Linked thermal and solute perturbation analysis of cooperative domain interactions in proteins. Structural stability of diphtheria toxin

The temperature and guanidine hydrochloride (GuHCl) dependence of the structural stability of diphtheria toxin has been investigated by high-sensitivity differential scanning calorimetry. In 50 mM phosphate buffer at pH 8.0 and in the absence of GuHCl, the thermal unfolding of diphtheria toxin is characterized by a transition temperature (Tm) of 54.9 degrees C, a calorimetric enthalpy change (delta H) of 295 kcal/mol, and a van't Hoff to calorimetric enthalpy ratio of 0.57. Increasing the GuHCl concentration lowers the transition temperature and the calorimetric enthalpy change. At the same time, the van't Hoff to calorimetric enthalpy ratio increases until it reaches a value of 1 at 0.3 M GuHCl and remains constant thereafter. At low GuHCl concentrations (0-0.3 M), the thermal unfolding of diphtheria toxin is characterized by the presence of two transitions corresponding to the A and B domains of the protein. At higher GuHCl concentrations (0.3-1 M), the A domain is unfolded at all temperatures, and only one transition corresponding to the B domain is observed. Under these conditions, the most stable protein conformation at low temperatures is a partially folded state in which the A domain is unfolded and the B domain folded. A general model that explicitly considers the energetics of domain interactions has been developed in order to account for the stability and cooperative behavior of diphtheria toxin. It is shown that this cooperative domain interaction model correctly accounts for the temperature location as well as the shape and area of the calorimetric curves. Under physiological conditions, domain-domain interactions account for most of the structural stability of the A domain.(ABSTRACT TRUNCATED AT 250 WORDS)     

Experimental modelling of thermal inactivation of urease

Thermal inactivation of jack bean urease (EC 3.5.1.5) was investigated in a 0.1 M phosphate buffer with pH 7. An injection flow calorimetry method was adapted for the measurement of the enzyme activity. The inactivation curves were measured in the temperature range of 55 to 87.5 degrees C. The curves exhibited a biphasic pattern in the whole temperature range and they were well fitted with a biexponential model. A simultaneous fit of all inactivation data was based on kinetic models that were derived from different inactivation mechanisms and comprised the material balances of several enzyme forms and the enthalpy balance characterizing the initial heating period of enzyme solution. The multitemperature evaluation revealed that an adequate model had to incorporate at least three reaction steps. It was concluded that the key reaction steps at urease thermal inactivation were the reversible dissociation/denaturation of native form into an inactive denatured form, and irreversible association reactions of both the denatured and native forms.     

Thermal stability and intersubunit interactions of cholera toxin in solution and in association with its cell-surface receptor ganglioside GM1

The thermal stability of cholera toxin free in solution and in association with its cell-surface receptor ganglioside GM1 has been studied by using high-sensitivity differential scanning calorimetry and differential solubility thermal gel analysis. In the absence of ganglioside GM1, cholera toxin undergoes two distinct thermally induced transitions centered at 51 and 74 degrees C, respectively. The low-temperature transition has been assigned to the irreversible thermal denaturation of the active A subunit. The second transition has been assigned to the reversible unfolding of the B subunit pentamer. The isolated B subunit pentamer exhibits a single transition also centered at 74 degrees C, suggesting that the attachment of the A subunit does not contribute to the stability of the pentamer. In the intact toxin, the A subunit dissociates from the B subunit pentamer at a temperature that coincides with the onset of the B subunit thermal unfolding. In aqueous solution, the denatured A subunit precipitates after dissociation from the B subunit pentamer. This phenomenon can be detected calorimetrically by the appearance of an exothermic heat effect. In the presence of ganglioside GM1, the B subunit is greatly stabilized as indicated by an increase of 20 degrees C in the transition temperature. In addition, ganglioside GM1 greatly enhances the cooperative interactions between B subunits. In the absence of ganglioside, each monomer within the B pentamer unfolds in an independent fashion whereas the fully ganglioside-bound pentamer behaves as a single cooperative unit. On the contrary, the thermotropic behavior of the A subunit is only slightly affected by the presence of increasing concentrations of ganglioside GM1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional and structural comparisons of cysteine residues in the Val108 wild type and Met108 variant of human soluble catechol O-methyltransferase

