Stoichiometry and arrangement of subunits in rod cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) ion channels mediate the response to light in retinal rods. They are tetramers of two homologous subunits (alpha and bet a), each of which is essential for the function of the channels in vivo. We have investigated the stoichiometry and arrangement of these two subunits to determine how they come together within an individual channel complex. W e exploited the very specific geometric and spatial requirements for formin g a high-affinity Ni2+-binding site to examine the number and relative posi tions of the subunits. We found that only an order of alpha/alpha/beta/beta could account qualitatively and quantitatively for the observed intersubun it coordination of Ni2+ in wild-type and mutant alpha/beta channels. Furthe rmore, our results suggest a structural dimerization among like subunits, a t least at the level of the Ni2+-binding site.