The CBD of EPAC and the domain organization.

a) The regulatory region consists of the DEP (disheveled Egl-10 pleckstrin) domain and the cAMP binding domain (CBD), colored grey. The catalytic region includes the CDC25 homology domain (CDC25HD), the Ras exchange motif (REM), and Ras association (RA) domain. The dashed red lines illustrate an expanded view of the sequence alignment of CBDs for the regions spanning the PBC α5 helix to the Hinge Helix. The corresponding secondary structure is shown above the sequence. The asterisks mark the site of termination in the deletion mutants. b) The structure of the CBD of apo-EPAC is shown in grey, whereas the major changes caused by cAMP (black spheres) binding are shown in orange. The curved black arrow illustrates the transition of the hinge helix from the apo form (grey; PDB ID: 2BYV) to the holo form (orange; PDB ID:3CF6). c)The thermodynamic cycle of cAMP dependent EPAC activation. Dashed lines encircle the equlibrium between the apo/inactive and apo/active states, i.e. the auto-inhibitory equilibrium. d) The hydrophobic “spine”, a network of residues involving the hydrophobic contacts between the hinge helix and adjacent helices (α4 and α5).