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  • Review Article
  • Published:

Towards a structural view of gating in potassium channels

Nature Reviews Neuroscience volume 5, pages 905–916 (2004)Cite this article

Key Points

  • The gate region in K+ channels controls whether ions can traverse the ion conduction pore. Studies using quaternary ammonium compounds and the X-ray structure of the KcsA K+ channels are consistent with the presence of a gate at the intracellular side of the pore.

  • There is substantial evidence that the structure of the gate region of Kv channels and the movements that occur during opening are different from that proposed for KcsA and MthK.

  • The S4 transmembrane segment of the channels is rich in positively charged basic residues, and S4 is agreed to be important in voltage sensing, although the conceptual and structural basis for gating charge movement is the subject of intense controversy.

  • There are two models for the movement of gating charges across the channels. The 'membrane translocation model' proposes that the gating charges completely translocate from one side of the hydrophobic phase of the membrane to the other, which corresponds to a movement of more than 20 Å. By contrast, the 'focused field model' proposes that charges move shorter distances between water-filled crevices in the protein, which serve to focus the electric field of the membrane.

  • Most studies favoured the focused field model until 2003 when the X-ray structure of the KvAP channel was revealed. The X-ray structure of the intact KvAP protein was solved to 3.2 Å resolution, using Fab fragments of monoclonal antibodies bound to the voltage-sensors. This provided important evidence, which is in line with the translocation model but contradictory to the focused field model.

  • Although MacKinnon and colleagues pointed out several reasons for suggesting that there could be distortions in the X-ray structure of KvAP, they concluded that the crystallized full-length channel is not far from a membrane-bound conformation.

  • At present, the extent to which the structure of KvAP is distorted remains a central issue in the debate. This review argues that there are many indications that the distortions are extensive.

  • The original paddle model for gating charge movement is a membrane translocation model that is supported by an intuitive interpretation of how and why the KvAP structure is distorted, and by functional experiments and electron microscopy reconstruction.

  • Unlike the X-ray structure of KvAP, the electron microscopy reconstruction of KvAP is compatible with the focused field model because the S3 and S4 segments have transmembrane orientations. The focused field model is also supported by experiments demonstrating that S4 can form a proton conducting channel, and by experiments showing that the C terminus of S3 and the N terminus of S4 are positioned near the extracellular side of the membrane in the resting conformation.

  • Although the membrane translocation and focused field models are useful for considering the mechanisms that underlie voltage-sensing, neither model has a solid structural framework. Future work should focus on the tertiary structure of the voltage sensing domain and the spatial proximity of specific residues within well-defined secondary structural elements of the voltage-sensors.

Abstract

Voltage-activated cation channels have pores that are selective for K+, Na+ or Ca2+. Neurons use these channels to generate and propagate action potentials, release neurotransmitters at synaptic terminals and integrate incoming signals in dendrites. Recent X-ray and electron microscopy studies of an archaebacterial voltage-activated K+ (Kv) channel have provided the first atomic resolution images of the voltage-sensing domains in Kv channels. Although these structures are consistent with previous biophysical analyses of eukaryotic channels, they also contain surprises, which have provoked new ideas about the structure and movements of these proteins during gating. This review summarizes our current understanding of these intriguing membrane proteins and highlights the open questions.

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Figure 1: Membrane folding model for Kv and related cation channels.
Figure 2: X-ray structures of KcsA and MthK potassium channels.
Figure 3: Illustration of the bent S6 model in an open Kv channel and two types of Cd2+ bridges.
Figure 4: Charge movement in Kv channels.
Figure 5: X-ray structures of the KvAP channel and the isolated S1–S4 domain.
Figure 6: Sequence alignment for Kv channels and comparison of helical assignments.
Figure 7: X-ray structure of KvAP indicating the positions of important residues and a Fab fragment, and comparison of the intracellular pore aperture.
Figure 8: Electron microscopy reconstruction of the KvAP channel with two possible orientations of the paddle relative to the pore.
Figure 9: A model showing the positions, in the membrane, of the voltage-sensor paddles during closed (resting) and opened (activated) conformations, and a hypothesis for coupling to pore opening.

