User:Laura Fountain/Chloride Ion Channel

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{{STRUCTURE_1k0o | PDB=1k0o | SCENE=User:Laura_Fountain/Sandbox_1/1k0o/1}}
== CLIC1: A Chloride Ion Channel ==
== CLIC1: A Chloride Ion Channel ==
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CLIC1 (NCC27) is a member of the highly conserved class of chloride ion channels that exist in both soluble and integral membrane forms. The CLIC family consists of seven distinct members: CLIC1, CLIC2, CLIC3, CLIC4, CLIC5, p64, and parchorin. The family is defined by a COOH-terminal core segment of ~230 amino acids that is highly conserved among all family members. CLIC1 has only a few amino acids upstream of this conserved core. CLIC1 is the most commonly studied member of the CLIC family because it is expressed to some extent in most tissues and cell types that have been studied and is particularly highly expressed in muscle.<ref name="Tulk">PMID:11940526</ref>CLIC1 has also been found in various intracellular membranes such as the mitochondrial, nuclear, and endoplasmic reticular membranes.<ref name="Transition">PMID:12202911</ref>
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The CLIC family consists of seven members: CLIC1-5, p64, and parchorin. CLIC1 is the most commonly studied member of the CLIC family because it is expressed to some extent in most tissues and cell types that have been studied and is particularly highly expressed in muscle.<ref name="Tulk">PMID:11940526</ref> Along with being present in the plasma membrane, CLIC1 has been found in various intracellular membranes, such as those of the mitochondria, nucleus (where it is designated NCC27), vesicles, and the endoplasmic reticulum.<ref name="Cromer">PMID:12202911</ref><ref name="Harrop">PMID:11551966</ref>
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Because of their wide array of locations within the cell there is still a lot of research being done to discover their various functions within the cell. Some of the possibilities currently listed are: cell signaling, cell division, apoptosis, and, of course, ion flow regulation.
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This wide range of locations in the cell causes a plausible reason to assume that the CLIC chloride channel family participate in an equally wide variety of physiological processes. Some of these include cell division, kidney function, bone resorption, transepithelial transport, and signal transduction. <ref name="Cromer">PMID:12202911</ref>
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CLIC1 is a member of the highly conserved class of chloride ion channels that exist in both soluble and integral membrane forms. When disrupted cells are washed approximately half of the CLIC1 proteins will remain within the fractioned membrane as would be expected from an integral membrane protein. Atypically, the other half will behave as a soluble cytoplasmic protein and exist within the aqueous extract.<ref name="Tulk">PMID:11940526</ref> Tulk et. al. showed that functionality of the protein isn't greatly effected by the method with which the protein inserts itself into the membrane.
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== About this Structure ==
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This is part of the evidence which leads Tulk et. al. to postulate that CLIC1 is among the small group of proteins which are assembled as soluble cytoplasmic proteins, which will then insert themselves into the appropriate membrane via their own mechanism.
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<applet load='1k0o' size='300' frame='true' align='right' caption='Soluble form of CLIC1' />
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== Structure ==
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Purified CLIC1 can integrate into synthetic lipid bilayers forming a chloride channel with similar properties to those observed in vivo. The structure of the soluble form of CLIC1 has been determined at 1.4-A resolution, and is shown to the right. It's a homodimeric structure with one pore per subunit, creating a "double barreled" channel. At its binding site in the pore, chloride interacts with the ends of four helices that come from both sides of the membrane. A <scene name='User:Laura_Fountain/Sandbox_1/Glutamate_residue/1'>glutamate residue</scene> that protrudes into the pore is proposed to participate in gating.<ref name="CLC">PMID:12163078</ref> Integration of CLIC1 into the membrane is likely to require a major structural rearrangement, probably of the N-domain (<scene name='User:Laura_Fountain/Sandbox_1/N-domain/3'>residues 1-90</scene>), with the putative transmembrane helix arising from residues in the vicinity of the redox-active site.<ref name="Crystal">PMID:11551966</ref>
