Fig. 1.

Different conductances of alternative ouabain-resistant PTX-bound pump-channels. (A and B) Currents at –50 mV in outside-out patches excised from oocytes expressing rat α1 (A) or Xenopus DR-α1 (B) Na/K pumps in symmetrical 125 mM Na solutions with 100 μM external ouabain but no pipette ATP, briefly exposed to PTX (50 pM in A and 100 pM in B) until the first channel opening was observed. Lower traces show gating transitions of PTX-bound pump-channels at indicated voltages after washout of unbound toxin; dotted lines mark 0, 1, 2, or 3 open channels. (C) Current amplitudes for channels in A (circles) and B (triangles) plotted against voltage; fits (straight lines) between –100 and –20 mV gave channel conductances γ = 7.5 pS for A and γ = 1.8 pS for B. (D) Sequence alignment of WT Xenopus α1, Xenopus DR-α1, rat α1, and sheep α1 Na,K-ATPases, and rabbit SERCA Ca-ATPase; all residues are numbered from Met 1. Loop 1-2 assignment is from cardiotonic steroid-binding studies (e.g., ref. 14), not later SERCA structures.