Single-channel kinetics of the rat olfactory cyclic nucleotide-gated channel expressed in Xenopus oocytes

Cyclic nucleotide-gated channels are nonselective cation channels activated by intracellular cAMP and/or cGMP. It is not known how the binding of agon ists opens the channel, or how the presumed four binding sites, one on each subunit, interact to generate cooperativity, We expressed the rat olfactor y cyclic nucleotide-gated channel cy subunit in Xenopus oocytes and recorde d the single-channel currents. The channel had a single conductance state, and flickers at -60 mV showed the same power spectrum for cAMP and cGMP. At steady state, the distribution patterns of open and dosed times were relat ively simple, containing one or two exponential components. The conductance properties and the dwell-time distributions were adequately described by m odels that invoke only one or two binding events to open the channel, follo wed by an additional binding event that prolongs the openings and helps to explain apparent cooperativity. In a comparison between cAMP and cGMP, we f ind that cGMP has clearly higher binding affinity than cAMP, but only modes tly higher probability of inducing the conformational transition that opens the channel.