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Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
The CLC Family of Proteins: Chloride Transporters and Channels, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555816452/9781555813284_Chap11-1.gif /docserver/preview/fulltext/10.1128/9781555816452/9781555813284_Chap11-2.gifAbstract:
Several members of the CLC family of proteins are voltage-gated, and this entire family is sometimes termed the voltage-gated family of Cl- channels (chloride channels). Bioinformatics screening of CLC channels suggests the existence of regions in the cytoplasmic carboxyl tail of these proteins that have the propensity to bind actin and possibly other cytoskeletal proteins. While the ClC-0, ClC-1, ClC-2, and ClC-Ka and -Kb branch of the CLC family are generally believed to be plasma membrane channels, the location of the others is controversial, being totally or partially confined to intracellular membranes under normal circumstances. The authors' recent analysis of the human CLCN7 promoter has identified several interesting consensus transcription-factor-binding sites. By mutation, their importance in the transcriptional regulation of this gene has been demonstrated. Transcription factor binding has been demonstrated, and the identification of these factors is under way. Mutations in CLC channels have now been associated with a number of diseases in both humans and other species. Myotonia, the best understood of the CLC diseases, is characterized by a peculiar muscle stiffness that is normally painless, an inability of the muscle to relax after a voluntary contraction. This is purely a muscle phenomenon and does not involve nerve dysfunction. It is sometimes accompanied by weakness, and the stiffness may improve after exercise. Genetic or pharmacological manipulation of the relevant Cl1 channels could treat CLC diseases or, conversely, mimicking some aspect of these diseases could point the way to therapies for other diseases.
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Dual topological diagram comparing the original 12 hydrophobic domains with the 18 helices (A to R) based on the X-ray structure. The lengthy eukaryotic C tail of largely indeterminate structure with its pair of CBS domains is also compared with the short prokaryotic C tail. Original hydrophobic domain D13 coincides roughly with CBS2.
Dual topological diagram comparing the original 12 hydrophobic domains with the 18 helices (A to R) based on the X-ray structure. The lengthy eukaryotic C tail of largely indeterminate structure with its pair of CBS domains is also compared with the short prokaryotic C tail. Original hydrophobic domain D13 coincides roughly with CBS2.
Illustration of CLC protein types in prokaryotes and eukaryotes. Prokaryotic ClC-Ec1 is shown as a Cl-/H+ exchanger. Eukaryotes have evolved a variety of CLC proteins. Some are confined to internal membranes, where they are channels associated with proton pumps and endosome acidification. Others are plasma membrane channels capable of accommodating the current flow necessary to stabilize muscle membrane potentials or to facilitate the stunning voltage discharges of electric rays.
Illustration of CLC protein types in prokaryotes and eukaryotes. Prokaryotic ClC-Ec1 is shown as a Cl-/H+ exchanger. Eukaryotes have evolved a variety of CLC proteins. Some are confined to internal membranes, where they are channels associated with proton pumps and endosome acidification. Others are plasma membrane channels capable of accommodating the current flow necessary to stabilize muscle membrane potentials or to facilitate the stunning voltage discharges of electric rays.
Diagram showing the relatedness of selected ClC sequences. Branch lengths are not proportional to evolutionary differences. Three main branches of the eukaryotic CLC family can be seen: that including ClC-0, ClC-1, ClC-2, ClC-Ka, and ClC-Kb; that including ClC-3, ClC-4, and ClC-5; and that including ClC-6 and ClC-7. Of the others, only those of the plant A. thaliana, the yeast S. cerevisiae, and the two prokaryotic CLCs, for which the crystal structures are known, have been included.
Diagram showing the relatedness of selected ClC sequences. Branch lengths are not proportional to evolutionary differences. Three main branches of the eukaryotic CLC family can be seen: that including ClC-0, ClC-1, ClC-2, ClC-Ka, and ClC-Kb; that including ClC-3, ClC-4, and ClC-5; and that including ClC-6 and ClC-7. Of the others, only those of the plant A. thaliana, the yeast S. cerevisiae, and the two prokaryotic CLCs, for which the crystal structures are known, have been included.
Whole-cell patch-clamp currents recorded from cultured HEK cells expressing the wild-type (WT) or mutant (C278G) human skeletal muscle chloride channel, ClC-Hs1. (A) A sequence of currents is shown (overlying each other) in response to conditioning voltage steps from −140 mV (inside negative compared to outside zero) to +100 mV. “Tail” currents are also shown in response to a constant test pulse of −100 mV. From the sizes of the tail currents, an apparent overall (for both gates) open probability, P o, can be calculated for these channels at each conditioning voltage. (B) P o for the WT channels is shown. Separated apparent open probabilities for the fast and common gates are shown in panel D for the C278G mutant whose currents are recorded in panel C.
Whole-cell patch-clamp currents recorded from cultured HEK cells expressing the wild-type (WT) or mutant (C278G) human skeletal muscle chloride channel, ClC-Hs1. (A) A sequence of currents is shown (overlying each other) in response to conditioning voltage steps from −140 mV (inside negative compared to outside zero) to +100 mV. “Tail” currents are also shown in response to a constant test pulse of −100 mV. From the sizes of the tail currents, an apparent overall (for both gates) open probability, P o, can be calculated for these channels at each conditioning voltage. (B) P o for the WT channels is shown. Separated apparent open probabilities for the fast and common gates are shown in panel D for the C278G mutant whose currents are recorded in panel C.
Whole-cell patch-clamp currents as in Fig 4 . It is apparent (A) that currents from the G284S mutant display almost purely fast gating, and (B) this is reinforced by the separated P o curves, where it can be seen that the P o for the common gates is close to 1 even at very negative potentials. (C) In complete contrast, mutant S289G seems to show only common (slow) gating. This is because, as the overall P o curve in panel D shows, the common gates do not begin to open in this mutant until around 0 mV, by which voltage the fast gates are fully open (cf. P o fast for G284S in panel B and P o for WT in Fig. 4B ).
Whole-cell patch-clamp currents as in Fig 4 . It is apparent (A) that currents from the G284S mutant display almost purely fast gating, and (B) this is reinforced by the separated P o curves, where it can be seen that the P o for the common gates is close to 1 even at very negative potentials. (C) In complete contrast, mutant S289G seems to show only common (slow) gating. This is because, as the overall P o curve in panel D shows, the common gates do not begin to open in this mutant until around 0 mV, by which voltage the fast gates are fully open (cf. P o fast for G284S in panel B and P o for WT in Fig. 4B ).
CLC nomenclature
CLC nomenclature