Characterization of tetanus toxins and toxin components by amino terminal analyses

Limited proteolysis of tetanus toxin. Relation to activity and identification of cleavage sites

Tetanus toxin is synthesized by Clostridium tetani as a 151-kDa peptide chain. The primary gene product is processed post-translationally by removal of the initiating methionine residue, formation of disulfide bridges and limited proteolysis by bacterial or exogenous proteinases. The mature toxins consist of a 52-kDa light chain and a 98-kDa heavy chain, linked together by a disulfide bond. Proteolytic nicking is accompanied by increased pharmacological potency. To identify the structural alterations involved, single-chain toxin has been subjected to limited proteolysis with various enzymes. The new N-termini have been determined by Edman degradation and the C-termini by isolation of short C-terminal peptide fragments and subsequent analysis of the sequence and composition. All two-chain toxins result from proteolytic nicking within the 17-residue segment of residues 445-461. Thus, the protease(s) of the culture broth cleave on the C-terminal side of Glu449 and partially Ala456, giving rise to two heavy chain N-termini. Trypsin and clostripain first attack the C-terminal of Arg454 and later Arg448, whereas endoproteinase Arg-C cleaves the former bond only. Chymotrypsin and endoproteinase Glu-C each split a single peptide bond, i.e. that located after Tyr452 and Glu449, respectively. Papain gives rise to a large number of cleavages within the 17-residue segment, the new C-terminus being Thr445 or Asn446 and the new N-terminus being Asp460 or Leu461. Further papain digestion leads to an additional cleavage within the heavy chain between Ser863 and Lys864. The original N-terminal Pro1 and C-terminal Asp1314, predicted from the nucleotide sequence, are conserved in all proteolytic digests. The pharmacological activity of the various two-chain toxins was 5-11 times that of the single-chain toxin, as estimated from the inhibition of [3H]noradrenaline release from rat-brain homogenate. The present data on the processing and activation by limited proteolysis prove the existence of several active tetanus isotoxins. These data, together with our previous data on the localization of disulfide bridges and sulfhydryl groups (Krieglstein, K., Henschen, A., Weller, U. & Habermann, E. (1990) Eur. J. Biochem. 188, 39-45), provide the detailed protein chemical characterization of the tetanus isotoxins.

Arrangement of disulfide bridges and positions of sulfhydryl groups in tetanus toxin

Tetanus toxin is a 151-kDa protein. The complete amino acid sequence is known. The mature toxin is made up of two peptide chains and contains 10 half-cystine residues. Treatment with 4-vinylpyridine in the presence of 6 M guanidine converted six of them into S-pyridylethyl cysteine residues as determined by amino acid analysis. When alkylation was preceded by mercaptolysis, all 10 half-cystine residues were recovered in the S-pyridylethylated form. It was therefore concluded that the toxin contains six sulfhydryl groups and two disulfide bonds. The positions of the residues carrying sulfhydryl groups and of those involved in disulfide bridges were determined by labelling of the toxin alternatively with 4-vinylpyridine or with 4-dimethylaminoazobenzene-4'-iodoacetamide (DABIA), directly or after mercaptolysis. The toxin derivatives were cleaved with cyanogen bromide and the elution patterns in reversed-phase HPLC compared. The chromatography components were identified by N-terminal amino acid sequence and amino acid composition. In the chromatography of the non-mercaptolysed, DABIA-treated sample four chromophore-carrying components were detected which could be demonstrated by N-terminal sequence analysis to correspond to six half-cystine-containing cyanogen bromide fragments. In the mercaptolysed, DABIA-treated sample three additional chromophore-carrying components were present, corresponding to two previously disulfide-linked cyanogen bromide fragments and one fragment which had contained an internal disulfide bridge. The HPLC patterns showed characteristic differences as the DABIA-labelled fragments were considerably more hydrophobic than the corresponding vinylpyridine-labelled fragments. It was established that the half-cystine residues in positions 26, 185, 198, 311, 868, and 1300 are present in the sulfhydryl form, that those in positions 438 and 466 are disulfide-bridged, thereby connecting the light and heavy chains of the toxin, and that those in positions 1076 and 1092 are disulfide-bridged, thereby giving rise to a loop in the heavy chain. During the progress of the investigations about 20% of the amino acid sequence previously predicted from DNA analysis was confirmed by protein-chemical methods.

