EXAMINATION OF SUBCONDUCTANCE LEVELS ARISING FROM A SINGLE ION CHANNEL

Single-channel records often show frequent currents at a main conductance level and occasional currents at subconductance levels. In some instances, the conductances occur at regular levels that are multiples of a minimum conductance. It is well-appreciated that multiple conductance levels may arise either from the cooperative gating of more than one pore or from changes that occur in a single pore. In this paper, we used theoretical models of ion permeation to examine subconductances arising in a single-pore channel. In particular, the work focuses on the following question: how can an ion channel that provides only one aqueous pore through the membrane produce regular subconductances and a main conductance that all have the same selectivity and the same ion binding affinity? The three types of ion permeation models used in this study showed that a single-pore channel can have subconductances because of long-lived conformational states, because of alterations in rapid fluctuations between conformational states, or because of slight alterations in the electrostatic properties in the channel's entrance vestibules. Regular subconductances with the same selectivity and binding affinity can arise in a single pore even if the energy profile changes do not meet the constant peak offset condition. The results show that the appearance of regular subconductance levels in a single-channel recording is not sufficient evidence to conclude that identical pores have co-operative gating, as would arise in a channel that is a multi-pore complex. © 1991 Academic Press Limited.