ROLE OF H5 DOMAIN IN DETERMINING PORE DIAMETER AND ION PERMEATION THROUGH CYCLIC NUCLEOTIDE-GATED CHANNELS

ION permeation through membrane channels is thought to be governed by a narrow region of the channel pore termed the selectivity filter1, wh ich has been proposed to discriminate among ions by both specific bind ing and molecular sieving, as determined by pore diameter. Recent evid ence suggests that a conserved domain (known as H5, P or SS1-SS2) in v oltage-gated potassium2-8, sodium9-13 and calcium12channels contribute s to the lining of the pore. Here we investigate whether the H5 domain determines pore diameter and examine the role of pore diameter in con trolling ion permeation. These studies rely on differences in single c hannel conductance, ion selectivity and apparent pore diameter between cyclic nucleotide-gated channels cloned from bovine retina14 and catf ish olfactory neurons15. Using chimaeric retinal-olfactory channels, w e find that the H5 domain determines these differences in permeation p roperties, providing structural evidence that the cyclic nucleotide-ga ted channels are indeed members of the voltage-gated channel family15- 17. Moreover, these results show directly that the H5 domain helps for m the selectivity filter and that molecular sieving is important in co ntrolling ion permeation.