Proteins that bind cyclic nucleotides (cAMP or cGMP) share a structural domain of about 120 residues [1,2, 3]. The best studied of these proteins is the prokaryotic catabolite gene activator (alsoknown as the cAMP receptor protein) (gene crp) where such a domain is known to be composed of three alpha-helices anda distinctive eight-stranded, antiparallel beta-barrel structure. There are six invariant amino acids in this domain,three of which are glycine residues that are thought to be essential for maintenance of the structural integrity ofthe beta-barrel. cAMP- and cGMP-dependent protein kinases (cAPK and cGPK) contain two tandem copies of the cyclicnucleotide-binding domain. The cAPK's are composed of two different subunits, a catalytic chain and a regulatory chain,which contains both copies of the domain. The cGPK's are single chain enzymes that include the two copies of the domainin their N-terminal section. Vertebrate cyclic nucleotide-gated ion-channels also contain this domain. Two suchcations channels have been fully characterised, one is found in rod cells where it plays a role in visual signaltransduction.

1. Phylogeny of the bacterial superfamily of Crp-Fnr transcription regulators: exploiting the metabolic spectrum by controlling alternative gene programs. FEMS Microbiol. Rev. 27, 559-92
2. Transcription activation by catabolite activator protein (CAP). J. Mol. Biol. 293, 199-213
3. The cyclic nucleotide-gated channels of vertebrate photoreceptors and olfactory epithelium. Trends Neurosci. 14, 150-7

Proteins that bind cyclic nucleotides (cAMP or cGMP) share a structural domain of about 120 resid
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