The Plasma Membrane H+-ATPase (A Highly Regulated Enzyme with Multiple Physiological Functions)

The proton-pump ATPase (H+-ATPase) of the plant plasma membrane acts as a primary transporter by pumping protons out of the cell, thereby creating pH and electrical potential differences across the plasmalemma (Fig. 1). Transport of many solutes (ions, metabolites, etc.) into and out of the cell involves secondary transporters whose ability to function is directly dependent on the proton-motive force created by the H+-ATPase. Depending on the electrical charge of the solute to be transported, the direction of its transport, and its concentration on either side of the membrane, it is possible to predict from Figure 1 the type of transport protein required. For instance, the uptake of a cation is energetically favorable because of the positive external electrical potential, and therefore requires only a diffusion facilitator, such as a channel protein or a uniport. Conversely, to be energetically favorable, the uptake of an anion must be accompanied by the uptake of one or more protons in a symport system. In addition to activating secondary transport, the H+-ATPase promotes more specialized physiological functions.