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<applet load='1k0o' size='300' frame='true' align='right' caption='Dimer view of CLIC1' />
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While this exact mechanism isn't known, it has been shown that functionality of the channel doesn't change whether it goes through 'normal' membrane integration via vesicles, or whether it's inserted into the intracellular space and allowed to integrate itself.<ref name="Tulk">PMID:11940526</ref> Littler et. al. propose that upon oxidation CLIC1 undergoes a reversible transition from a monomeric to a non-covalent dimeric state due to the formation of an intramolecular disulfide bond (<scene name='User:Laura_Fountain/Sandbox_1/Cys_visualization/1'>Cys-24-Cys-59</scene>). They have determined the crystal structure of this oxidized state and show that a major structural transition has occurred, exposing a large hydrophobic surface, which forms the dimer interface. The oxidized CLIC1 dimer maintains its ability to form chloride ion channels in artificial bilayers and vesicles, whereas a reducing environment prevents the formation of ion channels by CLIC1. Their mutational studies show that both Cys-24 and Cys-59 are required for channel activity.<ref name="Intracellular">PMID:14613939</ref>
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The CLIC family is defined by a COOH-terminal core segment of ~230 amino acids that are highly conserved among the family members. CLIC1 only contains a few amino acids upstream of the <scene name='User:Laura_Fountain/Sandbox_1/1k0o/3'>conserved core</scene>.<ref name="Tulk">PMID:11940526</ref>
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1K0O is a 2 chains structure of sequences from Homo sapiens. The protein is monomeric and structurally homologous to the glutathione S-transferase superfamily, and it has a redox-active site resembling glutaredoxin. The structure of the complex of CLIC1 with glutathione shows that glutathione occupies the redox-active site, which is adjacent to an open, elongated slot lined by basic residues. This structure indicates that CLIC1 is likely to be controlled by redox-dependent processes.<ref name="Crystal">PMID:11551966</ref>
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It has a homodimeric structure with one pore per subunit, which creates an incredibly unique "double barreled" channel. The <scene name='User:Laura_Fountain/Sandbox_1/1k0o/4'>N-domain</scene> of CLIC1 (~ amino acids 1-90) consists of <scene name='User:Laura_Fountain/Sandbox_1/1k0o/5'>4 beta-sheets</scene> and <scene name='User:Laura_Fountain/Sandbox_1/1k0o/6'>3 alpha-helices</scene>, and the <scene name='User:Laura_Fountain/Sandbox_1/1k0o/7'>C-domain</scene> consists entirely of alpha-helices. The <scene name='User:Laura_Fountain/Sandbox_1/1k0o/9'>long loop between helices</scene> at the foot of CLIC1 (Pro147–Gln164) is a distinctive feature of the CLICs. It is highly negatively charged with seven acidic residues.<ref name="Harrop">PMID:11551966</ref>
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Integration of CLIC1 into the membrane is a highly prospect mechanism, but it is likely to require a major structural rearrangement, probably of the <scene name='User:Laura_Fountain/Sandbox_1/1k0o/4'>N-domain</scene><ref name="Harrop">PMID:11551966</ref>, which would insert itself and then allow the <scene name='User:Laura_Fountain/Sandbox_1/1k0o/7'>C-domain</scene> helices to insert and form the pore.
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== Selectivity for Chloride ==
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Based on the similarity of CLIC1's <scene name='User:Laura_Fountain/Sandbox_1/Channel/1'>antiparallel alpha helical loop</scene> (residues 101-145) to those of other better understood proteins which are able to insert themselves into the membrane, Tulk et. al. propose that this is the single area of the protein which is able to transverse the membrane and also serve as a pore. The <scene name='User:Laura_Fountain/Sandbox_1/Channel/3'>5 positively</scene> and <scene name='User:Laura_Fountain/Sandbox_1/Channel/4'>5 negatively</scene> charged amino acids on each end of the alpha helices could be part of the ion selectivity of this channel.<ref name="Tulk">PMID:11940526</ref>
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== Potential Ion Gating ==
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At its binding site in the pore, chloride could interact with the ends of four helices that come from both sides of the membrane. A <scene name='User:Laura_Fountain/Sandbox_1/Channel/5'>glutamate residue</scene> that protrudes into the pore is proposed to participate in gating due to its negative charge.<ref name="CLC">PMID:12163078</ref>
== References ==
== References ==
<references/>
<references/>

