When present at nanomolar concentrations on one side of a lipid bilayer membrane, helianthus toxin (a protein of mol wt≈16,000) increases enormously membrane permeability to ions and nonelectrolytes by forming channels in the membrane. Membranes containing sphingomyelin are especially sensitive to toxin, but sphingomyelin is not required for toxin action. Conductance is proportional to about the 4th power of toxin concentration. Single channel conductances are approximately 2×10-10 mho in 0.1 m KCl. Toxin-treated membranes are more permeable to K+ and Na+ than to Cl- and SO4=, but the degree of selectivity is pH dependent. Above pH 7 membranes are almost ideally selective for K+ with respect to SO4=, whereas below pH 4 they are poorly selective. The channels show classical molecular sieving for urea, glycerol, glucose, and sucrose - implying a channel radius >5 Å. In symmetrical salt solutions above pH 7, the I-V characteristic of the channel shows significant rectification: below pH 5 there is very little rectfication. Because of the effects of pH on ion selectivity and channel conductance, and also because of the rectification in symmetrical salt solutions and the effect of pH on this, we conclude that there are titratable negative charge groups in the channel modulating ion permeability and selectivity. Since pH changes on the side containing the toxin are effective whereas pH changes on the opposite side are not, we place these negative charges near the mouth of the channel facing the solution to which toxin was added.
ASJC Scopus subject areas
- Cell Biology