Phosphorylation of the adenosine triphosphatase in a deoxycholate-treated plasma membrane fraction from corn roots

The plasma membrane H+ -ATPase, a simple polypeptide with a long history

The plasma membrane H+ -ATPase of fungi and plants is a single polypeptide of fewer than 1,000 residues that extrudes protons from the cell against a large electric and concentration gradient. The minimalist structure of this nanomachine is in stark contrast to that of the large multi-subunit FO F1 ATPase of mitochondria, which is also a proton pump, but under physiological conditions runs in the reverse direction to act as an ATP synthase. The plasma membrane H+ -ATPase is a P-type ATPase, defined by having an obligatory phosphorylated reaction cycle intermediate, like cation pumps of animal membranes, and thus, this pump has a completely different mechanism to that of FO F1 ATPases, which operates by rotary catalysis. The work that led to these insights in plasma membrane H+ -ATPases of fungi and plants has a long history, which is briefly summarized in this review.

Bacterial phytotoxin, syringomycin, induces a protein kinase-mediated phosphorylation of red beet plasma membrane polypeptides

Syringomycin, a peptide toxin and a virulence factor produced by the bacterial phytopathogen Pseudomonas syringae pv. syringae, stimulated the phosphorylation of several plasma membrane polypeptides of red beet storage tissue. Among these was a 100-kDa polypeptide, which corresponds in size to the proton pump ATPase. The phosphorylations were insensitive to hydroxylamine, indicating that the polypeptide phosphorylated intermediates involved phosphate ester bonds characteristic of protein kinase-mediated phosphorylation. Phosphorylation of the 100-kDa polypeptide and of most of the other polypeptides was reduced or eliminated by extraction of the membranes with 0.1% (wt/vol) sodium deoxycholate, a treatment that also eliminated the ability of the toxin to stimulate ATPase activity. Phosphorylation of the 100-kDa polypeptide was highest with 10-20 mug of syringomycin; the same amounts gave the highest degree of ATPase activity stimulation. Phosphorylation of some of the polypeptides was eliminated or decreased by the Ca(2+) chelator EGTA. Addition of excess Ca(2+) restored the phosphorylation of most of these polypeptides. We conclude that syringomycin acts by stimulating an endogenous membrane protein kinase activity, which results in the phosphorylation of several membrane polypeptides. One of the phosphorylated polypeptides corresponds in size to the proton pump ATPase.

Vanadate inhibition of stomatal opening in epidermal peels of Commelina communis : Cl(-) interferes with vanadate uptake

An H(+) ATPase at the plasma-membrane of guard cells is thought to establish an electrochemical gradient that drives K(+) and Cl(-) uptake, resulting in osmotic swelling of the guard cells and stomatal opening. There are, however, conflicting results regarding the effectiveness of the plasma-membrane H(+)-ATPase inhibitor, vanadate, in inhibiting both H(+) extrusion from guard cells and stomatal opening. We found that 1 mM vanadate inhibited light-stimulated stomatal opening in epidermal peels of Commelina communis L. only at KCl concentrations lower than 50 mM. When impermeant n-methylglucamine and HCl (pH 7.2) were substituted for KCl, vanadate inhibition was still not observed at total salt concentrations≥50 mM. In contrast, in the absence of Cl(-), when V2O5 was used to buffer KOH, vanadate inhibition of stomatal opening occurred at K(+) concentrations as high as 70 mM. Partial vanadate inhibition was observed in the presence of the impermeant anion, iminodiacetic acid (100 mM KHN(CH2CO2H)2). These results indicate that high concentrations of permeant anions prevent vanadate uptake and consequently prevent its inhibitory effect. In support of this hypothesis, an inhibitor of anion uptake, anthracene-9-carboxylic acid, partially prevented vanadate inhibition of stomatal opening. Other anion-uptake inhibitors (1 mM 4,4-diisothiocyanatostilbene-2,2'-disulfonic acid, 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, 200 μM Zn(2+)) were not effective. Decreased vanadate inhibition at high Cl(-)/vanadate ratios may result from competition between vanadate and Cl(-) for uptake. Unlike metabolic inhibitors, vanadate did not affect the extent of stomatal closure stimulated by darkness, further indicating that the observed action of vanadate represents a specific inhibition of the guard-cell H(+) ATPase.

