A Step Forward in Understanding the Mechanism of VDAC Voltage-Gating

Abstract

The voltage-dependent anion channel (VDAC) governs the exchange of ions and metabolites between the mitochondria and the rest of the cell. In its open state VDAC exhibits high conductance and selectivity for anions that facilitates the passage of ADP, ATP, and other metabolites. At increased voltages (>30mV) VDAC switches to lower conducting states, termed as “closed” states. Closed states are cation-selective and impermeable for ATP. The voltage-induced transition from the open to closed states is referred to as voltage-gating. Although it is well established that VDAC voltage-gating involves large structural rearrangements, the precise molecular mechanism of this process is still under debate. We investigated VDAC voltage-gating by systematically titrating VDAC charge residues and by using thermodynamic and kinetic approaches to study opening and closing of the channel. All the models proposed so far agree that N-terminal region plays a key role in VDAC voltage-gating. According to the original idea, the N-terminal region is a part of a mobile voltage sensor domain, which slides in and out of the channel lumen in response to the applied voltage. The alternative models consider independent movement of the N-terminal region upon gating. In order to test the role of VDAC N-terminal region in voltage-gating, we engineered a double Cys mutant of murine VDAC1 that cross-links the α-helix to the β-strand 11 of the pore wall. The cross-linked VDAC1 reconstituted into planar lipid membranes exhibited typical voltage gating, which suggests that the N-terminal α-helix is located inside the pore of VDAC in the open state and remains associated with the pore wall during voltage gating. Our findings support a model where β-barrel is not rigid but undergoes a conformational change that leads to a partial constriction upon transition to the closed states.