Covalent activation of retinal rod cGMP-gated channels reveals a functional heterogeneity in the ligand binding sites.

Ion channels gated by the binding of multiple ligands play a critical role in synaptic transmission and sensory transduction. It has been difficult to resolve the contribution of individual binding events to channel gating because ligands are continuously binding and unbinding at each site. In examining the allosteric mechanism of retinal rod cGMP-gated channels, we have circumvented this problem by making use of a cGMP derivative, 8-p-azidophenacylthio-cGMP (APT-cGMP), that can be covalently tethered to the binding sites in the presence of long-wavelength UV light. In excised membrane patches, a population of channels was isolated that contained covalently-attached ligands at all but one site. Activation of these channels by cGMP revealed a previously unknown heterogeneity in the ligand-binding sites. The dose-response relations were much shallower than predicted by single-site activation models, but were well described by models in which there are two populations of sites, in roughly equal proportion, that bind cGMP with apparent affinities that differ by a factor of 25. The two apparent affinities, incorporated into a four-site model of the channel, provided an accurate description of the patch's original dose-response relation. A comparison of results on native and expressed channels suggests that the heterogeneity in the native channel arises at least in part from the presence of two different cGMP-binding subunits.