Further Characterization of the Red Beet Plasma Membrane Ca-ATPase Using GTP as an Alternative Substrate

The GTP-driven component of Ca(2+) uptake in red beet (Beta vulgaris L.) plasma membrane vesicles was further characterized to confirm its association with the plasma membrane Ca(2+)-translocating ATPase and assess its utility as a probe for this transport system. Uptake of (45)Ca(2+) in the presence of GTP demonstrated similar properties to those previously observed for red beet plasma membrane vesicles utilizing ATP with respect to pH optimum, sensitivity to orthovanadate, dependence on Mg:substrate concentration and dependence on Ca(2+) concentration. Calcium uptake in the presence of GTP was also strongly inhibited by erythrosin B, a potent inhibitor of the plant plasma membrane Ca(2+)-ATPase. Furthermore, after treatment with EGTA to remove endogenous calmodulin, the stimulation of (45)Ca(2+)-uptake by exogenous calmodulin was nearly equivalent in the presence of either ATP or GTP. Taken together these results support the proposal that GTP-driven (45)Ca(2+) uptake represents the capacity of the plasma membrane Ca(2+)-translocating ATPase to utilize this nucleoside triphosphate as an alternative substrate. When plasma membrane vesicles were phosphorylated with [gamma-(32)P]-GTP, a rapidly turning over, 100 kilodalton phosphorylated peptide was observed which contained an acyl-phosphate linkage. While it is proposed that this peptide could represent the catalytic subunit of the plasma membrane Ca(2+)-ATPase, it is noted that this molecular weight is considerably lower than the 140 kilodalton size generally observed for plasma membrane Ca(2+)-ATPases present in animal cells.

Phenol-acetic acid-urea polyacrylamide gel electrophoresis of membrane proteins

A phenol-acetic acid-urea polyacrylamide gel electrophoretic system (PAU-PAGE) was simplified by adaptation to a slab gel format, allowing the simultaneous comparison of up to 12 samples. The system fractionated most proteins according to molecular mass, although chemical reduction was required since certain proteins (e.g., bovine serum albumin) showed reduction-dependent shifts in mobility. Sodium dodecyl sulfate-PAGE of partially purified membrane proteins can be adversely affected by protein aggregation and proteolysis. PAU-PAGE, which solubilized aqueous insoluble proteins and rapidly inactivated proteases, was useful for assessing the polypeptide composition of plasma membrane preparations.

Electrophoretic characterization of a detergent-treated plasma membrane fraction from corn roots

Experiments were conducted to determine conditions essential for electrophoretic characterization of a detergent-extracted plasma membrane fraction from corn (Zea mays L.) roots. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) initially gave poor resolution of polypeptides in the plasma membrane fraction and, upon detergent treatment for purification of the proton-pumping adenosine triphosphatase (ATPase), showed no enrichment for a 100 kilodalton catalytic subunit characteristic of the ATPase. In contrast to SDS-PAGE, phenol urea acetic acid (PAU)-PAGE clearly resolved two polypeptides in the 100 kilodalton region that were enriched during detergent treatment and indicated at least one polypeptide forms a phosphorylated intermediate characteristic of the ATPase. Problems with SDS-PAGE were found to be caused, in part, by a combination of endogenous proteases and heat-induced aggregation of high molecular weight proteins. The usually standard procedure of boiling the sample prior to SDS-PAGE caused the aggregation of the 100 kilodalton polypeptides. By controlling for proteases using chymostatin and/or phenylmethane sulfonyl floride, and not boiling the sample prior to electrophoresis, two polypeptides were clearly resolved by SDS-PAGE in the 100 kilodalton region of Triton X-114-extracted membranes from corn, oat, barley, and tomato.

Mechanism of Action of Pseudomonas syringae Phytotoxin, Syringomycin : Stimulation of Red Beet Plasma Membrane ATPase Activity

Syringomycin, a peptide toxin produced by the phytopathogen Pseudomonas syringae pv syringae preferentially stimulated (2-fold) the vanadate-sensitive ATPase activity associated with the plasma membrane of red beet storage tissue. The toxin had a very slight effect on the tonoplast ATPase and had no detectable effect on the mitochondrial ATPase. Optimal stimulation was achieved with 10 to 50 micrograms of syringomycin per 25 micrograms of membrane protein. Treatment of membranes with 0.1% (weight/volume) deoxycholate eliminated the activation effect, and enzyme solubilized with Zwittergent 3-14 was not affected by syringomycin. ATPase activity was activated to the same extent at KCl concentrations ranging from 0 to 50 millimolar. Valinomycin, nigericin, carbonylcyanide p-trifluoromethoxyphenylhydrazone, and gramicidin did not increase the plasma membrane ATPase activity. However, these ionophores did not hinder the ability of syringomycin to stimulate the activity. We suggest that syringomycin does not increase ATPase activity by altering membrane ion gradients nor directly interacting with the enzyme, but possibly through regulatory effectors or covalent modification of the enzyme.

