Ion channel receptors are multimeric proteins, which usually locate in the plasma membrane. Each of those receptor proteins arranges itself in order to form a passageway or pore, so that to extend from one side of the membrane to the other, and therefore lead to fundamental physiological processes such as muscle contraction. These passageways, or ion channel receptors, have the flexible ability to open and close in response to chemical or mechanical signals. Ion channels receptors, unlike ion transporters, are passive. It means that they do not require a source of energy in order to function. Flow of ions to occur depend on the electro-chemical gradient. Most ion channels are gated to selectively block the ions flow. Both the amino acids that line a channel and the physical width of the channel determine which ions are able to go through from the cell exterior to its interior, and vice versa. When an ion channel is open, ions move into or out of the cell in single-file fashion. Individual ion channels are specific to particular ions. It means that ion channel receptors usually allow only a single type of ion to pass through them. A special feature of voltage-activated or -gated channels is a sensor that detects the electrical potential across the membrane. Sensors are the parts of the channel that can detect certain conditions or signal. Understanding the physical structure of ion channels is undoubtedly the important thing to sorting out how they actually work in various situations. A lot of insight or explanations to ion channel receptors has come from recent X-ray crystallographic studies, or by electron microscopy.

Sodium Channel

Sodium channels have a modular architecture, with distinct regions for the pore and the gates, mediating fast depolarization and conduct electrical impulses throughout nerve, muscle and heart. The voltage-gated sodium channel is a large, multimeric complex, composed of an α subunit and one or more smaller β subunits. α subunit contains the ion-conducting aqueous pore, while β subunit is required for full reconstitution of the properties of native sodium channels. Genes encoding sodium channel receptors include SCN1A, SCN2A, SCN3A, SCN5A, SCN8A, etc.

Potassium Channel

Voltage-dependent K+ channels open and allow ion conduction in response to changes in cell membrane voltage, which contain the central ion-conduction pores surrounded by voltage sensors. Voltage sensors include hydrophobic, cationic, helix–turn–helix structures on the channel's outer perimeter, also named "voltage-sensor paddles". Genes encoding potassium channel receptors include KCNA1, KCNA2, KCNA3, KCNA4, KCNA5, KCNB1, KCND3, KCNQ1, etc.

Calcium Channel

The voltage-gated calcium channels (VGCCs/CaVs), are transmembrane ion channel proteins that selectively conduct calcium ions through the cell membrane. The Cav channels play an important role as drug targets. They are associated with diseases such as hypokalemic periodic paralysis, cardiac arrhythmia, and epileptic seizure. The β-subunit (CaVβ), as intracellular protein of Cav channels, binds the α-interaction domain (AID) between transmembrane domains I and II of the pore-forming α1 subunit. Genes encoding calcium channel receptors include CACNA1C, CACNB2, CACNA2D1, CACNA1G, CACNA1H, etc.

Chloride Channel

Chloride is the most abundant anion in organisms. The carboxy terminus of all eukaryotic CLC proteins has two CBS domains, while the membrane-associated part of the protein is composed of 17 α-helices. Inspection of the crystal reveals that most of these helices are severely tilted to the membrane, not perpendicular. Many of these helices do not span the width of the bilayer. Genes encoding chloride channel receptors include CFTR, CLCN1, CLCN2, GABRA1, GABRA2, etc.

Acetylcholine Receptor

An acetylcholine receptor forms a gated ion channel in the plasma membrane. This receptor is a membrane protein with an aqueous pore, meaning it allows soluble materials to travel across the plasma membrane when open. But when no external signal is present, the pore is closed. When acetylcholine molecules bind to the receptor, this triggers a conformational change that opens the aqueous pore and allows ions to flow into the cell. Genes encoding acetylcholine receptors include CHRNA4, CHRNA7, CHRNB2, etc.

ATP-gated P2X receptor

The vertebrate P2X receptors include seven subtypes (P2X1–P2X7) that form homo- or hetero-trimers. Each subunit of the P2X receptor is composed of the large extracellular domain that contains the ATP and other ligand binding sites, the two transmembrane helices that form a non-selective cation pore, and the intracellular N- and C-termini that modulate channel gating. Genes encoding ATP-gated P2X receptors include P2X1, P2X2, P2X3, P2X4, P2X5, P2X6, P2X7.

References

1. Purves D, Augustine GJ, Fitzpatrick D, et al. The Molecular Structure of Ion Channels. Neuroscience. 2001.
2. Jiang, Y., Lee, A., Chen, J. et al. X-ray structure of a voltage-dependent K+ channel. Nature. 423, 33–41 (2003).
3. Van Petegem, F., Clark, K., Chatelain, F. et al. Structure of a complex between a voltage-gated calcium channel β-subunit and an α-subunit domain. Nature. 429, 671–675 (2004).
4. Jianping Wu, et al. Structure of the voltage-gated calcium channel Cav1.1 complex. Science. 2015.
5. Kasuya, G., Yamaura, T., Ma, X. et al. Structural insights into the competitive inhibition of the ATP-gated P2X receptor channel. Nat Commun. 8, 876 (2017).
6. THOMAS J. JENTSCH, et al. Molecular Structure and Physiological Function of Chloride Channels. Physiol Rev. 82: 503–568, 2002.

Related Section

• Technical Methods
• Neuroscience
• Channelopathies