A variety of polyanions increase the steepness of the voltage dependence of VDAC. The steepness of the voltage dependence is the change in the probability of the channel being in the open state per unit change in transmembrane voltage. It is usually designated as the minimum number of charges that would have to traverse the entire transmembrane potential to account for the observed steepness of voltage dependence. This is designated as the "n" value. The steeper the voltage dependence the more precise is the regulation of the state of the channel. Certain polyanions can increase the steepness by over an order of magnitude to unprecedented values. These extraordinary findings are not only of biophysical interest but also have potentially-important biological implications.
Typical "n" values for VDAC range from 2.5 to 4. This is somewhat lower than the classical values for sodium channels, 5 to 7. By adding 8kDa dextran sulfate the n value can be increased to 50. This is truly "ultra-steep" voltage dependence (figure 15 from review article).
This increased voltage dependence does not arise from 50 charges on VDAC
traversing the transmembrane potential but by a process by which dextran
sulfate amplifies the existing voltage-gating process. Dextran sulfate
is proposed to partition into the access resistance region at the mouth
of the channel in a voltage dependent manner according to the Boltzmann
distribution. There it interacts electrostatically with the positively-charged
voltage sensor on VDAC favoring channel closure. According to this mechanism,
dextran sulfate does not bind to VDAC and can only amplify VDACís voltage
dependence when the dextran sulfate side is made negative. In addition,
the greater the valency of the polyanion, the more potent the effect and
the steeper the voltage dependence. Finally, increasing the viscosity
of the medium should increase the access resistance and further amplify
the effect. All these expectations are true. An example of the asymmetrical
effect is shown in the accompanying figure.
The mechanism does not require any specific chemical structure and thus
a variety of different polyvalent anions work. Note the greater potency
of dextran sulfate 500kDa. The weaker effect of RNA and pepsin may arise
from a defined 3-dimensional structure that prevents clustering of a large
amount of charge and/or its effective partitioning into the access resistance
Table: Augmentation of the voltage dependence of VDAC by polyanions
Mangan, P. S. and Colombini, M. 1987. Ultra-steep voltage
dependence in a membrane channel. Proceedings of the National
Academy of Sciences U.S.A.,
Thomas, L., Blachly-Dyson, E., Colombini, M., and Forte, M. 1993.
Mapping of residues forming the voltage sensor of the VDAC ion channel.