Conformational changes in S6 coupled to the opening of cyclic nucleotide-gated channels

In cyclic nucleotide-gated channels (CNG), direct binding of cyclic nucleot ides in the carboxy-terminal region is allosterically coupled to opening of the pore. A CNG1 channel pore was probed using site-directed cysteine subs titution to elucidate conformational changes associated with channel openin g. The effects of cysteine modification on permeation suggest a structural homology between CNG and KcsA pores. We found that intersubunit disulfide b onds form spontaneously between S399C residues in the helix bundle when cha nnels are in the closed but not in the open state. While MTSET modification of pore-lining residues was state dependent, Ag+ modification of V391C, in the inner vestibule, occurred at the same diffusion-limited rate in both o pen and closed states. Our results suggest that the helix bundle undergoes a conformational change associated with gating but is not the activation El ate for CNG channels.