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Voltage gating in VDAC is markedly inhibited by micromolar quantities of aluminum
E T Dill1, M J Holden, M Colombini
1Department of Zoology, University of Maryland, College Park 20742.
Abstract:
The mitochondrial outer membrane contains voltage-gated channels called VDAC that are responsible for the flux of metabolic substrates and metal ions across this membrane. The addition of micromolar quantities of aluminum chloride to phospholipid membranes containing VDAC channels greatly inhibits the voltage dependence of the channels' permeability. The channels remain in their high conducting (open) state even at high membrane potentials. An analysis of the change in the voltage-dependence parameters revealed that the steepness of the voltage dependence decreased while the voltage needed to close half the channels increased. The energy difference between the open and closed states in the absence of an applied potential did not change. Therefore, the results are consistent with aluminum neutralizing the voltage sensor of the channel. pH shift experiments showed that positively charged aluminum species in solution were not involved. The active form was identified as being either (or both) the aluminum hydroxide or the tetrahydroxoaluminate form. Both of these could reasonably be expected to neutralize a positively charged voltage sensor. Aluminum had no detectable effect on either single-channel conductance or selectivity, indicating that the sensor is probably not located in the channel proper and is distinct from the selectivity filter.
Insights
Aluminum chloride inhibits voltage-gated outer membrane channels (VDAC) by neutralizing their voltage sensor. This finding sheds light on VDAC channel regulation and aluminum
Area of Science:
- Biophysics
- Cell Biology
- Mitochondrial Function
Background:
- Voltage-gated anion channels (VDAC) regulate metabolite and ion transport across the mitochondrial outer membrane.
- VDAC channels exhibit voltage-dependent gating, controlling their permeability based on membrane potential.
- Understanding VDAC regulation is crucial for comprehending cellular bioenergetics and transport processes.
Purpose of the Study:
- To investigate the effect of aluminum chloride on the voltage-dependent properties of VDAC channels.
- To elucidate the mechanism by which aluminum interacts with and modulates VDAC channel function.
- To identify the specific aluminum species responsible for altering VDAC channel gating.
Main Methods:
- Incorporation of purified VDAC channels into artificial phospholipid bilayers.
- Electrophysiological recordings to measure channel current and voltage dependence.
- Analysis of VDAC gating parameters (steepness, closing voltage) under varying aluminum concentrations and pH.
- Determination of the effect of aluminum on single-channel conductance and ion selectivity.
Main Results:
- Micromolar concentrations of aluminum chloride significantly inhibited the voltage dependence of VDAC channels.
- VDAC channels remained in a high conducting (open) state even at high membrane potentials.
- Aluminum binding decreased the steepness of voltage dependence and increased the voltage required for channel closure, without altering the energy difference between open and closed states.
- Aluminum did not affect single-channel conductance or selectivity, suggesting the voltage sensor is distinct from the pore.
- pH shift experiments indicated that positively charged aluminum species were not involved; aluminum hydroxide or tetrahydroxoaluminate were identified as the active forms.
Conclusions:
- Aluminum acts by neutralizing the positively charged voltage sensor of VDAC channels, rather than directly blocking the pore.
- The voltage-sensing domain of VDAC is spatially distinct from the channel's selectivity filter and conductance pathway.
- These findings provide novel insights into the molecular mechanisms of VDAC channel regulation by metal ions.