PROBING THE MAJOR SKELETAL-MUSCLE CHLORIDE CHANNEL WITH ZN2-REACTIVE COMPOUNDS( AND OTHER SULFHYDRYL)

The sensitivity of the human skeletal muscle Cl- channel, hClC-1, towa rds various sulfhydryl-reactive agents was tested with the channel exp ressed in Xenopus oocytes and in human embryonic kidney cells. Externa l but not internal Zn2+, at 1 mM, substantially reduced the current wi thout affecting activation parameters. External Cd2+ and Hg2+ as well as organic mercurial compounds reduced the Cl- currents to a similar d egree. With the mutant channel hClC-1 D136G, presumed to have a defect ive voltage sensor, external Zn2+ also reduced the current without eff ect on the altered gating. These findings suggest that hClC-1 contains cysteine residues near the extracellular face that may directly influ ence ion conduction. Since Zn2+ can also bind to histidine side chains , we tested the effect of compounds with either more cysteine- or more histidine-specificity. The results confirm the involvement of cystein e(s) in the observed effects but do not exclude the involvement of his tidine(s).