Energization of Plant Cell Membranes by H+-Pumping ATPases: Regulation and Biosynthesis

Abstract

The circulation of ions across biological membranes is a fundamental process of cellular energetics (Harold, 1986). Indeed, ion currents play the key role in energy capture during respiration and photosynthesis. They mediate the interconversion of chemical, osmotic, and electrical forms of biological energy, and they support a range of physiological functions, including active transport and motility. Unlike animals, which use Na+ ions, plants use protons almost exclusively as the coupling ion (Figure 1). Regardless of the coupling ion used by an organism, the central theme of ion transport, which is based on Mitchell’s chemiosmotic theory, is conserved (Harold, 1986). A transport system generates an ion gradient whose electrochemical potential represents stored energy. Return of that ion across the membrane down its electrochemical gradient is mediated by a second transport system so as to link the “downhill” flux of the ion to the performance of some useful work, such as the “uphill” transport of another solute. The rate of work depends on the rate of current flow, and its capacity depends on the potential gradient established by the primary transport system. Coupling ions usually carry a positive charge. Unlike uncharged solutes, cations offer the advantage of contributing both an electrical and concentration gradient to the driving force. Furthermore, electrical imbalances can be rapidly transmitted over long distances along the surface of membranes and are thus effective in both energy transduction and communicating information. Currents of protons in plants predominantly couple metabolism to work. In addition, H+ as well as Ca2+ ions are used for transducing chemical and environmental signals. In addition to mitochondria (see Mackenzie and McIntosh, 1999, in this issue) and chloroplasts, there are three distinct pumps in plants that generate proton electrochemical gradients (Figure 1). The plasma membrane H+-ATPase (PM H+-ATPase) extrudes H+ from the cell to generate a proton motive force with a membrane potential of -120 to -160 mV (negative inside) and a pH gradient of 1.5 to 2 units (acid outside). The vacuolar-type H+-ATPase (V-ATPase) and the vacuolar H+-pumping pyrophosphatase (H+-PPase) acidify the vacuolar lumen and other endomembrane compartments. This review highlights the similarities and differences between the structures and modes of regulation of the two H+-pumping ATPases and points out future challenges in the functional analysis of these proteins. The H+-PPase is briefly discussed for comparative purposes. A key function of the PM H+-ATPase is to generate a proton electrochemical gradient, thereby providing the driving force for the uptake and efflux of ions and metabolites across the plasma membrane. Essential nutrients, such as nitrate and sulfate, are taken into cells against concentration and electrical gradients by H+-coupled anion symporters (Figure 1; Tanner and Caspari, 1996). The translocation of organic compounds from source tissues to sink tissues similarly involves energy-dependent steps, and various H+-coupled symporters for amino acids and sucrose have been identified and characterized (see Lalonde et al., 1999, in this issue). The electrical gradient across the plasma membrane also determines the direction and extent of passive ion flow through ion-specific channels. In general, cations rush into the cytosol according to the Nernst potential when cation-specific channels open; anions correspondingly leak out of the cytosol when anion-specific channels open. Three Types of H+ Pumps in Plant Cells. PM H+ ATPase extrudes H+ outside the cell generating a proton electrochemical gradient (-120 to -160 mV relative to the outside). Vacuolar-type ATPase and PPase acidify endomembrane compartments, including the vacuole. The proton motive force provides energy for uptake and release of solutes via symporters (open circles), antiporters (stippled circles), and channels (open squares). Pumps are indicated by closed circles and closed squares. S, C+, and A- refer to organic solute, cation, and anion, respectively. Modified from Sze (1985). NUC, nucleus. Inside the cell, the V-ATPase maintains a proton electrochemical gradient across endomembrane compartments, including the vacuole, and energizes secondary active transport. Relative to the cytosol, a vacuolar membrane potential of +30 mV is maintained, and the lumenal pH ranges from 3 to 6. The electrochemical gradient is used by proton-coupled antiporters to accumulate cations, such as Ca2+, inside the lumen (Hirschi et al., 1996), whereas cation release is favored via channels, such as the inositol (1,4,5) triphosphate–sensitive Ca2+ channel (see Franklin-Tong, 1999; Sanders et al., 1999, in this issue). Conversely, the electrical gradient drives anions into the lumen via channels; however, anion efflux into the cytosol would generally be energetically unfavorable and so should depend on proton-coupled anion symporters. It is now wi