SUBUNIT STOICHIOMETRY OF CYCLIC NUCLEOTIDE-GATED CHANNELS AND EFFECTSOF SUBUNIT ORDER ON CHANNEL FUNCTION

Cyclic nucleotide-gated (CNG) ion channels are multimeric structures c ontaining at least two subunits. However, the total number of subunits per functional channel is unknown. To determine the subunit stoichiom etry of CNG ion channels, we have coexpressed the 30 pS conductance bo vine retinal channel (RET) with an 85 pS conductance chimeric retinal channel containing the catfish olfactory channel P region (R0133). Whe n R0133 and RET monomers are coexpressed, channels with four distinct intermediate conductances are observed. Dimer constructs reveal that t wo of these conductance levels arise from channels with the same subun it composition (2 R0133:2 RET) but distinct subunit order (like subuni ts adjacent to each other versus like subunits across from each other) . Thus, the data demonstrate that cyclic nucleotide-gated ion channels are tetrameric like the related voltage-gated potassium ion channels; the order of subunits affects the conductance of the channel; and the channel has 4-fold symmetry in which four asymmetric subunits assembl e head to tail around a central axis.