side effects, and a narrow therapeutic window. The selectivity of targeting can be improved by intein-based toxin reconstitution from two dysfunctional fragments provided their cytoplasmic delivery via independent, selective pathways. While the reconstitution of proteins from genetically encoded elements has been explored, exploiting cell-surface receptors for boosting selectivity has not been attained. We designed a robust splitting algorithm and achieved reliable cytoplasmic reconstitution of functional diphtheria toxin from engineered intein-flanked fragments upon receptor-mediated delivery of one of them to the cells expressing the counterpart. Retargeting the delivery machinery toward different receptors overexpressed in cancer cells enables selective ablation of specific subpopulations in mixed cell cultures. In a mouse model, the transmembrane delivery of a split-toxin construct potently inhibits the growth of xenograft tumors expressing the split counterpart. Receptor-mediated delivery of engineered split proteins provides a platform for precise therapeutic and experimental ablation of tumors or desired cell populations while also greatly expanding the applicability of the intein-based protein transsplicing.

Anthrax Protective Antigen Retargeted with Single-Chain Variable Fragments Delivers Enzymes to Pancreatic Cancer Cells.

The nontoxic, anthrax protective antigen/lethal factor N-terminal domain (PA/LFN ) complex is an effective platform for translocating proteins into the cytosol of cells. Mutant PA (mPA) was recently fused to epidermal growth factor (EGF) to retarget delivery of LFN to cells bearing EGF receptors (EGFR), but the requirement for a known cognate ligand limits the applicability of this approach. Here, we render practical protective antigen retargeting to a variety of receptors with mPA single-chain variable fragment (scFv) fusion constructs. Our design enables the targeting of two pancreatic cancer-relevant receptors, EGFR and carcinoembryonic antigen. We demonstrate that fusion to scFvs does not disturb the basic functions of mPA. Moreover, mPA-scFv fusions enable cell-specific delivery of diphtheria toxin catalytic domain and Ras/Rap1-specific endopeptidase to pancreatic cancer cells. Importantly, mPA-scFv fusion-based treatments display potent cell-specific toxicity in vitro, opening fundamentally new routes toward engineered immunotoxins and providing a potential solution to the challenge of targeted protein delivery to the cytosol of cancer cells.

Site-Specific PEGylation of Recombinant Immunotoxin DAB389IL-2: Structural and Functional Assessment

BackgroundDAB389IL-2 is considered a fusion immunotoxin and it is used for the CTCL therapy. DAB389IL-2 includes of two distinct portions; the catalytic domain of diphtheria toxin and IL-2. DAB389IL-2 duo to the presence of a free cysteine residue (Cys 513 in IL-2 part) is prone to unwanted intramolecular and intermolecular disulfide bonds formation and aggregation problems. Aggregation is considered as the most common physical instability. PEGylation is an effective approach to increase the stability and half-life of therapeutic proteins.Materials and methodsIn this study, the PEGylation of recombinant DAB389IL-2 was performed by mPEG-vinylsulfone, through partial denaturation condition at 4 0C for 24 h. To confirm the PEGylation reaction, SDS-PAGE and Dynamic Light Scattering (DLS) was used. The structure of DAB389IL-2 and PEGylated immunotoxin DAB389IL-2 was analyzed using the circular dichroism (CD) and fluorescence methods. Also, K562 cells line were treated with various concentrations of DAB389IL-2 and conjugated form. In the following, the nuclease activity of DAB389IL-2 and PEGylated form was determined.ResultsThe SDS-PAGE result confirmed the site-specific PEGylation of DAB389IL-2. Spectroscopy results exhibited that the PEGylation doesn’t affect the protein native structure. Also, cytotoxicity assay and nuclease activity test confirmed that PEGylated protein induces death in K562 cells line and DNA degradation respectively. ConclusionIt is concluded that the PEGylated immunotoxin DAB389IL-2 has a proper structure and function; thus, PEGylated immunotoxin requires more survey due its unique properties.

