Cluster organization and pore structure of ion channels formed by beticolin 3, a nonpeptidic fungal toxin

Beticolin 3 (B3) belongs to a family of nonpeptidic phytotoxins produced by the fungus Cercospora beticola, which present a broad spectrum of cytotoxi c effects. We report here that, at cytotoxic concentration (10 mu M), B3 fo rmed voltage-independent, weakly selective ion channels with multiple condu ctance levels in planar lipid bilayew. In symmetrical standard solutions, c onductance values of: the first levels were, respectively, 16 +/- 1 pS, 32 +/- 2 pS, and 57 +/- 2 pS (n = 4) and so on, any conductance level being ro ughly twice the lower one. Whether a cluster organization of elementary cha nnels or different channel structures underlies this particular property wa s addressed by investigating the ionic selectivity and the pore size corres ponding to the first three conductance levels. Both selectivity and pore si ze were found to be almost independent of the conductance level. This indic ated that multiple conductance behavior resulted from a cluster organizatio n of "B3 elementary channels." According to the estimated pore size and ana lyses of x-ray diffraction of B3 microcrystals, a structural model for "B3 elementary channels" is proposed. The ability to form channels is likely to be involved in the biological activity of beticolins.