Abstract
CLC-0, a prototype Cl- channel in the CLC family, employs two gating mechanisms that control its ion-permeation pore: fast gating and slow gating. The negatively-charged sidechain of a pore glutamate residue, E166, is known to be the fast gate, and the swinging of this sidechain opens or closes the pore of CLC-0 on the millisecond time scale. The other gating mechanism, slow gating, operates with much slower kinetics in the range of seconds to tens or even hundreds of seconds, and it is thought to involve still-unknown conformational rearrangements. Here, we find that low intracellular pH (pHi) facilitates the closure of the CLC-0’s slow gate, thus generating current inhibition. The rate of low pHi-induced current inhibition increases with intracellular H+ concentration ([H+]i)—the time constants of current inhibition by low pHi = 4.5, 5.5 and 6 are roughly 0.1, 1 and 10 sec, respectively, at room temperature. In comparison, the time constant of the slow gate closure at pHi = 7.4 at room temperature is hundreds of seconds. The inhibition by low pHi is significantly less prominent in mutants favoring the slow-gate open state (such as C212S and Y512A), further supporting the fact that intracellular H+ enhances the slow-gate closure in CLC-0. A fast inhibition by low pHi causes an apparent inverted voltage-dependent activation in the wild-type CLC-0, a behavior similar to those in some channel mutants such as V490W in which only membrane hyperpolarization can open the channel. Interestingly, when V490W mutation is constructed in the background of C212S or Y512A mutation, the inverted voltage-dependent activation disappears. We propose that the slow kinetics of CLC-0’s slow-gate closure may be due to low [H+]i rather than due to the proposed large conformational change of the channel protein. Our results also suggest that the inverted voltage-dependent opening observed in some mutant channels may result from fast closure of the slow gate by the mutations.
Highlights
The CLC channel/transporter family consists of transmembrane proteins of two functional categories: Cl- channels and Cl-/H+ antiporters [1, 2]
We study the relation between the inverted voltage-dependent channel opening and the intracellular H+ effect on the slow gating in CLC-0
The cDNAs of the WT CLC-0 and various mutants of CLC-0 were subcloned in the pIRES2-effect on CLC-0’s slow gating fluorescent protein (EGFP) vector containing internal ribosome entry sites (IRES) and enhanced green
Summary
The CLC channel/transporter family consists of transmembrane proteins of two functional categories: Cl- channels and Cl-/H+ antiporters [1, 2]. These CLC proteins are expressed in various tissues to carry out critical physiological functions [3]. Mammalian CLCs other than those mentioned above, such as CLC-5 [11], function as Cl-/H+ antiporters [12] They are thought to be important in controlling the pH in the intracellular organelles, and mutations of these CLC proteins have been known to be associated with human hereditary diseases such as Dent’s disease [13, 14], osteopetrosis [15], and lysosomal storage diseases [16]
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