Abstract
Several cloned ClC-type Cl- channels open and close in a voltage-dependent manner. The Torpedo electric organ Cl- channel, ClC-0, is the best studied member of this gene family. ClC-0 is gated by a fast and a slow gating mechanism of opposite voltage direction. Fast gating is dependent on voltage and on the external and internal Cl- concentration, and it has been proposed that the permeant anion serves as the gating charge in ClC-0 (Pusch, M., U. Ludewig, A. Rehfeldt, and T.J. Jentsch. 1995. Nature (Lond.). 373:527-531). The deactivation at negative voltages of the muscular ClC-1 channel is similar but not identical to ClC-0. Different from the extrinsic voltage dependence suggested for ClC-0, an intrinsic voltage sensor had been proposed to underlie the voltage dependence in ClC-1 (Fahlke, C., R. Rüdel, N. Mitrovic, M. Zhou, and A.L. George. 1995. Neuron. 15:463-472; Fahlke, C., A. Rosenbohm, N. Mitrovic, A.L. George, and R. Rüdel. 1996. Biophys. J. 71:695-706). The gating model for ClC-1 was partially based on the properties of a point-mutation found in recessice myotonia (D136G). Here we investigate the functional effects of mutating the corresponding residue in ClC-0 (D70). Both the corresponding charge neutralization (D70G) and a charge conserving mutation (D70E) led to an inwardly rectifying phenotype resembling that of ClC-1 (D136G). Several other mutations at very different positions in ClC-0 (K165R, H472K, S475T, E482D, T484S, T484Q), however, also led to a similar phenotype. In one of these mutants (T484S) the typical wild-type gating, characterized by a deactivation at negative voltages, can be partially restored by using external perchlorate (ClO4-) solutions. We conclude that gating in ClC-0 and ClC-1 is due to similar mechanisms. The negative charge at position 70 in ClC-0 does not specifically confer the voltage sensitivity in ClC-channels, and there is no need to postulate an intrinsic voltage sensor in ClC-channels.
Highlights
From the Center for Molecular Neurobiology (ZMNH), Hamburg University, D-20246 Hamburg, Germany abstract Several cloned ClC-type ClϪ channels open and close in a voltage-dependent manner
While the gating charge for a single potassium channel was estimated to involve about 12 elementary charges (Schoppa et al, 1992), the voltage dependence in ClϪ channels is rather weak, involving only 1–2.2 elementary charges for the different gating processes
The fast gating of single pores is strongly dependent on the extracellular ClϪ concentration, and external permeant anions can effectively open the channel
Summary
From the Center for Molecular Neurobiology (ZMNH), Hamburg University, D-20246 Hamburg, Germany abstract Several cloned ClC-type ClϪ channels open and close in a voltage-dependent manner. The fast gating of single pores is strongly dependent on the extracellular ClϪ concentration, and external permeant anions can effectively open the channel. Despite these similarities, Fahlke et al (1995, 1996) concluded that fast gating of ClC-1 differs from ClC-0 in the sensitivity to external ClϪ: when ClϪ was reduced and replaced by MeSO3Ϫ, the open probability was increased compared with the pure ClϪ solution.
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