Current revision

PDB ID 1k0o

Drag the structure with the mouse to rotate
1k0o, resolution 1.75Å ()
Resources: FirstGlance, OCA, PDBsum, RCSB
Coordinates: save as pdb, mmCIF, xml


Contents

CLIC1: A Chloride Ion Channel

The CLIC family consists of seven members: CLIC1-5, p64, and parchorin. CLIC1 is the most commonly studied member of the CLIC family because it is expressed to some extent in most tissues and cell types that have been studied and is particularly highly expressed in muscle.[1] Along with being present in the plasma membrane, CLIC1 has been found in various intracellular membranes, such as those of the mitochondria, nucleus (where it is designated NCC27), vesicles, and the endoplasmic reticulum.[2][3]

This wide range of locations in the cell causes a plausible reason to assume that the CLIC chloride channel family participate in an equally wide variety of physiological processes. Some of these include cell division, kidney function, bone resorption, transepithelial transport, and signal transduction. [2]

CLIC1 is a member of the highly conserved class of chloride ion channels that exist in both soluble and integral membrane forms. When disrupted cells are washed approximately half of the CLIC1 proteins will remain within the fractioned membrane as would be expected from an integral membrane protein. Atypically, the other half will behave as a soluble cytoplasmic protein and exist within the aqueous extract.[1] Tulk et. al. showed that functionality of the protein isn't greatly effected by the method with which the protein inserts itself into the membrane.

This is part of the evidence which leads Tulk et. al. to postulate that CLIC1 is among the small group of proteins which are assembled as soluble cytoplasmic proteins, which will then insert themselves into the appropriate membrane via their own mechanism.

Structure

Dimer view of CLIC1

Drag the structure with the mouse to rotate

The CLIC family is defined by a COOH-terminal core segment of ~230 amino acids that are highly conserved among the family members. CLIC1 only contains a few amino acids upstream of the .[1]

It has a homodimeric structure with one pore per subunit, which creates an incredibly unique "double barreled" channel. The of CLIC1 (~ amino acids 1-90) consists of and , and the consists entirely of alpha-helices. The at the foot of CLIC1 (Pro147–Gln164) is a distinctive feature of the CLICs. It is highly negatively charged with seven acidic residues.[3]

Integration of CLIC1 into the membrane is a highly prospect mechanism, but it is likely to require a major structural rearrangement, probably of the [3], which would insert itself and then allow the helices to insert and form the pore.

Selectivity for Chloride

Based on the similarity of CLIC1's (residues 101-145) to those of other better understood proteins which are able to insert themselves into the membrane, Tulk et. al. propose that this is the single area of the protein which is able to transverse the membrane and also serve as a pore. The and charged amino acids on each end of the alpha helices could be part of the ion selectivity of this channel.[1]

Potential Ion Gating

At its binding site in the pore, chloride could interact with the ends of four helices that come from both sides of the membrane. A that protrudes into the pore is proposed to participate in gating due to its negative charge.[4]

References

  1. ↑ 1.0 1.1 1.2 1.3 Tulk BM, Kapadia S, Edwards JC. CLIC1 inserts from the aqueous phase into phospholipid membranes, where it functions as an anion channel. Am J Physiol Cell Physiol. 2002 May;282(5):C1103-12. PMID:11940526 doi:10.1152/ajpcell.00402.2001
  2. ↑ 2.0 2.1 Cromer BA, Morton CJ, Board PG, Parker MW. From glutathione transferase to pore in a CLIC. Eur Biophys J. 2002 Sep;31(5):356-64. Epub 2002 May 23. PMID:12202911 doi:10.1007/s00249-002-0219-1
  3. ↑ 3.0 3.1 3.2 Harrop SJ, DeMaere MZ, Fairlie WD, Reztsova T, Valenzuela SM, Mazzanti M, Tonini R, Qiu MR, Jankova L, Warton K, Bauskin AR, Wu WM, Pankhurst S, Campbell TJ, Breit SN, Curmi PM. Crystal structure of a soluble form of the intracellular chloride ion channel CLIC1 (NCC27) at 1.4-A resolution. J Biol Chem. 2001 Nov 30;276(48):44993-5000. Epub 2001 Sep 10. PMID:11551966 doi:10.1074/jbc.M107804200
  4. ↑ Estevez R, Jentsch TJ. CLC chloride channels: correlating structure with function. Curr Opin Struct Biol. 2002 Aug;12(4):531-9. PMID:12163078

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