State-dependent inter-repeat contacts of exceptionally conserved asparagines in the inner helices of sodium and calcium channels
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Voltage-gated sodium and calcium channels play key roles in the physiology of excitable cells. The alpha-1 subunit of these channels folds from a polypeptide chain of four homologous repeats. In each repeat, the cytoplasmic halves of the pore-lining helices contain exceptionally conserved asparagines. Such conservation implies important roles, which are unknown. Mutations of the asparagines affect activation and inactivation gating as well as the action of pore-targeting ligands, including local anesthetics and steroidal agonists batrachotoxin and veratridine. In the absence of the open-channel structures, underlying mechanisms are unclear. Here, we modeled the pore module of Cav1.2 and Nav1.4 channels and their mutants in the open and closed states using the X-ray structures of potassium and sodium channels as templates. The energy of each model was Monte Carlo-minimized. The asparagines do not face the pore in the modeled states. In the open-channel models, the asparagine residue in a given repeat forms an inter-repeat H-bond with a polar residue, which is typically nine positions downstream from the conserved asparagine in the preceding repeat. The H-bonds, which are strengthened by surrounding hydrophobic residues, would stabilize the open channel and shape the open-pore geometry. According to our calculation, the latter is much more sensitive to mutations of the asparagines than the closed-pore geometry. Rearrangement of inter-repeat contacts may explain effects of these mutations on the voltage dependence of activation and inactivation and action of pore-targeting ligands.
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