[Review of molecular structure and function of ion channels]

Recent research on various ion channel proteins have revealed that they can be classified into several groups of super-families from their structural similarity. Among them, the voltage-gated ion channel super-family, including K(+)-channel, Na(+)-channel and Ca(2+)-channel, consist of four homologous functional units of six transmembrane segments. Site directed mutagenesis applied on those proteins revealed that the 4th segment works as the voltage sensor and the linker (SS1-SS2) between 5th and 6th segments forms the pore structure passing ions. On the other hand, the ligand operated receptor channel super-family, including nicotinic acetylcholine receptor, the GABAA receptor and glutamate receptor, is composed of five subunits which are also homologous. Each subunit contains a large hydrophilic amino-terminal domain which binds ligands and/or competitive blockers, followed by four transmembrane segments. The 2nd segment (MII) of each subunit lines the internal surface of the channel pore as suggested from labelling studies with non-competitive blockers. The ryanodine receptor and the inositol-1,4,5-trisphosphate receptor are also homologous in their structure and are thus classified as a family catalyzing Ca2+ mobilization from intracellular Ca2+ stores. Furthermore, cGMP operated cation channel of vertebrate rod photoreceptor and cAMP operated cation channel of olfactory neuron are composed of almost identical polypeptide, which contains the remnant of the voltage sensor segment. Classification of these channel proteins may provide valuable clues to further understanding the structure/function relationships at molecular levels.