Cyclic nucleotide-gated channels (CNGs) are crucial for signal transduction in the visual and olfactory systems. Similar to other channels that have six transmembrane domains, CNGs are tetrameric. In the case of rod photoreceptor CNGs, the tetramers are heteromeric complexes of the A1 and B1 CNG subunits. But how many subunits of each type are there in a given channel? Although indirect evidence indicated that rod CNG channels might have two A1 and two B1 monomers, two recent papers reach the surprising conclusion that they contain three A1 and one B1 subunits.

In the first paper, Weitz et al. used chemical crosslinking to measure the stoichiometry of native channels. As A1 and B1 subunits have different molecular weights, the authors predicted the existence of complexes of different mass, which depended on the number of A1 and B1 monomers that are present in the channel. In the second report, Zheng et al. established the stoichiometry of recombinant channels by using fluorescence resonance energy transfer between tagged subunits. This method depends on the ability of a stimulated fluorophore to transfer energy to a second fluorophore located within a short radius, and provides an excellent estimate of intra- and intermolecular distance.

Both studies provided evidence that rod CNG channels have three A1 and one B1 monomers. As B1 subunits confer rod CNG channels with specific functional properties, such as their modulation by Ca2+/calmodulin, it will be important to discover how a single subunit fulfils this role. More importantly, the results pose questions about how channel assembly takes place and about the mechanisms that prevent two B1 subunits from integrating into a tetramer. The data also raise the intriguing possibility that other tetrameric channels have similar assembly restrictions. Last, as olfactory CNG channels consist of three different subunits (A2, A4 and B1b), the approaches of both groups might help unravel their stoichiometry.