Cyclic nucleotide Known binding proteins Pathway/Biological association cAMP protein kinase A cyclic nucleotide-gated ion channels Epac Catabolite Activator Protein (CAP) smooth muscle relaxation[14] photo/olfactory receptors[3] glucagon production in pancreatic beta cells[15] lac operon regulation in E. coli[16][17] cGMP cGMP-dependent protein kinase (PKG) cyclic nucleotide-gated ion channels smooth muscle relaxation[14] photo/olfactory receptors[3] cCMP cGMP kinase I protein kinase A smooth muscle relaxation[6][18]