The FMRFamide-gated Na+ channel (FaNaC) is a unique peptide-gated sodium channel and a member of the epithelial sodium channel/degenerin family. Previous studies have shown that an aspartate residue (Asp552) in the second transmembrane domain is involved in activation of the FaNaC. To examine the significance of a negative charge at position 552, we used a cysteine-modification method. Macroscopic currents of a cysteine mutant (D552C) were potentiated or inhibited by use of positively or negatively charged sulfhydryl reagents ([2-(trimethylammonium)ethyl]methanethiosulfonate bromide, MTSET, and sodium (2-sulfonatoethyl)methanethiosulfonate, MTSES, respectively). Dose-response analysis showed that treatment with MTSET increased the potency of the FMRFamide in the FaNaC whereas treatment with MTSES reduced the maximum response. Negative charge at position 552 was necessary for the characteristic inward rectification of the FaNaC. These results suggest that negative electric charge at position 552 is important to the activation and permeation properties of the FaNaC.
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