Catechol O-methyltransferase (COMT) plays an important role in the inactivation of biologically active and toxic catechols. This enzyme is genetically polymorphic with a wild type and a variant form. Numerous epidemiological studies have shown that the variant form is associated with an increased risk of developing estrogen-associated cancers and a wide spectrum of mental disorders. There are seven cysteine residues in human S-COMT, all of which exist as free thiols and are susceptible to electrophilic attack and/or oxidative damage leading to enzyme inactivation. Here, the seven cysteine residues were systematically replaced by alanine residues by means of site-directed mutagenesis. The native forms and cysteine/alanine mutants were assayed for enzymatic activity, thermal stability, methylation regioselectivity, and reactivity of cysteine residues to thiol reagent. Our data showed that although there is only one encoding base difference between these two COMT forms, this difference might induce structural changes in the local area surrounding some cysteine residues, which might further contribute to the different roles they might play in enzymatic activity, and to the different susceptibility to enzyme inactivation.     

Evidence that diphtheria toxin and modeccin enter the cytosol from different vesicular compartments

Inhibition of protein synthesis in Vero cells was measured at different periods of time after treatment with diphtheria toxin and the related plant toxin modeccin. Diphtheria toxin acted much more rapidly than modeccin. Cells were protected against both toxins with antiserum as well as with agents like NH4Cl, procaine, and the ionophores monensin, FCCP, and CCCP, which increase the pH of intracellular vesicles. Antiserum, which is supposed to inactivate toxin only at the cell surface, protected only when it was added within a short period of time after modeccin. Compounds that increase the pH of intracellular vesicles, protected even when added after 2 h, indicating that modeccin remains inside vesicles for a considerable period of time before it enters the cytosol. After addition of diphtheria toxin to the cells, compounds that increase the pH of intracellular vesicles protected only approximately to the same extent as antitoxin. This indicates that after endocytosis diphtheria toxin rapidly enters the cytosol. At 20 degrees C, the cells were more strongly protected against modeccin than against diphtheria toxin. The residual toxic effect of diphtheria toxin at 20 degrees C could be blocked with NH4Cl whereas this was not the case with modeccin. This indicates that at 20 degrees C the uptake of diphtheria toxin occurs by the normal route, whereas the uptake of modeccin occurs by a less efficient route than that dominating at 37 degrees C. The results indicate that after endocytosis diphtheria toxin rapidly enters the cytosol from early endosomes with low pH (receptosomes). Modeccin enters the cytosol much more slowly, possibly after fusion of the endocytic vesicles with another compartment.     

Cloning and expression in Escherichia coli of three fragments of diphtheria toxin truncated within fragment B.

We have constructed three different truncated versions of diphtheria toxin (a 535-amino-acid polypeptide) which correspond to the N-terminal 290, 377, and 485 amino acids of the toxin. These lengths include one, three, and all four of the putative membrane-spanning sequences of the toxin which are thought to play a role in the translocation of fragment A into cells. Each of these three genes has been modified at its 3' end to code for a C-terminal cysteine (to allow for disulfide linkage of a targeting ligand) or a gene fusion with alpha-melanocyte-stimulating hormone. We have also substituted the native diphtheria tox promoter (ptox) with the lambda pR promoter in an effort to overexpress these proteins. The truncated genes are expressed in Escherichia coli from both the tox promoter in a constitutive fashion and from the pR promoter by using the heat-inducible cI857 repressor. The clones produce proteins which react with anti-diphtheria toxin serum, which migrate at the anticipated Mr on Western blots, and which have ADP-ribosyltransferase activity. Constitutive synthesis from ptox leads to severe proteolytic degradation even in a protease-deficient strain. High-level expression from the pR promoter in the same lon htpR strain allows the full-length polypeptides to accumulate but also stops the growth of the cells. It appears that removal of as few as 50 amino acids from the C-terminus of diphtheria toxin alters its conformation, making it a target for proteases and causing overexpression lethality in the host cells.     

