Fig. 2.

The binding parameters for tubulin-induced VDAC closure depend on tubulin concentration, electrolyte concentration, and applied voltage. (A) (a and d) VDAC open time between successive blockages, τ_on, linearly decreases with tubulin concentration and depends on salt concentration. The medium consisted of 1 M KCl (a, filled circles) and 0.25 M KCl (d, filled diamonds). Both components of tubulin residence (closed) time, τ_off⁽¹⁾and τ_off⁽²⁾, are independent of tubulin or salt concentration (b, c, e, and f, open symbols) and are equal to 2.8 ± 0.5 ms and 23.7 ± 6.3 ms, respectively. The applied voltage was −20 mV. (B) Voltage dependence of VDAC residence times, τ_off⁽¹⁾ and τ_off⁽²⁾, in the presence of 50 nM tubulin in cis (open diamonds), trans (open circles), or both sides (filled squares) of the membrane. Residence time in extrapolation to 0 voltage does not depend on 1-side (cis or trans) or 2-side tubulin addition. (C) Voltage dependence of the on-rate, κ_on, with 50 nM tubulin added to the cis side. The line is an exponential fit to κ_on = κ₀exp(nVF/RT) with n = 4.97. Each time value presents the characteristic time of 9 different log probability fitting procedures ± SE. The medium consisted of 1 M KCl (B and C) and buffered with 5 mM Hepes at pH 7.4. VDAC was isolated from N. crassa mitochondria. Bilayer membranes were formed from DPhPC.