Figure 5.

Block of single KcsA channels by trans Na⁺. Single KcsA openings were recorded in 100 mM K⁺ solutions with or without Na⁺ added to either side. (Left) Raw current traces in the absence of Na⁺ (control) or with 10 mM Na⁺ added to the trans side. (Right) Open-channel I–V curves. ○, no Na⁺ added; •, 30 mM Na⁺ cis; ▪, 10 mM Na⁺ trans. Solid curve through the control points is a spline fit with no theoretical significance. Solid curve through the trans Na⁺ points is the prediction of a single-site Woodhull 1973 blocking mechanism, with an effective valence of 0.5 and a zero-voltage dissociation constant for Na⁺ of 322 mM.