Conformation of the four homologous domains showing the formation of a central pore<unk> s are generally composed of several subunits arranged in such a way that there is a central pore through which ions can travel down their electrochemical gradients. The channels tend to be ion-specific, although similarly sized and charged ions may sometimes travel through them. The functionality of<unk> s is attributed to its three main discrete units: the<unk>  sensor, the pore or conducting pathway, and the<unk>  Na+, K+, and Ca2+ channels are composed of four transmembrane domains arranged around a central pore; these four domains are part of a single α-subunit in the case of most Na+ and Ca2+ channels, whereas there are four α-subunits, each contributing one transmembrane domain, in most K+ channels. The membrane-spanning segments, designated S1-S6, all take the form of alpha helices with specialized functions. The fifth and sixth transmembrane segments (S5 and S6) and pore loop serve the principal role of ion conduction, comprising the<unk> and pore of the channel, while S1-S4 serve as the<unk> -sensing region. The four subunits may be identical, or different from one another. In addition to the four central α-subunits, there are also regulatory β-subunits, with oxidoreductase activity, which are located on the inner surface of the cell membrane and do not cross the membrane, and which are coassembled with the α-subunits in the endoplasmic reticulum.
Voltage-gated ion channel
Cyclic nucleotide–gated ion channel
G protein-gated ion channel
Mechanosensitive channels
Neurotransmitter transporter
Glutamate transporter
Carboxylate transporter
Sodium-chloride symporter