What are ion channels?

Ion channels are targets for selective drugs

Ion channels constitute a large and diverse group of membrane proteins that function as electrical signal transducers, and they govern the electrical properties of all living cells. For example, the coordinated activity of several ion channels underlies action potentials in excitable cells, such as those in the heart and brain.

Ion channels are generally heteromultimeric membrane proteins that constitute water-filled passageways for ions across the phospholipid bilayer membrane. The physical pore is shaped by an assembly of several subunits, and the pore is lined with hydrophilic amino acid residues. A narrow region of the pore constitutes a ‘selectivity-filter’ that determines which ions can pass through the pore.

Ion channels may open and close in response to e.g. membrane potential (voltage-gated ion channels) or chemical (ligand-gated ion channels) stimuli.

Classification of Ion Channels

Each ion channel is characterized by its ion selectivity sequence. It may be highly specific for a single ion, or it may be less specific, conducting a few or several different ions. The selectivity is reflected in the common classification of the channels:

• K⁺ channels
• Na⁺ channels
• Ca²⁺ channels
• Cl^– channels
• non-selective cation channels

Functionally, ion channels are broadly divided into voltage- and ligand-gated channels, referring to the type of physiological stimulus that activates the channel.