Voltage-gated potassium channel
Voltage-gated potassium channels are transmembrane channels specific for potassium and sensitive to voltage changes in the cell's membrane potential. They play a crucial role during action potentials in returning the depolarized cell to a resting state.
Alpha subunits form the actual conductance pore. Based on sequence homology of the hydrophobic transmembrane cores, the alpha subunits of voltage-gated potassium channels have been grouped into 12 classes labeled Kv1-12. The following is a list of the 40 known human voltage-gated potassium channel alpha subunits grouped first according to function and then subgrouped according to the Kv sequence homology classification scheme:
- Kvα1.x - Shaker-related: Kv1.1 (KCNA1), Kv1.2 (KCNA2), Kv1.3 (KCNA3), Kv1.4 (KCNA4), Kv1.5 (KCNA5), Kv1.6 (KCNA6), Kv1.7 (KCNA7), Kv1.8 (KCNA10)
- Kvα2.x - Shab-related: Kv2.1 (KCNB1), Kv2.2 (KCNB2)
- Kvα3.x - Shaw-related: Kv3.1 (KCNC1), Kv3.2 (KCNC2)
- Kvα7.x: Kv7.1 (KCNQ1) - KvLQT1, Kv7.2 (KCNQ2), Kv7.3 (KCNQ3), Kv7.4 (KCNQ4), Kv7.5 (KCNQ5)
- Kvα10.x: Kv10.1 (KCNH1)
A-type potassium channel
- Kvα3.x - Shaw-related: Kv3.3 (KCNC3), Kv3.4 (KCNC4)
- Kvα4.x - Shal-related: Kv4.1 (KCND1), Kv4.2 (KCND2), Kv4.3 (KCND3)
- Kvα10.x: Kv10.2 (KCNH2)
Unable to form functional channels as homotetramers but instead heterotetramerize with Kvα2 family members to form conductive channels.
- Kvα5.x: Kv5.1 (KCNF1)
- Kvα6.x: Kv6.1 (KCNG1), Kv6.2 (KCNG2), Kv6.3 (KCNG3), Kv6.4 (KCNG4)
- Kvα8.x: Kv8.1 (KCNV1), Kv8.2 (KCNV2)
- Kvα9.x: Kv9.1 (KCNS1), Kv9.2 (KCNS2), Kv9.3 (KCNS3)
Beta subunits are auxiliary proteins which associate with alpha subunits in a α4β4 stoichiometry. These subunits do not conduct current on their own but rather modulate the activity of Kv channels.
- Kvβ1 (KCNAB1)
- Kvβ2 (KCNAB2)
- Kvβ3 (KCNAB3)
- minK (KCNE1)
- MiRP1 (KCNE2)
- MiRP2 (KCNE3)
- MiRP3 (KCNE4)
- KCNE1-like (KCNE1L)
Proteins minK and MiRP1 are putative hERG beta subunits.
The voltage-gated K+ channels that provide the outward currents of action potentials have similarities to bacterial K+ channels.
These channels have been studied by X-ray diffraction, allowing determination of structural features at atomic resolution.
The function of these channels is explored by electrophysiological studies.
Genetic approaches include screening for behavioral changes in animals with mutations in K+ channel genes. Such genetic methods allowed the genetic identification of the "Shaker" K+ channel gene in Drosophila before ion channel gene sequences were well known.
Study of the altered properties of voltage-gated K+ channel proteins produced by mutated genes has helped reveal the functional roles of K+ channel protein domains and even individual amino acids within their structures.
Voltage-gated K+ channels of vertebrates typically are tetramers of four identical subunits arranged as a ring, each contributing to the wall of the trans-membrane K+ pore. Each subunit is comprised of six membrane spanning hydrophobic α-helical sequences. A high resolution crystallographic structure of the rat Kvα1.2/β2 channel has recently been solved (Protein Databank Accession Number ).
Voltage-gated K+ channels are selective for K+ over other cations such as Na+. There is a selectivity filter at the narrowest part of the transmembrane pore.
Channel mutation studies revealed the parts of the subunits that are essential for ion selectivity. They include the amino acid sequence (Thr-Val-Gly-Tyr-Gly) or (Thr-Val-Gly-Phe-Gly) typical to the selectivity filter of voltage-gated K+ channels. As K+ passes through the pore, interactions between potassium ions and water molecules are prevented and the K+ interacts with specific atomic components of the Thr-Val-Gly-X-Gly sequences from the four channel subunits.
Open and closed conformations
Attempts continue to relate the structure of the mammalian voltage-gated K+ channel to its ability to respond to the voltage that exists across the membrane. Specific domains of the channel subunits have been identified that are important for voltage-sensing and converting between the open conformation of the channel and closed conformations. There are at least two closed conformations; in one, the channel can open if the membrane potential becomes positive inside. Voltage-gated K+ channels inactivate after opening, entering a distinctive, second closed conformation. In the inactivated conformation, the channel cannot open, even if the transmembrane voltage is favorable. A domain at one end of the K+ channel protein mediates inactivation. This end of the protein can transiently plug the inner opening of the pore, preventing ion movement through the channel.
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