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1 Institut für Physiologie II, Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany
2 Fachhochschule Schmalkalden, Fachbereich Elektrotechnik, Blechhammer, D-98754 Schmalkalden, Germany
3 Forschungsinstitut für Molekulare Pharmakologie, Robert-Rössle-Str. 10, D-13125 Berlin, Germany
Cyclic nucleotide-gated (CNG) ion channels play a key role in the sensory transduction of vision and olfaction. The channels are opened by the binding of cyclic nucleotides. Native olfactory CNG channels are heterotetramers of CNGA2, CNGA4, and CNGB1b subunits. Upon heterologous expression, only CNGA2 subunits can form functional homotetrameric channels. It is presently not known how the binding of the ligands to the four subunits is translated to channel opening. We studied activation of olfactory CNG channels by photolysis-induced jumps of cGMP or cAMP, two cyclic nucleotides with markedly different apparent affinity. It is shown that at equal degree of activation, the activation time course of homotetrameric channels is similar with cGMP and cAMP and it is also similar in homo- and heterotetrameric channels with the same cyclic nucleotide. Kinetic models were globally fitted to activation time courses of homotetrameric channels. While all models containing equivalent binding sites failed, a model containing three binding sites with a ligand affinity high–low–high described the data adequately. Only the second binding step switches from a very low to a very high open probability. We propose a unique gating mechanism for homotetrameric and heterotetrameric channels that involves only three highly cooperative binding steps.
(Received 9 June 2005;
accepted after revision 29 July 2005;
first published online 4 August 2005)
Corresponding author K. Benndorf: Institut für Physiologie II, Friedrich-Schiller-Universität Jena, Kollegiengasse 9, D-07743 Jena, Germany. Email: klaus.benndorf{at}mti.uni-jena.de
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