Figure 5.

Ionic selectivity of the tubulin-blocked state of VDAC is cationic for both a recombinant construct and WT mammalian tubulin. Current–voltage relationships for a single VDAC channel in a membrane separating 200 mm (cis) and 1 m (trans) KCl solutions (2 mm HEPES, pH 7.4) are shown. Open state (black triangles) has anionic selectivity with the reversal potential ψ_rev = −9.4 ± 0.5 mV, marked by the black arrow. Currents for the tubulin-blocked states are denoted by red circles for the α1/β3 tubulin construct and by blue circles for WT mammalian tubulin. To be able to measure the reversal potential for the two tubulins on the same single channel, we added the recombinant tubulin to the cis side and WT tubulin to the trans side of the membrane, in concentrations of 10 and 20 nm, respectively. As indicated by the arrows, blockage of VDAC by either tubulin similarly reverses the original anionic selectivity of the channel to the cationic one with ψ_rev = 8.4 ± 0.4 mV and 9.5 ± 0.6 mV for α1/β3 and the WT mammalian tubulin, respectively.