LOW-CONDUCTANCE STATES OF A SINGLE-ION CHANNEL ARE NOT CLOSED

We have used a polymer-exclusion method to estimate the sizes of the h igh- and low-conductance states of Staphylococcus aureus alpha-toxin c hannels across planar lipid bilayers. Despite a > 10-fold difference i n conductance between high- and low-conductance states, the size diffe rs by < 2-fold. We conclude that factors other than the dimensions hav e a strong influence on the conductance of alpha-toxin channels. We al so show that the high conductance state is destabilized by the presenc e of high molecular weight polymers outside the channel, compatible wi th the removal of channel water as the high conductance state ''shrink s'' to the low conductance state.