MECHANISM OF VOLTAGE-DEPENDENT GATING IN SKELETAL-MUSCLE CHLORIDE CHANNELS

Voltage-dependent gating was investigated in a recombinant human skele tal muscle Cl- channel, hCIC-1, heterologously expressed in human embr yonic kidney (HEK-293) cells. Gating was found to be mediated by two q ualitatively distinct processes, One gating step operates on a microse cond time scale and involves the rapid rearrangement of two identical intramembranous voltage sensors, each consisting of a single titratabl e residue. The second process occurs on a millisecond time scale and i s due to a blocking-unblocking reaction mediated by a cytoplasmic gate that interacts with the ion pore of the channel. These results illust rate a rather simple structural basis for voltage sensing that has evo lved in skeletal muscle Cl- channels and provides evidence for the exi stence of a cytoplasmic gating mechanism in an anion channel analogous to the ''ball and chain'' mechanism of voltage-gated cation channels.