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CHAPTER 11 CYCLIC NUCLEOTIDE-GATED CHANNELS

Ionic currents sensitive to cyclic nucleotides have been described in a number of tissues, including photoreceptors, olfactory sensory neurones, neurones, cardiac cells and the kidney (Kaupp, 1995; Zimmerman, 1995). They share the common property that their activation is mediated by the direct binding of cyclic nucleotides. Since many of thesechannels have not yet been identified at the molecular level, it is not known whether they all share a common structural motif. Two families of channels regulated by cyclic nucleotides have, however, been cloned. These are the cyclic nucleotide-gated channels (CNG channels) and the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Both belong to the superfamily of voltage-gatedchannels. In addition to the CNG and HCN channels, two K channels that are activated by cyclic AMP have been cloned from the plant Arapidopsis thalia. These channels, called AKT1 and KAT1, resemble the KV channels in their molecular structure. In this chapter, we focus on the structure and function of CNG and HCN channels and their role in health and disease.

CYCLIC NUCLEOTIDE-GATED CHANNELS
Theimportance of the CNG channels was first recognised in vertebrate photoreceptors (rods and cones) and in the olfactory epithelium, where they play important roles in sensory transduction. Subsequently, they have been found in a variety of other tissues including spermatozoa, neurones, kidney, liver, lymphocytes and heart. Mutations in the rod CNG channel give rise to the degenerative disease of rodsknown as retinitis pigmentosa.

Structure
Despite the fact that their gating is only slightly voltage dependent, CNG channels are members of the superfamily of voltage-gated ion channels. The
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CHAPTER 11 Cyclic Nucleotide-Gated Channels

native channel is believed to be a tetramer composed of two homologous subunits, and (Kaupp et al., 1989; Chen et al., 1993; Kaupp, 1995; Liu etal., 1996b). Three main types of -subunit and one type of -subunit have been identified in mammals to date, with molecular masses of between 63 and 240 kDa (Fig. 11.1; Table 11.1). Related channels have been cloned from Drosophila and the worm Caenorhabditis elegans. The -subunit has six putative transmembrane (TM) domains and a hydrophobic loop (H5) that dips down into the membrane between thefifth and sixth TMs to form part of the lining of the pore (Fig. 11.2). The fourth TM shows significant homology to the S4 region of voltage-gated channels, although the gating of the channel is only slightly voltage dependent. Both N and C termini are located in the cytoplasm. The latter contains a domain of 80–100 amino acids, which is homologous to the cGMP-binding domains of cGMP kinases and isinvolved in the binding of cyclic nucleotides. In the olfactory CNG channel there is also a calmodulin-binding site in the N terminus. The -subunit has a similar topology to that of the -subunit, but is much larger (240 kDa as compared with 63 kDa; Korschen et al., 1995). This is ¨ because although both subunits are highly homologous in the C terminal and transmembrane regions, the amino-terminalregion of the -subunit contains an additional large domain which is nearly identical to a glutamic acidrich protein (GARP). The function of the GARP region is completely unknown. Both the - and -subunits are able to form functional homomeric ion channels, when expressed in heterologous systems (Kaupp et al., 1989; Broillet and Firestein, 1997). Homomeric -channels probably do not occur in vivo, sincetheir properties differ from those of native CNG channels. Although it was initially thought that -subunits do not form functional channels when expressed independently, this turns out not to be the case, for homomeric -channel activity can be observed in response to stimulation with nitric oxide (Broillet and Firestein, 1997). Homomeric -channels are expressed in
brCNGCα bcCNGCα roCNGCα1...