Introduction

Cyclic nucleotide-gated (CNG) channels play a central role in the conversion of sensory information, such as light and scent, into primary electrical signals. We have purified the CNG channel from bovine retina and have studied it using electron microscopy and image processing. We present the structure of the channel to 35Å resolution. This three-dimensional reconstruction provides insight into the architecture of the protein, suggesting that the cyclic nucleotide-binding domains that initiate the response to ligand, 'hang' below the pore forming part of the channel, attached by narrow linkers. The structure also suggests that the four cyclic nucleotide-binding domains present in each channel form two distinct domains, lending structural weight to the suggestion that the four subunits of the cyclic nucleotide-gated channels are arranged as a pair of dimers.

In the present study, we have purified the heteromeric CNG channel from bovine rod outer segments in microgram quantities and have studied it by electron microscopy and image processing of single particles. The resultant 35Å resolution structure shows three distinct domains. The larger domain has four corners and a width similar to that of reconstructions of voltage-gated ion channels (Sato et al., 2001; Sokolova et al., 2001). We propose that this forms the membrane-spanning region of the channel. Attached to this, by two narrow linkers, are two smaller domains. These are related to one another by approximate two-fold symmetry and we propose that they contain the ordered parts of the cytoplasmic regions of the channel. These include the cyclic nucleotide-binding domains and the ordered part of the N-terminus. The presence of two distinct domains formed from four channel subunits supports the proposal that the ligand-binding domains of CNG channels are arranged as a functional dimer in which each monomer contains two domains. The structure also suggests an architecture for the channel in which the cyclic nucleotide-binding domains 'hang' in the cytosol, below the pore forming part of the channel, modulating gating of the pore through narrow linkers without compromising part of the ion pathway of the channel. On ligand binding the structure could become tetrameric and open the channel.

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Gebhard Schertler, gfx@mrc-lmb.cam.ac.uk