Design and Production of Recombinant TAT Protein Structure, Catalytic Domain of Diphtheria Toxin, and Evaluation of Its Effect on Cell Line

Design and Production of Recombinant TAT Protein Structure, Catalytic Domain of Diphtheria Toxin, and Evaluation of Its Effect on Cell Line

Bispecific ligand-directed toxin targeting CD22 and CD19 (DT2219) for refractory B-cell malignancies: Results of phase I-II trial.

e19066 Background: The novel bispecific toxin DT2219 consists of a fusion between the catalytic domain of diphtheria toxin (DT) and single-chain variable fragments (scFV) of antibodies targeting human CD19 and CD22. Methods: The primary objective of Phase I Dose was to determine DT2219 dose which is well tolerated on an 8 dose schedule (days 1, 3, 5, 8 and day 15, 17, 19 and 22). Phase II Extension aimed to establish a preliminary estimate of overall response at day 29 in patients stratified to Arm A (relapsed/refractory B-cell lymphoma) and Arm B (B-cell leukemia). Baseline serum level of neutralizing antibody (NA) against diphteria toxin below 50% was required for eligibility. Results: Eighteen patients (NHL n = 12; ALL n = 6) at median age 65 years (range 50-83 yrs) with relapse/refractory disease and confirmed expression of CD19 and/or CD22 were enrolled. Treatment was well tolerated at 60 mcg/kg x 8 doses and the most common adverse events included capillary leak sy, elevated AST/ALT, low albumin, weight gain and leukopenia. All were Gr 1-2 and resolved after 3-5 days allowing day 15 DT2219 administration. There were no neutropenic fever or immune mediated adverse events. Four patients experienced dose limiting toxicity at dose 80 μg/kg/day: Gr 4 capillary leak syndrome (n = 1), Gr 3 LFTs abnormalities (n = 2) and Gr 4 thrombocytopenia > 7 days duration (n = 1). Thirteen patients were evaluable for response and 3 experienced objective clinical benefit. One patient with primary refractory pre-B acute lymphoblastic leukemia achieved complete remission after 1st cycle. Two patients with transformed lymphoma demonstrated transient tumor shrinkage, however, DT2219 therapy was discontinued due to DLT and increased NA titer after 1st cycle (pre C1 28%, pre C2 108%). Correlative studies showed a low incidence of neutralizing antibody in NHL patients recently exposed to Rituximab. Conclusions: We have established the biologically active dose of DT2219 at 60 μg/kg/day ×8 doses. Further development of DT2219 includes combination with rituximab (to reduce NA formation) and synergistic combinations against B-cell malignancies. Clinical trial information: NCT02370160.

In Silicoandin VitroEvaluation of Deamidation Effects on the Stability of the Fusion Toxin DAB389IL-2

Background:DAB389IL-2 (Denileukin diftitox) as an immunotoxin is a targeted pharmaceutical protein and is the first immunotoxin approved by FDA. It is used for the treatment of various kinds of cancer such as CTCL lymphoma, melanoma, and Leukemia but among all of these, treatment of CTCL has special importance. DAB389IL-2 consists of two distinct parts; the catalytic domain of Diphtheria Toxin (DT) that genetically fused to the whole IL-2. Deamidation is the most important reaction for chemical instability of proteins occurs during manufacture and storage. Deamidation of asparagine residues occurs at a higher rate than glutamine residues. The structure of proteins, temperature and pH are the most important factors that influence the rate of deamidation.Methods:Since there is not any information about deamidation of DAB389IL-2, we studied in silico deamidation by Molecular Dynamic (MD) simulations using GROMACS software. The 3D model of fusion protein DAB389IL-2 was used as a template for deamidation. Then, the stability of deamidated and native form of the drug was calculated.Results:The results of MD simulations were showed that the deamidated form of DAB389IL-2 is more unstable than the normal form. Also, deamidation was carried by incubating DAB389IL-2, 0.3 mg/ml in ammonium hydrogen carbonate for 24 h at 37o C in order to in vitro experiment.Conclusion:The results of in vitro experiment were confirmed outcomes of in silico study. In silico and in vitro experiments were demonstrated that DAB389IL-2 is unstable in deamidated form.

