Ligand-gated and voltage-gated ion channels are separate types of structure that were described in Chapter 6. Both are formed by complexes of proteins that make controlled passages into the cell. The ion channel may be composed of a single protein with multiple domains that pass through the cell membrane or of homo- or heteromultimers of various channel proteins. Voltage-gated channels typically involve four subunits, with the main subunit having multiple (e.g., five or six), similar transmembrane complexes, arranged in a circular fashion around the pore. Changes in the molecular configuration of the channel complex open or close the channel to ion flow. Whole families of genes code for the ion channels that preferentially transmit specific ions. That is, there are related genes that specify channels for Na+ or K+ or Ca2+.
Ligand-gated channels also have multiple copies of components (often 5), including multiple membrane-spanning elements around the pore, along with receptor and response domains. There are three known superfamilies of ligand-gated channel genes (cys-loop receptors responsive to GABA and other signaling molecules, ATP responsive, and glutamate activated). Each class is coded by its own gene family, whose members serve different physiological functions but via shared mechanisms and channel-structure characteristics.
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