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Acidic Residues on the Voltage-Sensor Domain Determine the Activation of the NaChBac Sodium Channel

Research Centre and Department of Medicine, Hôpital Laval, Quebec City, Quebec, Canada G1V 4G5

Correspondence: Address reprint requests to Mohamed Chahine, PhD, Le Centre de recherche Université Laval Robert-Giffard, Local F-6539, 2601 chemin de la Canardière, Québec (Québec) G1J 2G3 Canada. Tel: 418-663-5747 ext. 4723; Fax: 418-663-8756; E-mail: mohamed.chahine{at}phc.ulaval.ca.

The voltage-sensing domain of voltage-gated ion channels is characterized by specific, conserved, charged residues. Positively charged residues on segment S4 are the main contributors to voltage-sensing and negatively charged residues on the S2 and S3 segments are believed to participate to the process. However, their function in the voltage sensor is not well understood. To probe the role of three acidic residues in NaChBac (D-58 and E-68 in S2, and D-91 in S3), we employed site-directed mutagenesis to substitute native acidic residues with cysteine (neutral), lysine (positive charge), or either aspartate or glutamate (negative charge). We used a combination of the patch-clamp technique to record Na⁺ currents and molecular modeling to visualize interacting amino acid residues. We suggest that the acidic residues on the S2 and S3 segments form specific interactions with adjacent amino acids in the voltage-sensor domain. The main interactions in NaChBac are D-58 (S2) with A-97-G-98 (S3) and R-120 (S4), E-68 (S2) with R-129 (L4-5), and D-91 (S3) with R-72 (S2). Changing these acidic residues modified the interactions, which in turn altered the sensitivity of the voltage sensor.