NMR study of mutations of glycine-52 of the catalytic domain of diphtheria toxin

Cross-reacting-material 197 (CRM197) is a naturally occurring non-toxic mutant of diphtheria toxin (DT) that is one of the few carrier protein used in the manufacture of polysaccharide vaccines targeting bacterial pathogens such as Neisseria meningitidis, Streptococcus pneumaniae and Haemophilus influenzae. A detailed explanation in structural terms for the lack of toxicity has started to emerge with the report of the X-ray structure of CRM197. Here, we present an NMR spectroscopy study of the wild-type catalytic domain of diphtheria toxin and the effects of mutations at residue 52 on its conformation. We utilized a strategy that consisted of gradually inducing steric perturbations by increasing the side chain size of the residue. Results show that the catalytic domain does not tolerate even the smallest perturbation, such as a glycine to alanine substitution, resulting in the destabilization of domain fold leading to protein aggregation. The observed behavior is further exacerbated with the substitution of amino acids with larger side chains. These findings support the concept that the lack of toxicity observed for CRM197 is the result of a highly unstable conformation of its catalytic domain that, upon insertion into the cell, cannot properly refold and perform its catalytic activity responsible for the arrest of all cellular protein synthesis.

Docking and molecular dynamics studies for developing Microcin C7 as an alternative drug against diphtheria toxin

Some conventional drugs are becoming ineffective these days for the treatment of diseases. This problem arises due to increase in the incidences of drug-resistant pathogens. Thus, there is a need for the development of alternative drugs. These alternative drugs should have the properties against which the microorganisms will not be able to develop resistance. The most promising candidates to be developed as alternative drugs are the antimicrobial peptides. In the present study, the docking studies were performed between the antimicrobial peptide Microcin C7 and the catalytic domain of diphtheria toxin. His21, Gly22, Thr42, Tyr46, Ala62, Tyr65 and Asn69 formed non-bond interactions with Microcin C7. Furthermore, the simulation studies were done to find the stability of the docked complex. The root mean square deviation and root mean square fluctuation results were significant and hence it was predicted that further work can be done on antimicrobial peptide Microcin C7 to develop it as an alternative drug against diphtheria toxin.

Polylysine-mediated translocation of the diphtheria toxin catalytic domain through the anthrax protective antigen pore.

The protective antigen (PA) moiety of anthrax toxin forms oligomeric pores in the endosomal membrane, which translocate the effector proteins of the toxin to the cytosol. Effector proteins bind to oligomeric PA via their respective N-terminal domains and undergo N- to C-terminal translocation through the pore. Earlier we reported that a tract of basic amino acids fused to the N-terminus of an unrelated effector protein (the catalytic domain diphtheria toxin, DTA) potentiated that protein to undergo weak PA-dependent translocation. In this study, we varied the location of the tract (N-terminal or C-terminal) and the length of a poly-Lys tract fused to DTA and examined the effects of these variations on PA-dependent translocation into cells and across planar bilayers in vitro. Entry into cells was most efficient with ∼12 Lys residues (K12) fused to the N-terminus but also occurred, albeit 10–100-fold less efficiently, with a C-terminal tract of the same length. Similarly, K12 tracts at either terminus occluded PA pores in planar bilayers, and occlusion was more efficient with the N-terminal tag. We used biotin-labeled K12 constructs in conjunction with streptavidin to show that a biotinyl-K12 tag at either terminus is transiently exposed to the trans compartment of planar bilayers at 20 mV; this partial translocation in vitro was more efficient with an N-terminal tag than a C-terminal tag. Significantly, our studies with polycationic tracts fused to the N- and C-termini of DTA suggest that PA-mediated translocation can occur not only in the N to C direction but also in the C to N direction.

Abstract 5703: Efficient targeted delivery of protein toxins using self-manufacturing nanoparticles (minicells) derived from bacteria

