Our knowledge of the crystal structures of two related ligand binding domains (Weber and Steitz 1987; Su et al. 1995) and a related K+ channel pore–forming region (Doyle et al. 1998), together with detailed knowledge of function, makes cyclic nucleotide–gated (CNG) channels a unique model system for studying allostery in proteins. Already, we have learned much about the molecular basis for ligand specificity (Varnum et al. 1995), how subunits interact (Gordon and Zagotta 1995c; Liu et al. 1998; Shammat and Gordon 1999), and the features that distinguish one subfamily of CNG channel subunits from the next (Goulding et al. 1994; Gordon and Zagotta 1995b).
One approach that has been used successfully with CNG channels, and many other types of proteins, is that of perturbing...
