June 22, 2014
The proton gradient is a principal energy source for respiration-dependent active transport, but the structural mechanisms of proton-coupled transport processes are poorly understood. YiiP is a proton-coupled zinc transporter found in the cytoplasmic membrane of Escherichia coli. Its transport site receives protons from water molecules that gain access to its hydrophobic environment and transduces the energy of an inward proton gradient to drive Zn(ii) efflux. This membrane protein is a well-characterized member of the family of cation diffusion facilitators that occurs at all phylogenetic levels. Here we show, using X-ray-mediated hydroxyl radical labelling of YiiP and mass spectrometry, that Zn(ii) binding triggers a highly localized, all-or-nothing change of water accessibility to the transport site and an adjacent hydrophobic gate. Millisecond time-resolved dynamics reveal a concerted and reciprocal pattern of accessibility changes along a transmembrane helix, suggesting a rigid-body helical re-orientation linked to Zn(ii) binding that triggers the closing of the hydrophobic gate. The gated water access to the transport site enables a stationary proton gradient to facilitate the conversion of zinc-binding energy to the kinetic power stroke of a vectorial zinc transport. The kinetic details provide energetic insights into a proton-coupled active-transport reaction.
Figure: L152 controls the opening of an inter-cavity water portal. a: A structural water molecule (W, red sphere) near the transport-site occupied by a tetrahedral coordinated Zn(II) (Z1, magenta sphere), viewed from the periplasm. Relevant residues are drawn in s.. cks and labeled accordingly. TM5 is colored in red as indicated. b: Intracellular and extracellular cavity as outlined by dash lines. c: L152 gate viewed from the extracellular cavity along the arrow as indicated in b. The sidechains of L152, I90 and coordina.. on residues in the transport-site (s.. cks) are excluded from the protein surface drawing. M197 is shown as a yellow patch at the cytoplasmic entrance to the inter-cavity portal. d: L152 gate viewed from the intracellular cavity along the arrow as indicated in b. M151 and M197 are visible as yellow patches on the protein surface.
Results from: Gupta S, Chai J, Cheng J, D'Mello R, Chance MR, Fu D. Nature. 2014 Jun 22. doi: 10.1038/nature13382. [Epub ahead of print]