Cooperativity in cyclic nucleotide-gated ion channels.

Cooperativity is one of the marvelous "emergent" behaviors of macro-molecules undergoing multimolecular reactions (Perutz, 1990). Cooperativity sculpts the potential energy landscape of such a reaction so as to create a seemingly teleological pathway for reaching and stabilizing a particular state for example, a multimeric protein with multiple ligand sites occupied. Ligandgated ion channels provide excellent opportunities for examination of the function and the mechanistic nature of cooperative behavior (Jackson, 1989, 1994; Changeux and Edelstein, 1994). Cooperative ligand activation of ion channels can serve to create exquisitely sensitive molecular switches at chemical synapses. And single-channel recording can provide unique possibilities for investigating the mechanism(s) underlying cooperativity in single molecules (e.g., Jackson, 1994). While full understanding of the physical mechanisms underlying cooperative gating will require tertiary structure information of multiple structural forms of a channel--information unlikely to be generally available in the near term--partial insights into the phenomenon obtained with functional assays are greatly to be welcomed. In this issue of The Journal of General Physiology, Karpen and Brown (1996) refine their application of the technique of covalent activation of the cyclic nucleotidegated channel of retinal rods with the photoaffinity analogue 8-p-azidophenacylthio-cGMP, and in doing so advance the investigation of cooperative channel gating. The novel technical feat of the effort of Brown and Karpen (1996) is the production of membrane patches reasonably argued to contain only fully liganded channels, or channels having one unliganded nucleotide binding site. Although cyclic nucleotide-gated ion channels (CNCs) took their seat in the congress of known ligand-gated channels just a little over a decade ago (Fesenko et al., 1985), work on them has proceeded very rapidly. They are well characterized functionally as nondesensitizing, cooperatively gated, and weakly specific cation channels with rectifying properties dependent on divalent cation blocking (Yau and Baylor, 1989; Kaupp, 1991). Beginning with the bovine retinal rod channel (Kaupp et al., 1989) more than 10 distinct CNCs have been cloned and sequenced; of these most have been characterized functionally after re-expression (Yau and Chen, 1995). The generic native CNC is likely a heteromultimer comprising four (or possibly five) subunits, each having six transmembrane segments, with a ligand binding domain on the cytoplasmic side (Fig. 1; Kaupp, 1991; Molday and Hsu, 1994; Yau and Chert, 1995). CNCs constitute a distinct and archetypal family of ligand-gated channels, likely formed by the combination of a general nucleotide binding domain with an ur-channel subunit having a domain closely resembling the $4 voltage-sensing domain of some classes of K channels (Jan and Jan, 1990; Yau and Chen, 1995). This view is now strongly supported by recent work on mutant and chimeric CNCs (Heginbotham et al., 1992; Goulding et al., 1994), and a growing body of sequence homology data. CNCs have two known subunits: the ~-subunit, originally cloned by Kaupp et al. (1989), can form homomultimeric channels; the [3-subunit, discovered later (Chen et al., 1993) forms functional channels only in conjunction with the ~-subunit. (Excellent reviews of CNC structure and functional properties can be found in Kaupp, 1991; Molday and Tsu, 1994; Yau and Chen, 1995; Menini, 1995.) The nucleotide dependence of steady state currents through CNCs is usually well described by the Hill equation