Fig. 2.

Structural comparison. (A) Comparisons of the C-lobe conformation from this study (PDB ID code 6MUD, blue), a C-lobe from a previously proposed Na_VCT:Ca²⁺/CaM complex (PDB ID code 4JQ0, gray), and an apoC-lobe (PDB ID code 4OVN, magenta), which shows that the previously proposed Ca²⁺/C-lobe from a Na_V CT complex (13) is an apoC-lobe, not a Ca²⁺/C-lobe. The letters in A–C correspond to the CaM helices. (B) Comparisons of the N-lobe from this study (green) with the N-lobe from PDB ID code 4JQ0 (gray), confirming that the latter represents a Ca²⁺-occupied N-lobe. (C) Superpositions showing the relative C-lobe positions found in this study (6MUD, cyan) and three other previous CaM complex with the Na_V1.5 CT. (D) Side-by-side comparison of the current structure (PDB ID code 6MUD) and three other CaM complexes, including two apo-CaM complexes (PDB ID codes 4OVN and 4DCK) and one previously proposed Ca²⁺/CaM complex, which is a mixed CaM (PDB ID code 4JQ0). The structures are shown in the exact same view, based on a superposition of the sixth helix immediately after the EF-hand domain. (E) Superposition of the Ca²⁺/CaM (PDB ID code 6MUD) and mixed CaM (PDB ID code 4JQ0) complexes with Na_V1.5CT, based on the sixth helix containing the IQ domain, using the same colors as in D. This shows that the Ca²⁺/C-lobe (cyan) from the Na_V1.5 CT: Ca²⁺/CaM structure would clash with the Na_V1.5 EF-hand domain in the mixed CaM complex (PDB ID code 4JQ0). Therefore, binding of the Ca²⁺/C-lobe to the IQ domain is prevented by the FHF, which utilizes reorientation of the EF-hand domain as a mechanism to prevent Ca²⁺/C-lobe binding.