GATING CURRENT NOISE PRODUCED BY ELEMENTARY TRANSITIONS IN SHAKER POTASSIUM CHANNELS

Gating currents provide a direct record of the spatial rearrangement of charges occurring within the protein of voltage-sensitive ion channels. If the elementary charges move as very brief discrete pulses of current, they will produce fluctuations in the macroscopic gating current. The variance of such fluctuations in gating currents was measured in Shaker potassium channels expressed in Xenopus oocytes with a sufficiently high recording bandwidth to estimate the magnitude and time distribution of the elementary transition charge movements. Channel activation occurred in two sequential stages. The first stage consisted of numerous, fast transitions, each moving small amounts of charge that contributed little to the fluctuation in gating current, whereas the second stage, which contributed the bulk of the fluctuation, was represented by a number of discrete, correlated transitions, one or more of which carried a charge of at least 2.4 elementary charges across the membrane field.