Thermostability of bacteriophage T4 fibritin and its deletion mutants]

The thermal stability of a series of recently obtained mutants of fibritin from bacteriophage T4 (a superhelical fibrous homotrimer with parallel-packed subunits each containing 486 amino acid residues) progressively truncated from the subunit N-end was studied during incubation at 40-90 degrees C in the presence of a surfactant (2% SDS). The mutant fibritins, G, B, C, and E, contained 443, 276, 231, and 120 amino acid residues, respectively. One more truncated mutant (fibritin S1, 108 amino acid residues) was obtained. The 2% SDS-PAGE showed that the migration mobilities of all these proteins corresponded to apparent molecular masses substantially greater than those of the preliminarily heated samples (3 min at 100 degrees C). The heating of the intact fibritin and the mutant G at 50-70 degrees C for 10 min resulted in the formation of a form with an apparent molecular mass higher than 200 kDa. This form probably represented a trimeric protein with a partly denatured N-terminal part. Fibritins B and C were more stable and were only partly denatured into monomers even at 70-90 degrees C. The short mutants E and S1 dissociated into monomers at temperatures from 45 to 50 degrees C. The denaturation of mutants B, C, E, and S1 proceeded in one stage without formation of any intermediate form. The stability of the trimeric molecules of native fibritin under PAGE denaturing conditions and the behavior of the intact protein during heating in the temperature range of 50-70 degrees C might be used for the identification of fibritin intermediate forms upon folding in vivo. The refolding capability was found for fibritin and its mutants denatured by heating at low temperatures in the presence of 2% SDS.     

Indices of diphtheria (antitoxic) immunity in the dynamics of the disease and its treatment in adults

A total of 1,034 serum samples from 618 persons, including patients with different forms of diphtheria, carriers of the toxigenic forms of Corynebacterium diphtheriae, and angina patients, were studied. Analysis of the incidence of antibodies to C. diphtheriae toxin and their titers revealed that in more than half of all diphtheria patients no antibodies to C. diphtheriae toxin were detected upon admission to hospital. At the same time in 26% of the patients no antibodies were detected during the whole period of the disease; in such patients the toxic and subtoxic forms of diphtheria were registered twice as often as in seropositive patients. In 31% of the patients seronegative by the moment of hospitalization a rapid increase in the titers of antibodies C. diphtheriae toxin was observed in the course of the disease, which was indicative of the secondary character of immune response in patients who had been immunized earlier.     

Energetics of diphtheria toxin membrane insertion and translocation: calorimetric characterization of the acid pH induced transition

The pH and temperature stabilities of diphtheria toxin and its fragments have been studied by high-sensitivity differential scanning calorimetry. These studies demonstrate that the pH-induced conformational transition associated with the mechanism of membrane insertion and translocation of the toxin involves a massive unfolding of the toxin molecule. At physiological temperatures (37 degrees C), this process is centered at pH 4.7 at low ionic strength and at pH 5.4 in the presence of 0.2 M NaCl. At pH 8, the thermal unfolding of the nucleotide-bound toxin is centered at 58.2 degrees C whereas that of the nucleotide-free toxin is centered at 51.8 degrees C, indicating that nucleotide binding (ApUp) stabilizes the native conformation of the toxin. The unfolding profile of the toxin is consistent with two transitions most likely corresponding to the A fragment (Tm = 54.5 degrees C) and the B fragment (Tm = 58.4 degrees C), as inferred from experiments using the isolated A fragment. These two transitions are not independent, judging from the fact that the isolated A fragment unfolds at much lower temperatures (Tm = 44.2 degrees C) and that the B fragment is insoluble in aqueous solutions when separated from the A fragment. Interfragment association contributes an extra -2.6 kcal/mol to the free energy of stabilization of the A fragment. Whereas the unfolding of the entire toxin is irreversible, the unfolding of the A fragment is a reversible process. These findings provide a thermodynamic basis for the refolding of the A fragment after reexposure to neutral pH immediately following translocation across the lysosomal membrane.     

Entry of diphtheria toxin-protein A chimeras into cells

Fusion proteins consisting of diphtheria toxin and a duplicated Fc-binding domain of protein A were made in vitro after amplification of the DNA template by the polymerase chain reaction. The fusion proteins bound avidly to Vero cells coated with antibodies. A fusion protein containing full-length diphtheria toxin was toxic at lower concentrations than diphtheria toxin alone, apparently due to more efficient binding. The enzymatic part of the fusion protein was translocated across the surface membrane upon exposure to low pH. Like authentic diphtheria toxin, the fusion protein formed cation selective channels at low pH. Excess amounts of unlabeled diphtheria toxin inhibited formation of pronase-protected fragments derived from radiolabeled fusion protein. Furthermore, conditions that down-regulate the diphtheria toxin receptors reduced the sensitivity of the cells to the fusion protein, supporting the notion that authentic diphtheria toxin receptors are required. At temperatures below 18 degrees C the toxicity of the fusion protein was strongly reduced, whereas there was no temperature block for authentic diphtheria toxin. Brefeldin A protected Vero cells against the fusion protein but not against diphtheria toxin. The results indicate that the diphtheria toxin receptor is required for efficient toxin translocation even under conditions where the toxin is bound by an alternate binding moiety, and they suggest that the intracellular routing of the fusion protein is different from that of diphtheria toxin.     

