Figure 8.

Proton sensitivity of Xenopus δβγ ENaC is further modulated by a lysine in the δ-ENaC β1–β2 linker. a, homology model of the Xenopus δ-ENaC subunit indicating the location of δLys-89 in the linker region between the β1 and β2 sheets in the palm domain. b, fractional channel activation ((I_{M, x} − I_{M, min})/Δ_max I_M) resulting from a stepwise reduction of the extracellular pH (pH 8.0–5.5; pH 0.5 increments) in WT and δ_K89Lβγ ENaC. Introduction of the δ_K89L mutation leads to a significant acidic-shift of the channel's pH EC₅₀ (Student's unpaired t test). c, representative I_M recording of an oocyte expressing δ_K89Lβγ-ENaC. Macroscopic currents mediated by mutant channels display “stimulus-activated” characteristics as they are low at neutral pH 7.4 and reversibly increased by extracellular acidification (pH 6.0, 30 s; a = amiloride, 100 μm).