NMR structural determination of viscotoxin A3 from Viscum album L.

The high-resolution three-dimensional structure of the plant toxin viscotox in A3, from Viscum album L., has been determined in solution by H-1 NMR spe ctroscopy at pH 3.6 and 12 degrees C (the structure has been deposited in t he Protein Data Bank under the id. code 1EDO). Experimentally derived restr aints including 734 interproton distances from nuclear Overhauser effect me asurements, 22 hydrogen bonds, 32 phi angle restraints from J coupling meas urements, together with three disulphide bridge constraints were used as in put in restrained molecular dynamics, followed by minimization, using DYANA and Discover. Backbone and heavy atom root-mean-square deviations were 0.4 7+/-0.11 Angstrom (1 Angstrom = 10(-10) m) and 0.85+/-0.13 Angstrom respect ively. Viscotoxin A3 consists of two alpha-helices connected by a turn and a short stretch of antiparallel beta-sheet. This fold is similar to that fo und in other thionins, such as crambin, hordothionin-alpha and -beta, phora toxin A and purothionin-alpha, and -beta. The difference in the observed bi ological activity for thionins of known structure is discussed in terms of the differences in the calculated surface potential distribution, playing a n important role in their function through disruption of cell membranes. In addition, the possible role in DNA binding of the helix-turn-helix motif o f viscotoxin A3 is discussed.