Abstract Significant progress has been made with targeted delivery of small molecule cancer therapeutics. The key to clinical success has been the use of extremely potent drugs conjugated to targeting antibodies. There are also classes of very potent and promising therapeutic macromolecules, including protein toxins and RNAi. However, there are several challenges unique to macromolecular payloads that cannot be adequately addressed when using antibodies alone for targeting. These include manufacturing complexity, blood-based catabolism, low payload capacity, poor loading efficiency, and inefficient endosomal escape. Several of these issues have also impeded clinical success with antibody-decorated liposomes. We are addressing these issues by using self-manufacturing nanoparticles, or minicells, derived from E. coli. Minicells are spherical, non-viable ∼400 nm nanoparticles containing all bacterial components, except the chromosome. Minicells bud off the end of rod-shaped bacteria as a result of unequal cell division after induced overexpression of a gene involved in septum formation. We have developed procedures for purification of minicells, including removal of parental bacteria. Our minicell-producing bacterial strains are genetically engineered to prevent growth outside the laboratory. In addition, we have introduced a lipopolysaccharide biosynthesis mutation, which significantly attenuates endotoxin-related inflammatory responses. We have produced minicells with surface expression of the beta1 integrin-targeting protein invasin. The minicells also contain a bacterially-expressed cholesterol-dependent endosomal membrane disrupting protein, such as listeriolysin O (LLO) or perfringolysin O (PFO), and a potent protein toxin, such as the catalytic domain of diphtheria toxin (DT). Invasin binds with high affinity to certain β1-containing integrins, including α5β1, which is overexpressed in tumor vasculature and several solid tumors. Surface expression of invasin on minicells led to binding and endocytosis into human endothelial cells (HUVECs) and tumor cells lines that express α5β1 integrin. Despite significant internalization, invasin-expressing minicells were not toxic to mammalian cells at doses of ≥10,000 minicells per cell. Inclusion of LLO or PFO and DT led to targeted killing of HUVECs and tumor cells (melanoma, colorectal, pancreatic, lung and hepatocellular carcinoma) with IC50s in the range of 50-500 minicells per mammalian cell. In vivo evaluation of minicell toxicity, immunogenicity, tissue distribution and efficacy is in progress. In summary, our minicell-based targeted delivery platform for macromolecular therapeutics reduces manufacturing time and complexity, significantly increases payload capacity, protects payload from blood-based catabolism, and provides a critically enabling endosomal escape mechanism. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5703. doi:1538-7445.AM2012-5703

Backbone and side chain 1H, 13C and 15N resonance assignments of the catalytic domain of diphtheria toxin

Diphtheria is a serious upper respiratory tract disease caused by the diphtheria toxin (DT) secreted from the bacteria Corynebacterium diphtheriae. Vaccination is the best way to protect against this infectious disease. Diphtheria vaccines are prepared by isolating, purifying and chemically deactivating DT. Although toxoids have been used for decades in immunization, there is still little understanding at the molecular level of the process of toxoid preparation, and how chemical treatment enhances their immunogenicity. We have undertaken an NMR study of the catalytic domain as a first step in understanding the molecular details involved in vaccine antigen preparation. Here we report a near complete assignment for the backbone and side chain resonances of the diphtheria toxin catalytic domain.

Mechanism of Diphtheria Toxin Catalytic Domain Delivery to the Eukaryotic Cell Cytosol and the Cellular Factors that Directly Participate in the Process

Research on diphtheria and anthrax toxins over the past three decades has culminated in a detailed understanding of their structure function relationships (e.g., catalytic (C), transmembrane (T), and receptor binding (R) domains), as well as the identification of their eukaryotic cell surface receptor, an understanding of the molecular events leading to the receptor-mediated internalization of the toxin into an endosomal compartment, and the pH triggered conformational changes required for pore formation in the vesicle membrane. Recently, a major research effort has been focused on the development of a detailed understanding of the molecular interactions between each of these toxins and eukaryotic cell factors that play an essential role in the efficient translocation of their respective catalytic domains through the trans-endosomal vesicle membrane pore and delivery into the cell cytosol. In this review, I shall focus on recent findings that have led to a more detailed understanding of the mechanism by which the diphtheria toxin catalytic domain is delivered to the eukaryotic cell cytosol. While much work remains, it is becoming increasingly clear that the entry process is facilitated by specific interactions with a number of cellular factors in an ordered sequential fashion. In addition, since diphtheria, anthrax lethal factor and anthrax edema factor all carry multiple coatomer I complex binding motifs and COPI complex has been shown to play an essential role in entry process, it is likely that the initial steps in catalytic domain entry of these divergent toxins follow a common mechanism.

