Water movement during channel opening and closing.

By applying the osmotic stress of a nonpenetrating polymer, we have measured the change delta nu in polymer-inaccessible internal water volume of a voltage-gated ionic channel. The voltage-dependent anion channel (VDAC) from mitochondrial outer membranes shows a delta nu comparable in magnitude to the full channel volume estimated from solute penetrability, single-channel conductance, or image reconstruction. It thus appears that channel "gating" involves significant structure reorganization and water movement rather than the minimal changes caused by a local constriction or blockade. Hydration of the inner channel surface may be an important factor in channel gating as is the hydration of molecular surfaces in controlling macromolecular interaction in solution.