ULTRACYTOCHEMICAL AND BIOCHEMICAL STUDIES OF PLASMA MEMBRANE ATPASE IN THE INDIVIDUAL PARTS OF THE MATURE, DORMANT SEED OF PISUM SATIVUM

The location and activity of a K⁺ -ATPase in mature, dormant peas were investigated using two ultracytochemical techniques, as well as biochemical assays of plasma membrane fractions from separate seed parts. Both the Wachstein and Meisel (1957) and the Ernst (1972) cytochemical methods showed plasma membrane-associated reaction product located primarily on the exterior surfaces of the entire pea embryo, except for the stem apex and tip-most cells of the radicle. No plasma membrane-assocated reaction product was found in the seed coat, which typically consists of cells with degenerating protoplasts. Biochemical results showed the highest specific K⁺ -ATPase activity in the radicles (775 nmol Pi/mg protein/hr), followed by epicotyls (168 nmol Pi/mg protein/hr) and cotyledons (147 nmol Pi/mg protein/hr). It is proposed that the entire pea embryo may function in the active absorption of nutrients during the initial phases of germination. Additional functions of the enzyme may include cell wall loosening prior to cell elongation, regulation of cytoplasmic pH, and the generation of turgor.

ADP Is a Competitive Inhibitor of ATP-Dependent H Transport in Microsomal Membranes from Zea mays L. Coleoptiles

An anion-sensitive ATP-dependent H(+) transport in microsomal membranes from Zea mays L. coleoptiles was partially characterized using the pH gradient-dependent decrease of unprotonated neutral red. The following criteria strongly suggest a tonoplast origin of the H(+) transport observed: strict dependence on Cl(-); inhibition by SO(4) (2-) and NO(3) (-); insensitivity against vanadate, molybdate, and azide; reversible inhibition by CaCl(2) (H(+)/Ca(2+) antiport); inhibition by diethylstilbestrol. The substrate kinetics revealed simple Michaelis Menten kinetics for ATP in the presence of 1 millimolar MgCl(2) with a K(m) value of 0.56 millimolar (0.38 millimolar for MgATP). AMP and c-AMP did not influence H(+) transport significantly. However, ADP was a potent competitive inhibitor with a K(i) value of 0.18 millimolar. The same inhibition type was found for membranes prepared from primary roots by the same procedure.

The proton pumps of the plasmalemma and the tonoplast of higher plants

Studies on intact cells, membrane vesicles, and reconstituted proteoliposomes have demonstrated in higher plants the existence of an ATP-driven electrogenic proton pump operating at the plasmalemma. There is also evidence of a second ATP-driven H+ pump localized at the tonoplast. The characteristics of both these ATP-driven pumps closely correspond to those of the plasmalemma and tonoplast proton pumps of Neurospora and yeasts.

Characterization of the solubilized plasma membrane ATPase of red beet

The plasma membrane ATP-phosphohydrolase (ATPase) from red beet (Beta vulgaris L.) storage tissue was solubilized with the zwitterionic detergent Zwittergent 3-14 from a plasma membrane-enriched fraction which was extracted with the anionic detergent, sodium deoxycholate. For both the extraction of extraneous proteins by deoxycholate and the solubilization of active plasma membrane ATPase by Zwittergent 3-14, the optimal concentration of detergent was 0.1% (weight per volume) with a detergent to protein ratio of 1.0 (milligram per milligram). The properties of the solubilized ATPase were found to be similar to the membrane-bound enzyme with respect to pH optimum, substrate specificity, inhibitor sensitivity, and kinetics of K(+) stimulation. The solubilized ATPase preparation formed a rapidly turning over phosphoenzyme, the breakdown velocity of which was increased in the presence of 50 millimolar KCl. Solubilization with 0.1% Zwittergent 3-14 following extraction with 0.1% deoxycholate resulted in an increase in both ATPase activity and steady state phosphoenzyme level; however, a direct correspondence between the increase in ATPase activity and phosphorylation level did not exist. It is proposed that this discrepancy may be the result of a detergent-mediated modification of kinetic rate constants in the mechanism of the enzyme.

Evidence for a beta-Aspartyl Phosphate Residue in the Phosphorylated Intermediate of the Red Beet Plasma Membrane ATPase

A borohydride reduction method was used to identify the phosphorylated amino acid in the phospho-enzyme of the red beet (Beta vulgaris L.) plasma membrane ATPase. Plasma membrane fractions were phosphorylated with unlabeled ATP in the presence of MgSO(4) at pH 6.5 and then treated with sodium [(3)H]borohydride. The borohydride-treated samples were subjected to hydrolysis in 6 normal HCl at 110 degrees C for 22 hours and then analyzed by high voltage paper electrophoresis and thin layer chromatography. This analysis demonstrated the formation of labeled homoserine as the major reduction product when phosphorylated membrane samples were treated with sodium [(3)H]borohydride. This suggests that the phosphoryl group in the plasma membrane ATPase of red beet storage tissue is attached to the beta-carboxyl side chain of an aspartic acid residue in the active site of the enzyme.