Role of CypA and Hsp90 in membrane translocation mediated by anthrax protective antigen

Bacillus anthracis lethal toxin consists of the protective antigen (PA) and the metalloprotease lethal factor (LF). During cellular uptake PA forms pores in membranes of endosomes, and unfolded LF translocates through the pores into the cytosol. We have investigated whether host cell chaperones facilitate translocation of LF and the fusion protein LF(N)DTA. LF(N) mediates uptake of LF(N)DTA into the cytosol, where DTA, the catalytic domain of diphtheria toxin, ADP-ribosylates elongation factor-2, allowing for detection of small amounts of translocated LF(N)DTA. Cyclosporin A, which inhibits peptidyl-prolyl cis/trans isomerase activity of cyclophilins, and radicicol, which inhibits Hsp90 activity, prevented uptake of LF(N)DTA into the cytosol of CHO-K1 cells and protected cells from intoxication by LF(N)DTA/PA. Both inhibitors, as well as an antibody against cyclophilin A blocked the release of active LF(N)DTA from endosomal vesicles into the cytosol in vitro. In contrast, the inhibitors did not inhibit cellular uptake of LF. In vitro, cyclophilin A and Hsp90 bound to LF(N)DTA and DTA but not to LF, implying that DTA determines this interaction. In conclusion, cyclophilin A and Hsp90 facilitate translocation of LF(N)DTA, but not of LF, across endosomal membranes, and thus they function selectively in promoting translocation of certain proteins, but not of others.

Essential lysine residues within transmembrane helix 1 of diphtheria toxin facilitate COPI binding and catalytic domain entry

The translocation of the diphtheria toxin catalytic domain from the lumen of early endosomes into the cytosol of eukaryotic cells is an essential step in the intoxication process. We have previously shown that the in vitro translocation of the catalytic domain from the lumen of toxin pre-loaded endosomal vesicles to the external medium requires the addition of cytosolic proteins including coatomer protein complex I (COPI) to the reaction mixture. Further, we have shown that transmembrane helix 1 plays an essential, but as yet undefined role in the entry process. We have used both site-directed mutagenesis and a COPI complex precipitation assay to demonstrate that interaction(s) between at least three lysine residues in transmembrane helix 1 are essential for both COPI complex binding and the delivery of the catalytic domain into the target cell cytosol. Finally, a COPI binding domain swap was used to demonstrate that substitution of the lysine-rich transmembrane helix 1 with the COPI binding portion of the p23 adaptor cytoplasmic tail results in a mutant that displays full wild-type activity. Thus, irrespective of sequence, the ability of transmembrane helix 1 to bind to COPI complex appears to be the essential feature for catalytic domain delivery to the cytosol.

COPI coatomer complex proteins facilitate the translocation of anthrax lethal factor across vesicular membranes in vitro

The delivery of the diphtheria toxin catalytic domain (DTA) from acidified endosomes into the cytoplasm of eukaryotic cells requires protein-protein interactions between the toxin and a cytosolic translocation factor (CTF) complex. A conserved peptide motif, T1, within the DT transmembrane helix 1 mediates these interactions. Because the T1 motif is also present in the N-terminal segments of lethal factor (LF) and edema factor (EF) in anthrax toxin, we asked whether LF entry into the cell might also be facilitated by target cell cytosolic proteins. In this study, we have used LFnDTA and its associated ADP-ribosyltransferase activity (DTA) to determine the requirements for LF translocation from the lumen of endosomal vesicles to the external medium in vitro. Although low-level release of LFnDTA from enriched endosomal vesicles occurs in the absence of added factors, translocation was enhanced by the addition of cytosolic proteins and ATP to the reaction mixture. We show by GST-LFn pull-down assays that LFn specifically interacts with at least zeta-COP and beta-COP of the COPI coatomer complex. Immunodepletion of COPI coatomer complex and associated proteins from cytosolic extracts blocks in vitro LFnDTA translocation. Translocation may be reconstituted by the addition of partially purified bovine COPI to the translocation assay mixture. Taken together, these data suggest that the delivery of LF to the cytosol requires either COPI coatomer complex or a COPI subcomplex for translocation from the endosomal lumen. This facilitated delivery appears to use a mechanism that is analogous to that of DT entry.