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Acknowledgements

I thank M. Holmgren, Z. Lu, J. Mindell, E. Perozo, S. Silberberg and the members of the Swartz laboratory for helpful discussions.

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  1. Molecular Physiology and Biophysics Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 35 Convent Drive, MSC 3701, Bethesda, 20892-3701, Maryland

    Kenton J. Swartz

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FURTHER INFORMATION

Encyclopedia of Life Sciences

Calcium channels

Ion channels

Membrane potential

Sodium, calcium and potassium channels

Voltage-gated potassium channels

Swartz's laboratory

Glossary

MEMBRANE CONDUCTANCE

The movement of charged ions across biological membranes gives rise to an electrical current. Conductance is a measure of how readily these currents flow across the membrane.

HYPERPOLARIZATION/DEPOLARIZATION

In most electrically polarized cell membranes the intracellular side is more negatively charged relative to the extracellular side, and the voltage across the membrane is said to be negative. Depolarization signifies a change in membrane voltage whereby the inside becomes more positive; hyperpolarization a change whereby the inside becomes more negative.

X-RAY STRUCTURE

In X-ray crystallography of proteins, a crystallized protein is bombarded with X-rays and the diffraction pattern is used to develop a three dimensional model of the protein's atomic structure. This model is often termed the X-ray structure of the protein.

ELECTRON PARAMAGNETIC RESONANCE

(EPR). A spectroscopic technique based on the magnetic moment of an unpaired free electron. Although proteins typically have little EPR signal, spin-labels with unpaired electrons can be attached to the protein and EPR used to provide information about the mobility and solvent accessibility of the spin-label. EPR can also be used to obtain distances between spin labels and another paramagnetic atom.

METAL BRIDGE

A bridge formed by the coordination of a metal ion by two or more amino acid side chains. The most common bridges use the metals Cd2+ and Zn2+, and involve coordination by cysteine and histidine residues.

FLUORESCENCE

The process by which light is emitted from a substance, typically an aromatic molecule, when an electron in an excited singlet state returns to the ground state. Fluorescence emissions tend to be sensitive to the environment surrounding the fluorophore.

FAB FRAGMENTS

Cleavage of an immunoglobulin with papain releases two Fab fragments, each capable of recognizing antigen, and a single Fc fragment. Fab fragments that recognize motifs in membrane proteins facilitate the production of well-ordered crystals because they can provide additional protein contacts within the crystal and can preclude the protein–detergent micelle from participating in crystal contacts.

SECONDARY STRUCTURE

The local conformation of the polypeptide backbone of a protein can adopt two types of secondary structure that are stabilized by regular hydrogen-bonding interactions between backbone carbonyl and amide groups. The most common secondary structure is the right-handed α-helix, which contains 3.6 residues per turn. The other secondary structure is the β-sheet, which is stabilized by hydrogen bonds between carbonyl and amide groups on two separate β-strands.

TERTIARY STRUCTURE

Tertiary structure refers to the higher order arrangement of secondary protein structures, including loops and linkers, to form the three dimensional structure of the protein.

AVIDIN

A 57 kDa tetrameric protein from egg white that binds the vitamin biotin with extremely high affinity (Kd ∼10−15 M). Biotin derivatives are available that can be tethered to proteins using various reactions, including malemide chemistry for attachment to cysteine residues.

FLUORESCENCE RESONANCE ENERGY TRANSFER

(FRET). The process by which an excited fluorophore (the donor) transfers energy to another molecule (the acceptor) when the emissions spectrum of the donor overlaps the absorption spectrum of the acceptor. The distance over which energy transfer is 50% efficient, the Förster distance, is typically in the range of 20–60 Å, making FRET useful as a spectroscopic ruler for distance measurements in proteins.

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Swartz, K. Towards a structural view of gating in potassium channels. Nat Rev Neurosci 5, 905–916 (2004). https://doi.org/10.1038/nrn1559

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