X-ray grade crystals of the enzymatic fragment of diphtheria toxin

The enzymatic fragment of diphtheria toxin, fragment A (Mr = 21,167), complexed to the dinucleotide adenosine 3',5'-uridine (ApU), has been crystallized at two different values of pH by hanging drop vapor diffusion. Crystals grown at a pH value of 5.0 (from I) belong to the orthorhombic space group P2(1)2(1)2(1), with unit cell parameters a = 71.2 A, b = 73.0 A, c = 139.8 A and four protomers in the asymmetric unit. Crystals grown at a pH value of 8.1 (form II) belong to the monoclinic space group C2, with unit cell parameters a = 65.2 A, b = 85.6 A, c = 34.6 A, beta = 103.0 degrees and one protomer in the asymmetric unit. Both crystal forms diffract to 2.5 A resolution. The molecular structures of fragment A obtained from these two crystal forms may illuminate the pH-dependent transition of diphtheria toxin during membrane translocation.     

Human platelet phenol sulfotransferase: Familial variation in thermal stability of the TS form

Phenol sulfotransferase (PST) catalyzes the sulfate conjugation of catechol and phenolic drugs and xenobiotic compounds. Platelets and other tissues contain at least two forms of PST, forms that have been designated the "TL" and the "TS" forms. We measured the thermal stability of platelet TS PST in blood samples from 218 randomly selected unrelated subjects by heating platelet homogenates at 44 degrees C for 15 min. Thermal stability was expressed as the ratio of the enzyme activity remaining after preincubation to that in an unheated sample, a heated/control (H/C) ratio. The frequency distribution of H/C ratios for this population sample was bimodal, with a nadir at an H/C ratio of 0.33. Of the 218 subjects studied, 29 (13.3%) had thermolabile TS PST (H/C less than 0.33). Platelet samples were then obtained from subjects with thermolabile and thermostable TS platelet PST. PST activity in these platelet samples had similar apparent Km constants for substrates. IC50 values for inhibition of TS PST by 2,6-dichloro-4-nitrophenol in these samples were also nearly identical. The results of experiments in which platelet homogenates from subjects with thermolabile and thermostable TS PST were mixed and the results of experiments in which platelet homogenates were subjected to gel filtration chromatography were compatible with the conclusion that individual differences in TS PST thermal stability were properties of PST itself. Finally, there was a significant familial aggregation of the trait of thermolabile TS PST when H/C ratios were measured in platelet homogenates from 231 members of 49 randomly selected families.     

Conformational stability and activity of ribonuclease T1 with zero, one, and two intact disulfide bonds

Ribonuclease T1 has two disulfide bonds linking cysteine residues 2-10 and 6-103. We have prepared a derivative of ribonuclease T1 in which one disulfide bond is broken and the cysteine residues carboxymethylated, (2-10)-RCM-T1, and three derivatives in which both disulfides are broken and the cysteine residues reduced, R-T1, carboxamidomethylated, RCAM-T1, or carboxymethylated, RCM-T1. The RNA hydrolyzing activity of these proteins has been measured, and urea and thermal denaturation studies have been used to determine conformational stability. The activity, melting temperature, and conformational stability of the proteins are: ribonuclease T1 (100%, 59.3 degrees C, 10.2 kcal/mol), (2-10)-RCM-T1 (86%, 53.3 degrees C, 6.8 kcal/mol), R-T1 (53%, 27.2 degrees C, 3.0 kcal/mol), RCAM-T1 (43%, 21.2 degrees C, 1.5 kcal/mol), and RCM-T1 (35%, 16.6 degrees C, 0.9 kcal/mol). Thus, the conformational stability is decreased by 3.4 kcal/mol when one disulfide bond is broken and by 7.2-9.3 kcal/mol when both disulfide bonds are broken. It is quite remarkable that RNase T1 can fold and function with both disulfide bonds broken and the cysteine residues carboxymethylated. The large decrease in the stability is due mainly to an increase in the conformational entropy of the unfolded protein which results when the constraints of the disulfide bonds on the flexibility are removed. We propose a new equation for predicting the effect of a cross-link on the conformational entropy of a protein: delta Sconf = -2.1 - (3/2)R 1n n, where n is the number of residues between the side chains which are cross-linked. This equation gives much better agreement with experimental results than other forms of this equation which have been used previously.     

