ΔYY and ΔKESYY C-terminal truncations impair Ko-induced closure of palytoxin-bound Na/K pump channels. (A and B) Representative recordings of activation and deactivation of palytoxin-bound Na/K pump channels in outside-out membrane patches (with 125 mM Na but no ATP in the pipette–cytoplasmic side) excised from oocytes expressing C113Y (A) or C113Y-ΔYY truncated (B) pumps; the labeled bars indicate bath (extracellular side) solution changes. A high concentration, 100 nM, of palytoxin (PTX) rapidly opened pump channels in both cases with similar time courses (activation time constant, τ_act = 21 s for C113Y and 26 s for C113Y-ΔYY). Replacement of all Nao with Ko caused, after a small instantaneous current increase (asterisks), biexponential complete decay of palytoxin-activated current (dotted lines mark current levels in Ko before PTX). (C) Superimposed normalized traces show reduced fractional amplitude of the faster component of Ko-induced decay for the truncated pumps (C113Y-ΔYY, olive; C113Y-ΔKESYY, blue) than for the parent pump (C113Y, red); dotted line as in A and B; breaks in the traces are to show that the decay is complete.