Topology of the Yeast Plasma Membrane Proton-translocating ATPase

A transgene encoding a plasma membrane H+-ATPase that confers acid resistance in Arabidopsis thaliana seedlings

Proton pumps (H+-ATPases) are the primary active transport systems in the plasma membrane of higher plant cells. These enzymes are encoded by a large gene family expressed throughout the plant, with specific isoforms directed to various specialized cells. While their involvement in membrane energetics has been suggested by a large body of biochemical and physiological studies, a genetic analysis of their role in plants has not yet been performed. We report here that mutant Arabidopsis thaliana plants containing a phloem-specific transgene encoding a plasma membrane H+-ATPase with an altered carboxy terminus show improved growth at low pH during seedling development. These observations provide the first genetic evidence for a role of the plasma membrane H+-ATPase in cytoplasmic pH homeostasis in plants.

Reduction of ATPase activity accompanied by photodecomposition of ergosterol by near-UV irradiation in plasma membranes prepared from Saccharomyces cerevisiae

When plasma membranes prepared from the yeast Saccharomyces cerevisiae were exposed to near-UV radiation, photodecomposition of ergosterol and reduction of ATPase-activity occurred simultaneously. The Vmax for ATPase activity decreased markedly with increasing near-UV dosage while the Km value remained constant. When ATPase solubilized from the plasma membrane was exposed to near-UV, the activity remained constant irrespective of dosage, indicating that the ATPase molecule itself was not damaged by near-UV irradiation. The relationship between content of ergosterol and ATPase activity was examined using liposomes constructed with lipids extracted from the membrane. Maximum activity of ATPase was seen at 5% ergosterol in liposomes; this activity was 2.5 times greater than that in liposomes without ergosterol. Activity of ATPase bound to liposomes with 5% ergosterol was reduced after near-UV irradiation, while the activity remained unchanged in the case of the liposomes without ergosterol. Fluidity of the liposomes with 5% ergosterol also decreased with increasing near-UV dosage. Dosage-response curves for reduction of ATPase activity and for decrease in fluidity were similar to that for photodecomposition of ergosterol. These results suggested that the reduction of ATPase activity in the membrane by near-UV irradiation was not caused by photochemical degradation of the primary structure of the ATPase molecule, but was attributable to conformational change resulting from an alteration in the higher-order structure of the membrane due to photochemical decomposition of ergosterol.

Epitope mapping and accessibility of immunodominant regions of yeast plasma membrane H(+)-ATPase

Immunodominant regions of yeast plasma membrane H(+)-ATPase have been mapped by two different approaches. A rabbit polyclonal antibody was used to screen a library of random fragments of the ATPase gene in a bacterial expression plasmid. In addition, the epitopes recognized by a panel of mouse monoclonal antibodies against the ATPase were mapped by reactions with defined fragments of the enzyme expressed in Escherichia coli. Both methodologies indicated that two regions within the amino-terminal part of the ATPase (at amino acid positions 5-105 and 168-255) contain most of the antigenic determinants. The accessibility of the monoclonal antibodies to their epitopes in native and solvent-perturbed ATPase preparations was investigated by immunofluorescence studies on yeast protoplasts. Cells fixed and permeabilized with formaldehyde were either treated with or without detergents and organic solvents. ELISA competition tests with plasma membrane vesicles and with detergent-purified ATPase incubated in solution with the monoclonal antibodies gave similar results. All the epitopes were accessible in detergent-treated ATPase preparations. In contrast, only the epitopes at amino acids 24-56 were accessible in ATPase preparations not treated with detergents or organic solvents. These epitopes were cytoplasmic because protoplast permeabilization was required for decoration by the reactive monoclonal antibodies.

Mercaptoethanol and dithiothreitol decrease the difference of electrochemical proton potentials across the yeast plasma and vacuolar membranes and activate their H(+)-ATPases

Mercaptoethanol and dithiothreitol (DTT) inhibited the acidification of external medium by Saccharomyces carlsbergensis cells and protoplasts during glucose oxidation. The inhibition was also observed when cells were incubated with mercaptoethanol or when mercaptoethanol and DTT were used to prepare protoplasts. Experiments with S. carlsbergensis plasma membrane vesicles and vacuoles showed these thiol reagents to inhibit ATP-dependent generation of delta pH and Em across plasma membrane vesicles and vacuoles but to activate their H(+)-ATPases. Mercaptoethanol and DTT are suggested to de-energize plasmalemma as well as tonoplast by increasing their H(+)-permeability and to disturb the cell ion homeostasis.