Channels are large proteins in which multiple subunits are arranged in a cluster so as to form a pore that passes through the membrane. Each subunit consists of multiple transmembrane domains. These transmembrane channels are also found in the membranes of organelles including the nucleus, mitochondria and lysosome.
Channels perform passive transport of materials known as facilitated diffusion, differentiating solutes primarily by size and ionic charge. Most of the channels that we will consider are ion channels. The two factors that affect the flow of ions through an open ion channel are the membrane potential and the concentration gradient. Another important type of channel protein is an aquaporin. Aquaporins are channels that allow water to move rapidly across cell membranes.
There are several modes by which membrane channels operate.
- Ungated channels
A few types of channels are ungated, meaning they are open all the time. For instance, some K+ and some Cl– channels are ungated. By contrast, Ca2+ and Na+ ion channels are never ungated.
- Voltage gated channel
This require a trigger, such as a change in membrane potential to unlock or lock the pore opening. Voltage-gated ion channels are key in the generation of electrical signals in nerve, muscle, and cardiac cells.
- Ligand-gated channel
Many ion channels open or close in response to binding a small signaling molecule or “ligand”. Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens. Many neurotransmitter receptors are ligand gated channels. An example is the nicotinic acetylcholine receptor. This is the receptor that is found at the neuromuscular junction on skeletal muscle cells, and also at synapses in autonomic ganglia.
- Stress-gated channel
This require a mechanical force applied to the channel for opening. Mechanically-gated channels are found in skin and also in the specialized sensory cells of the auditory and vestibular system.
Examples of channel proteins
- Ion channels
Ion channels are activated by changes in the electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, regulating their opening and closing. Cell membranes are generally impermeable to ions, thus they must diffuse through the membrane through transmembrane protein channels. They have a crucial role in excitable cells such as neuronal and muscle tissues, allowing a rapid and co-ordinated depolarization in response to triggering voltage change. Found along the axon and at the synapse, voltage-gated ion channels directionally propagate electrical signals.
Voltage-gated ion-channels are usually ion-specific, and channels specific to sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl–) ions have been identified. The opening and closing of the channels are triggered by changing ion concentration, and hence charge gradient, between the sides of the cell membrane.
- Acetylcholine receptor channel
Acetylcholine receptor (abbreviated AChR), is a channel that mediates the transmission of nerve signals across synapses, the functional junctions between neurons. The acetylcholine receptor is a ligand-gated channel in that the channel opens in response to the binding of acetylcholine.
Acetylcholine receptors are classified according to their “pharmacology,” or according to their relative affinities and sensitivities to different molecules.
- Nicotinic acetylcholine receptors (nAChR, also known as “ionotropic” acetylcholine receptors) are particularly responsive to nicotine. The nicotine ACh receptor is also a Na+, K+ and Ca2+ ion channel.
- Muscarinic acetylcholine receptors (mAChR, also known as “metabotropic” acetylcholine receptors) are particularly responsive to muscarine.
Aquaporins also called water channels, facilitates the transport of water between cells, playing a critical roles in controlling the water contents of cells. Water moves through the cells in an organized way, most rapidly in tissues that have aquaporin water channels.
Aquaporins selectively conduct water molecules in and out of the cell, while preventing the passage of ions and other solutes. Water molecules traverse through the pore of the channel in single file. Some of them, known as aquaglyceroporins, also transport other small uncharged dissolved molecules including ammonia, CO2, glycerol, and urea.
In mammalian cells, more than 10 isoforms of aquaporins (AQP0-AQP10) have been identified so far. They are differentially expressed in many types of cells and tissues in the body. AQP0 is abundant in the lens. AQP1 is found in the blood vessels, kidney proximal tubules, eye, and ear. AQP2 is expressed in the kidney collecting ducts, where it shuttles between the intracellular storage sites and the plasma membrane under the control of antidiuretic hormone (ADH). Mutations of AQP2 result in diabetes insipidus. AQP3 is present in the kidney collecting ducts, epidermis, urinary, respiratory, and digestive tracts. AQP4 is present in the brain astrocytes, eye, ear, skeletal muscle, stomach parietal cells, and kidney collecting ducts. AQP5 is in the secretory cells such as salivary, lacrimal, and sweat glands. AQP5 is also expressed in the ear and eye. AQP6 is localized intracellular vesicles in the kidney collecting duct cells. AQP7 is expressed in the adipocytes, testis, and kidney. AQP8 is expressed in the kidney, testis, and liver. AQP9 is present in the liver and leukocytes. AQP10 is expressed in the intestine. The diverse and characteristic distribution of aquaporins in the body suggests their important and specific roles in each organ.