The N-terminal half of the heavy chain of botulinum type A neurotoxin forms channels in planar phospholipid bilayers.

The heavy chain of botulinum type A neurotoxin forms channels in planar phospholipid bilayer membranes. Channel activity is confined to the N-terminal half of this chain; the C-terminal half is inactive. Channel activity is stimulated by low pH (4.5-5.5) on the cis side (the side to which protein is added), neutral pH on the opposite (trans) side, and cis positive voltages. These findings are strikingly similar to those previously reported for analogous fragments of diphtheria and tetanus toxins.

Differentiation of two distinct K conductances in the basolateral membrane of turtle colon.

The K conductance of the basolateral membrane of turtle colon was measured in amphotericin-treated cell layers under a variety of ionic conditions. Changing the composition of the bathing solutions changed not only the magnitude but also the physical properties of the basolateral K conductance. The results are consistent with the notion that altered ionic environments can lead to changes in the relative abundance of two different populations of K channels in the basolateral membrane, which can be differentiated on the basis of pharmacological specificity, ion selectivity, and tracer kinetics. In the following article (Germann, W. J., S. A. Ernst, and D. C. Dawson, 1986, Journal of General Physiology, 88:253-274), we present evidence consistent with the hypothesis that one of these conductances was due to the same channels that give rise to the normal resting basolateral K conductance of the transporting cells, while the other was associated with experimental maneuvers that led to extreme swelling of the epithelial cells.

Resting and osmotically induced basolateral K conductances in turtle colon.

Two types of K conductance can be distinguished in the basolateral membranes of polyene-treated colonic epithelial cells (see Germann, W. J., M. E. Lowy, S. A. Ernst, and D. C. Dawson, 1986, Journal of General Physiology, 88:237-251). The significance of these two types of K conductance was investigated by measuring the properties of the basolateral membrane under conditions that we presumed would lead to marked swelling of the epithelial cells. We compared the basolateral conductance under these conditions of osmotic stress with those observed under other conditions where changes in cell volume would be expected to be less dramatic. In the presence of a permeant salt (KCl) or nonelectrolyte (urea), amphotericin-treated colonic cell layers exhibited a quinidine-sensitive conductance. Light microscopy revealed that these conditions were also associated with pronounced swelling of the epithelial cells. Incubation of tissues in solutions containing the organic anion benzene sulfonate led to the activation of the quinidine-sensitive gK and was also associated with dramatic cell swelling. In contrast, tissues incubated with an impermeant salt (K-gluconate) or nonelectrolyte (sucrose) did not exhibit a quinidine-sensitive basolateral conductance in the presence of the polyene. Although such conditions were also associated with changes in cell volume, they did not lead to the extreme cell swelling detected under conditions that activated the quinidine-sensitive gK. The quinidine-sensitive basolateral conductance that was activated under conditions of osmotic stress was also highly selective for K over Rb, in contrast to the behavior of normal Na transport by the tissue, which was supported equally well by K or Rb and was relatively insensitive to quinidine. The results are consistent with the notion that the basolateral K conductance measured in the amphotericin-treated epithelium bathed by mucosal K-gluconate solutions or in the presence of sucrose was due to the same channels that are responsible for the basolateral K conductance under conditions of normal transport. Conditions of extreme osmotic stress, however, which led to pronounced swelling of the epithelial cells, were associated with the activation of a new conductance, which was highly selective for K over Rb and was blocked by quinidine or lidocaine.