Structure-function relationships in a P-type H+ pump

The P-type plasma membrane (PM) H⁺-ATPase is found in fungi and plants and is the physiological equivalent of the animal Na⁺,K⁺-ATPase by being responsible for generating the essential electrochemical gradient across the plasma membrane.

 

The PM H⁺-ATPase transports one proton per ATP hydrolyzed, and pumps protons from the cytosol to the extracellular space apparently without any ion being counter-transported. Consequently, P-type PM H⁺-ATPases maintain high electrochemical gradients across the fungal and the plant plasma membranes (-300 mV and -200 mV, respectively), which is much higher than the approximately -70 mV reported for the Na⁺,K⁺-ATPase.

 

Protons have very high mobility in aqueous solutions compared to other cations, and an essential feature of proton pumps is the ability to transport protons against an electrochemical gradient without protons back-slipping. Thus, proper closure of the proton pathways and occlusion of the proton is crucial for function. We aim at understanding the detailed mechanism of pumping by the P-type H⁺ pump.

 

In the proton pump, a specific aspartate residue serve as a key proton donor/acceptor and an arginine residue appear to control the pKa of that aspartate residue. Besides this minimal proton pumping apparatus, not much is known about how proton transport is achieved by PM H⁺-ATPases.

 

Recently, the first three-dimensional structure for a P-type H⁺-ATPase was published, showing the plant PM H⁺-ATPase in a quasi-occluded E1 conformation at a resolution of 3.6 Å. Although this structure has been an important step forward in the understanding of the PM H⁺-ATPase, the resolution is too low to detect any water molecules present and to unambiguously determine the orientation of amino acid side chains, and can thus only provide indications for the proton translocation mechanism of the PM H⁺-ATPase.

 

Selected publications

Ekberg K, Pedersen BP, Sørensen DM, Nielsen AK, Veierskov B, Nissen P, Palmgren MG, Buch-Pedersen MJ (2010) Structural identification of cation binding pockets in the plasma membrane proton pump. Proc Natl Acad Sci U S A. 107: 21400-5.

 

Pedersen BP, Buch-Pedersen MJ, Morth JP, Palmgren MG, Nissen P (2007) Crystal structure of the plasma membrane proton pump. Nature. 450: 1111-4.