Structural Basis of Ion Channel Function

The successful crystallization and structural determination of the constitutively open charmel of Streptomyces lividans (Doyle et al., 1998) represented a large step forward in the understanding of function and selectivity of ion channels. 

The charmel is a tetramer in which each subunit has two a-hehcal transmembrane elements. One helix of each subunit forms the irmer coating of the pore whilst the other helices form contacts to the phosphohpid bUayer, via hydrophobic residues. Hie helices are in the form of an. .inverted teepee with the broad opening oriented to the outside, into the extracellular region (Fig. 16.6). The loops between the helix pairs are on the extracellular side and are oriented inwards to the pore. These form the selectivity filter that discriminates between K and Na The narrow selectivity filter, which is only 12 A long, joins a large hydrophobic cavity and the inner pore, which is hydrophi-lically coated. 

It is to be expected that these basic principles of ion selectivity and ion conduction for the charmel of Streptomyces lividans also apply to other types of ion charmels. 

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