In contrast to porins and toxins exhibiting weak ion selectivity, ion channels are highly specific filters regulating the ion balance of living cells. Up to now it was only possible to elucidate the structure of the potassium specific channel from Streptomyces lividans. Because of sequence homologies and similar physiologic properties it may be regarded as a prototype for this kind of channel. It's structural characteristics may be seen by clicking here.

The cytoplasmic membranes of Gram negative bacteria contain ion channels which regulate the ionic content of the cells in response to mechanical stress. Upon changes of pressure these mechanosensitive channels alter the diameter of the pores without discrimination of the kind of ions passing through. The channel from Mycobacterium tuberculosis could be crystallized, here is it's structure.

References:
DE Clapham, More pieces of the K⁺ ion channel puzzle, Nature struct. biol. 6 (1999) 807-810
L Heginbotham, Growing momentum in the molecular study of ion channels, Nature struct. biol. 6 (1999) 811-814