Picosecond to hour time scale dynamics of a "three finger" toxin:: Correlation with its toxic and antigenic properties

Toxin alpha from Naja nigricollis (61 amino acids, four disulfide bridges) belongs to the "three finger" fold family, which contains snake toxins with various biological activities and nontoxic proteins from different origins. In this paper, we report an extensive H-1 and N-15 NMR study of the dynamics of toxin ct in solution. N-15 relaxation,H-1 off-resonance ROESY, and H-D exchange experiments allowed us to probe picosecond to hour motions in the protein. Analysis of these NMR measurements demonstrates that toxin or exhibits various time scale motions, i.e., particularly large amplitude picosecond to nanosecond motions at the tips of the loops, observable microsecond to millisecond motions around two disulfide bridges, second time scale motions around the C-N bonds of asparagine and glutamine side chains which are more or less rapid depending on their amino acid solvent accessibility, and minute to hour motions in the beta-sheet structure. The less well-defined regions of toxin ct solution structures are subject to important picosecond to nanosecond motions. The toxic site is organized around residues belonging to the rigid core of the molecule but also comprises residues exhibiting dynamics on various time scales. The M alpha 1 epitope is subject to large picosecond to millisecond motions, which are probably modified by the interaction with the antibody. This phenomenon could be linked to the neutralizing properties of the antibody.