(A) Schematic representation of the targeted Epac and CUTie sensors. CNBD, cyclic nucleotide-binding domain; TD, targeting domain. (B) Cartoon model of the Epac sensor constructed using the Apo structure of Epac2 [49]. The model is coloured by secondary structure (red: helix, grey: coil, light blue: extended beta) and cAMP is green. The helical regions at the amino- and carboxy-termini of the CNBD are shown in semi-transparent materials to highlight that the allosteric mechanism responsible for the increase in FRET on cAMP release is uncertain (see text for details). The surfaces around the florescent modules are coloured yellow and cyan fort YFP and CFP, respectively. (C) The CUTie sensor in its cAMP-bound and free conformations, respectively. Models are taken from simulations presented in [9] and coloured as in (B). The grey arrow at the amino-terminus indicates the site where the sensor can be fused to the carboxy-terminus of targeting proteins. The black circle marks the hinge position. (D) Neonatal cardiac myocytes expressing targeted CUTie reporters. The identity of the targeting domain is indicated in the prefix. Scale bar: 10 µm. PLM, phospholemman; PLB, phospholamban; TPNI, troponin I.