Stability of HAMLET--a kinetically trapped alpha-lactalbumin oleic acid complex

The stability toward thermal and urea denaturation was measured for HAMLET (human alpha-lactalbumin made lethal to tumor cells) and alpha-lactalbumin, using circular dichroism and fluorescence spectroscopy as well as differential scanning calorimetry. Under all conditions examined, HAMLET appears to have the same or lower stability than alpha-lactalbumin. The largest difference is seen for thermal denaturation of the calcium free (apo) forms, where the temperature at the transition midpoint is 15 degrees C lower for apo HAMLET than for apo alpha-lactalbumin. The difference becomes progressively smaller as the calcium concentration increases. Denaturation of HAMLET was found to be irreversible. Samples of HAMLET that have been renatured after denaturation have lost the specific biological activity toward tumor cells. Three lines of evidence indicate that HAMLET is a kinetic trap: (1) It has lower stability than alpha-lactalbumin, although it is a complex of alpha-lactalbumin and oleic acid; (2) its denaturation is irreversible and HAMLET is lost after denaturation; (3) formation of HAMLET requires a specific conversion protocol.     

Monoclonal antibody against diphtheria toxin. Effect on toxin binding and entry into cells

A number of monoclonal antibodies against diphtheria toxin were isolated. Some of their properties were determined. Antibody 2 reacts with the region of between 30 and 45 kDa from the NH2 terminus of toxin. Antibody 7 reacts with the COOH-terminal 17-kDa region of toxin. These two antibodies show sharp contrasts in their effects on toxin action in cultured cells. When antibody 2 or 7 and toxin were mixed, incubated at 37 degrees C, and then added to sensitive Vero cells, antibody 7 blocked toxin action, but antibody 2 did not. When antibody 2 or 7 was added to cells to which toxin had been prebound at 4 degrees C, and the cells were then shifted to 37 degrees C, antibody 7 did not block toxin action, but antibody 2 inhibited intoxication. Antibody 7 blocked binding of 125I-toxin to cells and did not block degradation of toxin associated with cells. Antibody 2 did not block binding of 125I-toxin to cells, and was able to bind to cells in the presence of toxin. The results obtained from the effect of antibody 2 on degradation of 125I-toxin associated with cells resemble those seen with amines, which block toxin action but do not inhibit binding of toxin to cells. These facts show that antibody 2 does not block binding of toxin to cell surfaces, but blocks the entry of toxin into the cytosol at a step after binding of toxin to the receptor. Antibodies 14 and 15 react with fragment A of diphtheria toxin, but have no effect on any activity of toxin. The other monoclonal antibodies have effects on toxin binding and entry intermediate between those of 2 and 7.     

Artificial hybrid protein containing a toxic protein fragment and a cell membrane receptor-binding moiety in a disulfide conjugate. II. Biochemical and biologic properties of diphtheria toxin fragment A-S-S-human placental lactogen.

The biochemical and biologic properties of a purified disulfide conjugate of diphtheria toxin fragment A and human placental lactogen (toxin A-hPL) have been studied by (a) assaying the ADP-ribosyltransferase activity of the intact conjugate, (b) assaying the binding of the intact conjugate to mammary gland plasma membrane lactogenic receptors, and (c) assaying the effect of the conjugate on the rate of protein synthesis in rabbit mammary gland explants maintained in organ culture. The toxin A-hPL conjugate retains one-third of the NAD+:EF-2 ADP-ribosyltransferase activity of toxin A, and 26% of the hPL-binding activity to lactogenic receptors. Binding activity was demonstrated by radioreceptor assay and by assaying toxin A activity bound to membranes which was competitively displaced by excess hPL. Since the toxin A-hPL conjugate retained activities of its separate subunits, it could be regarded as a structural analogue of nicked diphtheria toxin with replacement of the original membrane-binding chain by another binding chain that is specific for lactogenic receptor. However, the conjugate failed to inhibit protein synthesis in organ-cultured mammary gland explants, although these were sensitive to native diphtheria toxin and could bind hPL. It is concluded from these results that the toxin A-hPL conjugate does not act as a functional analogue of diphtheria toxin with altered receptor specificity, and that the hPL receptor cannot mediate the entry of toxin A or toxin A-hPL from membrane-bound conjugate into the cytosol site of action of toxin A.     