Phosphorylated intermediate of a transport ATPase and activity of protein kinase in membranes from corn roots

A maize-root microsomal fraction was enriched in ATPase by treatment with Triton X-100. This activity, which reached 1.2-2.0/mumol Pi x min-1 x mg protein-1, was specific for ATP, very slightly stimulated by K+, inhibited by orthovanadate and diethylstilbestrol, resistant to oligomycin and azide, and had a Km of 1.2 mM MgATP. Incubation of the microsomal fraction with [gamma 32-P]ATP followed by electrophoresis in acid conditions revealed the presence of several phosphoproteins. The phosphorylation of a 110000-Mr polypeptide reached the steady-state level in less than 5 s and rapidly turned over the phosphate group. The phosphorylation level was an hyperbolic function of the [ATP] with a Km of 0.6 mM, suggesting that the rate of Pi production was proportional to the phosphoprotein concentration. The extent of phosphoprotein was decreased by vanadate and diethylstilbestrol. The phosphorylation level was 30% decreased by 50 mM K+ or Na+ while the ATPase activity was slightly stimulated (12% and 5%, respectively). The polypeptide could not be phosphorylated in reverse by Pi. This phosphorylated intermediate from maize-root microsomes exhibits molecular properties characteristic of transport ATPases such as the yeast plasma membrane H+-translocating ATPase. This similarity indicates existence of a transport ATPase in plant plasma membranes. Three other plant microsomal polypeptides (Mr = 52000, 17000 and 16000) and a low molecular weight component (Mr less than 1000) were phosphorylated much more slowly, were not undergoing a rapid turnover and were not hydrolysed by hydroxylamine. These phosphoproteins and the Mr less than 1000 phosphorylated component were inhibited by vanadate and diethylstilbestrol. These properties are similar to those of the protein kinase activity recently described in yeast plasma membranes.

Role of magnesium in the plasma membrane ATPase of red beet

The phosphorylation technique was used to assess the role of Mg in the red beet (Beta vulgaris L.) plasma membrane ATPase. When an excess of ethylenediaminetetraacetate (Tris salt, pH 6.5) was added to phosphorylation reactions at steady-state, the phosphorylation level declined exponentially and the rate constant for dephosphorylation was similar to that observed when phosphorylation reactions were chased with unlabeled ATP. When KCl was included with the EDTA chase, a 2.4-fold increase in the turnover of the phosphoenzyme was observed. Thus, the formation of the phosphorylated intermediate but not its breakdown requires free Mg to be present. When an excess of unlabeled ATP containing MgSO(4) was added to plasma membranes incubated for 20 seconds with [gamma-(32)P]ATP in the absence of MgSO(4), a burst of phosphorylation was observed that declined exponentially. The rate constant for this decline was similar to that observed for phosphoenzyme turnover after initial labeling in the presence of MgSO(4). Extrapolation of this kinetic plot to zero time indicated that ATP binding can occur when MgSO(4) is absent. It is proposed that Mg has a specific role in the transphosphorylation reaction of the terminal phosphate group of ATP to the enzyme.

Plasma membrane ATPase of red beet forms a phosphorylated intermediate

When a plasma membrane-enriched fraction isolated from red beet (Beta vulgaris L.) was incubated in the presence of 40 micromolar [gamma-(32)P] ATP, 40 micromolar MgSO(4) at pH 6.5, a rapidly turning over phosphorylated protein was formed. Phosphorylation of the protein was substrate-specific for ATP, sensitive to diethylstilbestrol and vanadate, but insensitive to azide. When the dephosphorylation reaction was specifically studied, KCl was found to increase the turnover of the phosphorylated protein consistent with its stimulatory effect upon plasma membrane ATPase. The protein-bound phosphate was found to be most stable at a pH between 2 and 3 and under cold temperature, suggesting that the protein phosphate bond was an acyl-phosphate. When the phosphorylated protein was analyzed with lithium dodecyl sulfate gel electrophoresis, a labeled polypeptide with a molecular weight of about 100,000 daltons was observed. Phosphorylation of this polypeptide was rapidly turning over and Mg-dependent. It is concluded that the phosphorylation observed represents a reaction intermediate of the red beet plasma membrane ATPase.