Trojan horse or proton force: Finding the right partner(s) for toxin translocation

Much is known about the structure function relationships of a large number of bacterial protein toxins, the nature of their cell surface receptors, and their enzymatic activities which lead to the inactivation of their respective cytosolic targets. Despite this wealth of knowledge a detailed understanding of the mechanisms which underlie translocation of the catalytic domain across the eukaryotic cell membrane to the cytosol, the penultimate event in the intoxication process, have been slow in developing. In the case of diphtheria toxin, two prominent hypotheses have been advanced to explain how the catalytic domain is translocated from the lumen of endocytic vesicles to the target cell cytosol. We discuss each of these hypotheses and provide an overview of recent observations that tend to favor a mechanism employing a Cytosolic Translocation Factor complex in the entry process. This facilitated mechanism of translocation appears to rely upon protein-protein interactions between conserved domains within the transmembrane domain of diphtheria toxin with host cell factors to effect delivery of the enzymatic moiety. We have recently identified a 10 amino acid motif in the transmembrane domain of diphtheria toxin that is conserved in anthrax Lethal and Edema Factors, as well as in botulinum neurotoxins A, C and D. Stable eukaryotic cell transfectants that express a peptide containing this motif become resistant to the toxin, and sensitivity is completely restored by co-expression of siRNA which inhibits peptide expression. Data obtained from use of the protein fusion toxin DAB(389)IL-2 in cytotoxicity assays using susceptible Hut 102/6TG and resistant transfectant Hut102/6TG-T1 cells, as well as pull down assays have led to the formulation of a working model of facilitated delivery of the diphtheria toxin catalytic domain to the cytosol of target cells which is discussed in detail.

Whole-cell Voltage Clamp Measurements of Anthrax Toxin Pore Current

Protective antigen (PA) of anthrax toxin binds cellular receptors and forms pores in target cell membranes, through which catalytic lethal factor (LF) and edema factor (EF) are believed to translocate to the cytoplasm. Using patch clamp electrophysiological techniques, we assayed pore formation by PA in real time on the surface of cultured cells. The membranes of CHO-K1 cells treated with activated PA had little to no electrical conductivity at neutral pH (7.3) but exhibited robust mixed ionic currents in response to voltage stimuli at pH 5.3. Pore formation depended on specific cellular receptors and exhibited voltage-dependent inactivation at large potentials (>60 mV). The pH requirement for pore formation was receptor-specific as membrane insertion occurs at significantly different pH values when measured in cells specifically expressing tumor endothelial marker 8 (TEM8) or capillary morphogenesis protein 2 (CMG2), the two known cellular receptors for anthrax toxin. Pores were inhibited by an N-terminal fragment of LF and by micromolar concentrations of tetrabutylammonium ions. These studies demonstrated basic biophysical properties of PA pores in cell membranes and served as a foundation for the study of LF and EF translocation in vivo.

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 °C compared to 37 °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.

Reversible refolding of the diphtheria toxin T-domain on lipid membranes.

The catalytic domain of diphtheria toxin (DT) is translocated across endosomal membranes by the T-domain (DTT) in response to acidification. Understanding the energetics of translocation, besides clarifying the mechanism of translocation, should provide insights into general principles of membrane protein stability and assembly. As a first step, we have evaluated the energetics of DTT binding to lipid vesicles using three single-cysteine mutants (L350C, Q369C, and Y280C) labeled with a 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) fluorophore sensitive to polarity changes. Remarkably strong association with the vesicles was detected for all mutants, even at pH 7 at which DTT is believed to be in a fully folded membrane-incompetent state. Lowering the pH in the presence of anionic membranes resulted in a strong but reversible increase in emission of NBD-labeled mutants, consistent with reversible membrane insertion. This reversibility permitted free energies of DTT interactions with vesicles to be determined for the first time. Free energy values for the three mutants ranged from -8 to -10 kcal mol(-1) at pH 4.3 and from -7 to -8 kcal mol(-1) at pH 7. Insights into the disposition of DTT on membranes were obtained using a novel hydropathy analysis that considers the relative free energies of transmembrane and interfacial interactions as a function of pH. This analysis suggests that interactions at the membrane interface dominate pH-triggered insertion of DTT, implying that the folding pathway involves interfacial intermediates.