FIG. 3.

Ion selectivity of the beta-toxin channel. (A) Current-voltage (I/V) curves of a BLM modified with beta-toxin under bi-ionic conditions (i.e., the concentration of NaCl is held constant in the trans side and varied in the cis compartment, or vice versa). In all cases the trans side of the membrane was maintained at 100 mM NaCl whereas the cis side was consecutively washed with 100 mM NaCl (circles), 10 mM NaCl (triangles), and finally 100 mM NaCl (squares). Beta-toxin was introduced to the cis compartment and allowed to form channels prior to changing of the bathing solution for the cis compartment. The reversal potential was calculated from the intercept between the I/V plots obtained with NaCl concentrations of 100 mM (circles) and 10 mM (triangles) on the cis side, with the trans side maintained at 100 mM NaCl. At zero current the reversal potential was +57 mV. (Inset) Current-voltage relationships elicited after the addition of beta-toxin to the trans side of a membrane in a symmetric solution of 100 mM NaCl. Each panel represents a different membrane. For both experiments the final concentration of beta-toxin was 129 nM. (B) A single-channel record was obtained, and the membrane was held at a potential of −60 mV (lower trace) or +60 mV (upper trace). The bilayer was bathed in a symmetric bathing solution of 100 mM NaCl, and the beta-toxin was added to the cis side of the membrane to a final concentration of 10.8 nM.