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Proton exclusion by an aquaglyceroprotein: a voltage clamp study
Sapar M Saparov1, Satoshi P Tsunoda, Peter Pohl
1Forschungsinstitut für Molekulare Pharmakologie, Robert-Rössle-Str. 10, 13125 Berlin, Germany.
Biology of the Cell
|April 27, 2005
Summary
The Escherichia coli glycerol facilitator (GlpF) does not conduct protons, aligning with computational predictions. This aquaglyceroporin exhibits low proton permeability, suggesting it does not contribute to proton leakage in cell membranes.
Area of Science:
- Membrane Biology
- Protein Channels
- Biophysics
Background:
- Aquaporins and aquaglyceroporins are water channels with debated proton permeability.
- Experimental evidence for proton exclusion is limited to orthodox aquaporins.
- Aquaglyceroporins' ability to exclude protons and conduct ions remains unclear.
Purpose of the Study:
- To experimentally determine proton permeability of the Escherichia coli glycerol facilitator (GlpF).
- To resolve discrepancies regarding proton and ion conductivity in aquaglyceroporins.
- To validate in silico predictions of GlpF proton exclusion.
Main Methods:
- Purified GlpF was reconstituted into planar lipid bilayers.
- Osmotic gradients were used to confirm functional reconstitution and measure water permeability.
- Proton conductivity was measured under a pH gradient to determine H+ transport stoichiometry.
Main Results:
- GlpF demonstrated a single channel water permeability of 0.7x10(-14) cm(3).subunit(-1).s(-1).
- The upper limit for proton (or hydroxide) to water molecule transport stoichiometry was 2x10(-9).
- No significant GlpF-mediated ion conductivity was detected.
Conclusions:
- The measured low proton conductivity of GlpF aligns with molecular dynamics simulations.
- GlpF does not appear to facilitate significant proton or ion transport.
- These findings support the role of GlpF as a selective glycerol facilitator.