Phototransduction relies on the precise balance of speed and sensitivity to achieve optimal performance. The cyclic nucleotide-gated (CNG) ion channels, with their Ca2+ permeability, high sensitivity to changes in cytosolic cGMP, rapid gating kinetics, and Ca2+-calmodulin modulation, are beautifully optimized for their role in light detection. Many of these specializations come about from the heteromeric composition of the native channel, comprised of CNGA1 and CNGB1 subunits. However, the stoichiometry and arrangement of these subunits is unknown. Here we have used an approach based on fluorescence resonance energy transfer (FRET) to determine the composition of the intact functional channel in the surface membrane. We find, surprisingly, that the channel contains three CNGA1 subunits and only one CNGB1 subunit. These results have implications for CNG channel function in particular and assembly of membrane proteins in general.
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