In voltage-gated ion channels and in the homologous cyclic nucleotide- gated (CNG) channels, the loop between the S5 and S6 transmembrane segments (P region) is thought to form the lining of the pore. To investigate the structure and the role in gating of the P region of the bovine retinal CNG channel, we determined the accessibility of 11 cysteine-substituted P region residues to small, charged sulfhydryl reagents applied to the inside and outside of membrane patches in the open and closed states of the channel. The results suggest that the P region forms a loop that extends toward the central axis of the channel, analogous to the L3 loop of bacterial porin channels. Furthermore, the P region, in addition to forming the ion selectivity filter, functions as the channel gate, the structure of which changes when the channel opens.
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