Ion channels constitute a large and diverse group of membrane proteins that function as electrical signal tranducers, governing the electrical properties of all living cells. For example, the coordinated activity of several ion channels governs the mechanism underlying action potentials in excitable cells. The function of ion channels is typically regulated by electric potentials or by a number of signalling molecules. Ion channels control a large part of the rapid physiological mechanisms that occur in the body.
Ion channels are membrane spanning proteins that form passageways for ions to travel across the phospholipid bilayer. The physical pore is shaped by an assembly of several subunits, and is lined with hydrophilic amino acids residues. A narrow region of the pore is typically charged and constitutes a selectivity-filter that determines the specificity of the channel. Ion channels may open and close in response to changes in membrane potential (known as voltage-gated ion channels) or chemical stimuli (ligand-gated ion channels). Each ion channel species is characterized by its ion selectivity sequence. For example, it may be highly specific for a single ion species or it may be less specific, conducting a few or several ion species. The selectivity is reflected in the common classification of the channels:
K+ channels
Na+ channels
Ca2+ channels
Cl- channels
non-selective cation channels
Ion channels control physiological processes such as generation of electrical activity in nerves, control of contractile activity in the heart and muscle, uptake of nutrients, immune responses and hormone secretion. They work either as chemical or electrical signal transducers or as trans-membrane routes for the bulk transport of salts. A typical mammalian cell has in the order of hundreds to thousands of ion channels of different types.
A multitude of human and animal diseases are caused by the dysfunction of ion channels, including cystic fibrosis, epilepsy and cardiac arrhythmias (such as the long QT syndrome)
page top
