Structure of tetanus toxin. I. Breakdown of the toxin molecule and discrimination between polypeptide fragments.

Tetanus toxin was digested with papain, yielding one major polypeptide (Fragment C) with a molecular weight corresponding to 47,000 +/- 5%, thus comprising about one-third of the toxin molecule. Fragment C was antigenically active, atoxic, and stimulated the formation of antibodies neutralizing the lethal action of tetanus toxin in vivo. Furthermore, a second split product (Fragment B) was isolated from the papain digest, containing two polypeptide chains linked together via a disulfide bond. Fragment B (Mr = 95,000 +/- 5%) was atoxic and showed a reaction of nonidentity with Fragment C on immunodiffusion analysis against tetanus antitoxin. The basic two-chain structure (heavy and light chain polypeptide, cf. Matsuda, M., and Yoneda, M. (1975) Infect. Immun. 12, 1147-1153) of tetanus toxin has been confirmed and the relationship between Fragments B and C within this framework has been established. Fragment C was distinguished from the light chain by electrophoresis in sodium dodecyl sulfate and by immunodiffusion analysis, indicating that this fragment constitutes a portion of the heavy chain polypeptide. Fragment B showed a reaction of partial identity with the light as well as the heavy chain from tetanus toxin. Reduction of Fragment B with dithiothreitol followed by gel chromatography yielded a fraction which was indistinguishable from the light chain portion of the toxin molecule. It is concluded that Fragment B comprises the complementary portion of the heavy chain (remaining after scission of the polypeptide bond(s) releasing Fragment C) linked to the light chain by a disulfide bond.