Partial amino acid sequence of the heavy and light chains of botulinum neurotoxin type A

The dichain (nicked) type A botulinum neurotoxin is a protein (mol. wt. 145,000) composed of a heavy and a light chain (mol. wt. 97,000 and 53,000, respectively) that are held together by disulfide bond(s). We report here the sequence of the first 17 amino acid residues of the light chain, and the first 10 residues of the heavy chain. The heavy chain was isolated from the neurotoxin by two different methods, while the light chain was isolated by the only available method. The identical amino acid sequence was found in both preparations of heavy chain. Two samples of the light chain isolated from two separately prepared batches of the neurotoxin also had identical sequences.

Analysis of the immune response to papain digestion products of tetanus toxin

Tetanus toxin was degraded by papain and the previously characterized Fragment B and Fragment C were isolated. The induction of protective immunity against tetanus toxin was subsequently investigated in two animal models using Fragment B and Fragment C, as well as conventional tetanus toxoid for comparison. In guinea pigs, Fragment B showed an immunizing potency similar to that recorded for an equal amount of tetanus toxoid, whereas Fragment C was considerably less efficient, on an equal weight basis. In contrast, mice immunized with Fragment C reached a protection level similar to that achieved after immunization with the complete tetanus toxoid antigen. In both animal models, Fragments B and C acted in a synergistic fashion when injected together. Analysis of sera from guinea pigs using ELISA, after administration of a low dose of tetanus immunogen corroborated these results. The data indicate that antigenic determinants in distinct regions of the toxin molecule independently may induce antibodies neutralizing the lethal action of tetanus toxin. The immune response to the complete toxoid antigen may be subject to influence by intramolecular competition between antigenic determinants. The immunogenicity of the fragments as measured by the protection rate after active immunization may depend upon the animal species used.

Affinity chromatographic purification and characterization of two iodinated tetanus toxin fractions exhibiting different binding properties

Highly purified iodinated tetanus toxin preparations separate on ganglioside affinity columns into two distinct (A and B) fractions representing about 20% and 75% of the iodinated toxin, respectively. Fraction A, eluted by 1% NaCl, migrates like native tetanus toxin (150,000 mol. wt) on SDS polyacrylamide gel electrophoresis. It forms an aggregate of molecular weight approximately 360,000 on Sephacryl S-300 gel permeation chromatography in the presence of detergent and contains two isoforms on preparative chromatofocusing. Fraction A binds poorly to neurons in tissue culture or to synaptosomal membrane preparations. It retains, however, its antigenicity and biotoxicity. Fraction B, eluted by 6% NaCl, binds effectively to gangliosides and also to neurons or synaptosome preparations. It has a similar molecular weight and chain composition to the native toxin and displays two isoforms, precipitable during chromatofocusing. Fraction B possesses similar binding, immunological and toxic properties to the original iodinated tetanus toxin. Following excessive iodination (4-6 mCi/mg protein), toxicity can be remarkably reduced. Unlabeled toxin shows a similar chromatographic pattern to the iodinated toxin on affinity columns, suggesting that a large portion (30% by protein and 55% by toxicity) of the toxin has a poor affinity for gangliosides. The molecular pharmacokinetics of tetanus toxin with respect to affinity toward ganglioside-dependent and ganglioside-independent receptors needs re-evaluation.

A review of the molecular structure of tetanus toxin

A discontinuous preparative polyacrylamide gel electrophoresis system has been developed and used to purify both the nicked and unnicked forms of tetanus toxin. The system was also used to prepare purified H and L chain peptides from the nicked toxin. The results show that the endogenous protease(s), which convert unnicked toxin to the nicked form, produce multiple species of nicked toxin, and heterogeneity in the H and L chains. The major amino termini of the toxins and their peptide components are: extract toxin, proline; filtrate toxin, proline, serine and asparagine; L chain, proline; and H chain, serine and asparagine. The L chain is located in the amino terminal position of the toxin molecule and the H chain the carboxy terminal end. A model is proposed to explain these results. Using the analytical ultracentrifuge, we have determined the molecular weights of extract and filtrate toxins to be 140000 +/- 5000 and 128000 +/- 3000, respectively. Using SDS-polyacrylamide gel electrophoresis we estimate the molecular weights of the H and L chains to be 87000 and 48000 daltons, respectively. Circular dichroic spectra of the toxins and their peptide components indicate that: the major tryptophanyl band in the toxin is contributed almost entirely by the H chain, the microenvironments of all the aromatics and disulfides in the two toxins appear to have small if any differences, the two toxins show little difference in their ordered secondary structure, and the two peptides when separated from one another still retain 80% of the helical structure that is present in the intact toxin but show a considerable loss of beta-structure. The crystalline form of the nicked toxin has a hexagonal symmetry with two dimensional reciprocal lattice constants of 1/150 A-1 and 1/150 A-1. The crystals appear to belong to the two dimensional plane group P6 suggesting that each unit cell contains 6 or a multiple of 6 toxin molecules.