Fig. 2. (A) Three-dimensional structure of a cyclic-nucleotide-gated (CNG) channel. Four subunits are arranged to form a common pore. There is a cyclic-nucleotide-binding site on each subunit. (B) Diagram of the primary structure of the -subunit. The cylinders indicate hydrophobic segments thought to represent transmembrane domains. The S4 segment is the voltage sensor for voltage-dependent transition in these channels; the region between S5 and S6 is thought to be part of the channel pore (P domain). The cyclic-nucleotide-binding domain is in the C-terminal region of the CNG channel.