Crystallization of diphtheria toxin.

Two new crystal forms (forms III and IV) have been grown of diphtheria toxin (DT), which kills susceptible cells by catalyzing the ADP-ribosylation of elongation factor 2, thereby stopping protein synthesis. Forms III and IV diffract to 2.3 A and 2.7 A resolution, respectively. Both forms belong to space group C2; the unit cell parameters for form III are a = 107.3 A, b = 91.7 A, c = 66.3 A and beta = 94.7 degrees and those for form IV are a = 108.3 A, b = 92.3 A, c = 66.1 A and beta = 90.4 degrees. Both forms have one protein chain per asymmetric unit with the dimeric molecule on a twofold axis of symmetry. Form IV is exceptional among all crystal forms of DT in that it can be grown reproducibly. Thus the form IV crystals should yield a crystallographic structure giving insight into the catalytic, receptor-binding and membrane-insertion properties of DT.     

Molecular cloning and expression of gene fragments from corynebacteriophage beta encoding enzymatically active peptides of diphtheria toxin

Two restriction fragments from corynebacteriophage beta vir tox+ that encode peptides similar to diphtheria toxin fragment A and the chain termination fragment, CRM45, have been cloned into Escherichia coli in plasmid pBR322. Clones containing the recombinant plasmids produced gene products that were active in catalyzing the ADP ribosylation of elongation factor 2 and were reactive with diphtheria toxin antiserum. Toxin-related peptides were found primarily in the periplasmic compartment and were degraded to nonimmunoreactive forms within 1 to 2 h of synthesis. The expression of both gene fragments appears to have originated from the diphtheria toxin promoter.     

Circular dichroism of diphtheria toxin, Pseudomonas aeruginosa exotoxin A, and various derivatives

We have recorded the near- and far-ultraviolet circular dichroism spectra of diphtheria toxin, Pseudomonas aeruginosa exotoxin A, and derivatives of these toxins. The far-ultraviolet spectra of various forms of diphtheria toxin were virtually identical, implying that no major changes in secondary structure accompany proteolytic nicking or dimerization of toxin, or binding of the endogenous dinucleotide, adenylyl-(3'-5')-uridine 3'-monophosphate (AdoPUrdP). Alpha-helix content was estimated to be 29%, as compared with 8% for fragment A. Near-ultraviolet spectra were identical between nicked and intact diphtheria toxin. A broad negative transition with a minimum at 304 nm was assigned to the intrachain disulfide bridge within the B moiety. Dimeric diphtheria toxin showed perturbations of aromatic residues. Binding of AdoPUrdP to monomeric diphtheria toxin or of adenylyl-(3',5')-uridine (AdoPUrd) to fragment A perturbed one or more tryptophans. The latter results correlate with evidence for involvement of a tryptophan in NAD binding. Native exotoxin A was estimated to have 16% alpha-helix, and the activated form of exotoxin A, 11%. An enzymically active, 31 kDa proteolytic fragment of exotoxin A showed similar alpha-helix content (7%) to that of diphtheria toxin fragment A.     

Precursor in cotranslational secretion of diphtheria toxin.

By extracellular labeling of peptides of intact Corynebacterium diphtheriae, followed by fractionation of the cells and chain completion by isolated polysomes, it is shown that diphtheria toxin is formed and secreted cotranslationally by membrane-bound polysomes; free polysomes from none. Moreover, when the chains on these polysomes were completed in vitro, in the absence of membrane they were found to include not only diphtheria toxin of a molecular weight of 62,000, but also a larger precursor of a molecular weight of 68,000. The precursor was identified by several properties: immune precipitation; conversion into toxin fragments A and B; adenosine diphosphate ribosyl-transferase activity after activation with trypsin; and cleavage to 62,000 daltons by membrane enzymes. The precursor yields an N-terminal A fragment with a broadened molecular weight distribution, compared with that from authentic toxin, thus supporting the expectation that the extra segment of the precursor is N-terminal.