| Tetanus neurotoxin (TeNT), produced by the anaerobic bacterium, Clostridium tetani, is the causative agent of the neuroparalytic syndromes of tetanus. Its mode of action, and subsequent prevention, are of interest in medicine and biodefense. The 52kDa carboxyl-terminal receptor-binding fragment heavy chain of tetanus toxin crystallizes as a monomer in orthorhombic space group P212121, with cell parameters a = 71.180, b = 79.380, c = 93.810 A. The 1.6 A structure, determined by a combination of multiple isomorphous replacement and multi-wavelength anomalous dispersion phasing using a gold derivative, has been refined to a final R-value = 0.192 (Rfree = 0.227). A 1.8 A native structure, crystallized in the same unit cell and determined by molecular replacement, has been refined after iterative model building and phase bias removal to a final R-value = 0.195 (Rfree = 0.242). These near atomic resolution TeNT heavy chain structures differ from previously reported ligand bound structures and polymorphs mainly in loop regions. Comparison with other structures suggests that loop rearrangements result largely from crystal contacts, and that these alternate conformations should be considered in functional analyses and in structure-guided drug design. Solvent-accessible gold sites show no significant changes in backbone conformation. Conserved structural features implicated in ganglioside binding, as well as a new potential ligand binding site, have been identified. |
