Structure derived from a Bd1971∆EAL construct (yellow/orange), in comparison to cAMP-bound full-length structure (dark/light blue, interdomain linker helices in red/pink). Residues from the opposing monomer labeled by a '. (A) The apo sensor overlaid with the cAMP-bound sensor indicates a relative swing out of the cNMP body relative to C-helix central stator (motion indicated by arrow); the apo form would now sterically clash with the linker helix in the full-length bound structure. (B and C) Comparison of P-loop environment in both apo and bound states. In the apo form, the P-loop (selected residues in green) migrates out of a hydrophobic pocket formed by the hinge and C-helices, breaking an interaction between R108' and D63. In the bound state, M59 stacks with the nucleotide base and the P-loop shifts upward to contact both sides of the pocket. (D) An alignment via the cNMP body demonstrates that swing out is licensed in part by P-loop twisting/migration - L61 in the bound state allows close packing of the hinge and C-helices, whereas L61 in the apo state would clash with V87 and L88, forcing a relative shift of the body and helical subdomains. (E and F) Engagement of the cAMP base and M59 with the C-helix interface. The two orthogonal views demonstrate that L105 and L106 form a hydrophobic plane at the C-helix interface, under which the M59 sidechain can pack against the cAMP base and allow the cNMP body to adopt the swung in conformation. 