EXPOSURE OF RESIDUES IN THE CYCLIC NUCLEOTIDE-GATED CHANNEL PORE - P-REGION STRUCTURE AND FUNCTION IN GATING

In voltage-gated ion channels and in the homologous cyclic nucleotide- gated (CNG) channels, the loop between the S5 and S6 transmembrane seg ments (P region) is thought to form the lining of the pore. To investi gate the structure and the role in gating of the P region of the bovin e retinal CNG channel, we determined the accessibility of 11 cysteine- substituted P region residues to small, charged sulfhydryl reagents ap plied to the inside and outside of membrane patches in the open and cl osed states of the channel. The results suggest that the P region form s a loop that extends toward the central axis of the channel, analogou s to the L3 loop of bacterial porin channels. Furthermore, the P regio n, in addition to forming the ion selectivity filter, functions as the channel gate, the structure of which changes when the channel opens.