Featured Article - December 2014
Short description: NMR studies reveal how UCP2 facilitates translocation of protons across the mitochondrial membrane via fatty acids.
The proton gradient across the inner mitochondrial membrane stores energy that can be used for ATP synthesis. This energy can also be dissipated via uncoupling proteins (UCPs), which shuttle protons down the electrochemical gradient in the presence of fatty acids, generating heat, regulating metabolism and reducing reactive oxygen species.
Berardi and Chou (PSI MPSbyNMR) had solved the structure of UCP2 (PDB 2LCK, highlighted previously at the PSI-SBKB) using NMR residual dipolar coupling (RDC) and paramagnetic relaxation enhancement (PRE) data. UCP2 forms a channel composed of three helical pseudo-repeats within the inner mitochondrial membrane.
To dissect the mechanism by which UCP2 works with fatty acids to transfer protons across the membrane, the authors employed chemical shift mapping and PRE to map the site of interaction of fatty acids on UCP2. The data indicated that the fatty acid acyl chain binds to a hydrophobic groove on the periphery of the protein, and the negatively charged head group interacts with basic residues near the matrix side. Subsequent RDC analysis showed that guanosine diphosphate binding within the hydrophilic cavity of UCP2 induces changes in the structure and/or dynamics of the hydrophobic groove that displace the fatty acid, consistent with its role in inhibiting UCP-mediated proton translocation.
The authors tested their model by mutating basic residues proposed to interact with the fatty acid head group. Such mutations reduced the ability of UCP2 to translocate protons across the membrane. However, basic residues within the cavity were also important for channel activity, suggesting that they create an electrostatic path that guides the fatty acid head group through the mitochondrial membrane.
They propose that fatty acids on the outer side of the inner membrane bind to protons and readily flip-flop across the membrane, depositing the proton into the mitochondrial matrix. The fatty acid then binds to UCP2, and basic residues within the hydrophilic cavity of the protein guide the acidic head group of the fatty acid across the membrane.
M. J. Berardi and J. J. Chou Fatty acid flippase activity of UCP2 is essential for its proton transport in mitochondria.
Cell Metab. 20, 541-552 (2014). doi:10.1016/j.cmet.2014.07.004