Figure 3.

Blocking the hydrolytic cycle slows channel closing. (a) Because opening rate falls to virtually zero after ATP removal, time course of current decay measures channel closing rate. (i) Time constant for current decay in control conditions is less than 1 s, but a slow component (τ_slow=21 s) appears if opening occurs in the presence of orthovanadate (VO₄). In other ABC transporters VO₄ inhibits ATPase activity by acting as an analogue of the pentacovalent transition state intermediate of hydrolysis. (ii) Adenosine 5′-(β,γ-imido)triphosphate (AMPPNP) can also ‘lock’ channels open. (b,c) Mean (±s.e.m., n≥15) (i) time constant (τ_slow) and (ii) fractional amplitude of the slow component.