6. Vacuolar H+ ATPase and Pyrophosphatase

  1. Vacuolar H+-ATPase
    • an electrogenic proton pump - transports protons from the cytoplasm to the vacuole
    • gradient allows pH of the vacuolar sap to be lower and more mV postive than the cytoplasm
    • gradient drives the uptake of cations and sugars into the vacuole via secondary transport antiporter systems
    • electrical potential difference is responsible for uptake of anions such as chloride or malate
  2. V-ATPase structure
    • 12 different subunits
    • 1. Peripheral catalytic complex, V1 - stalk
    • 2. Integral membrane channel complex, V0 -rotor
  3. V-ATPase mechanism of operation
    • like tiny rotary motors
    • hydrolysis of ATP drives rotation of the V1 stalk ->
    • drives the rotation of the V0 complex ->
    • protons are transported generating a PMF across the tonoplast
  4. characteristics of the vacuolar H+-ATPase
    • differs structurally and functionally from the plasma membrane H+-ATPase
    • distinguished physiologically by the action of inhibitors
    • P-ATPase - orthovanadate = phosphate analog that competes with phosphate from ATP to block phosphorylation
    • V-ATPase - antibiotic bafilomycin, and high concentrations of nitrate
    • Neither inhibits the other ATPase
  5. role of the vacuolar H+-ATPase
    • tonoplast - regulates the traffic of ions and metabolites
    • V-ATPase - provides the proton gradient that drives solute accumulation into the vacuoles -> lowers the osmotic potential of the cell
    • critical for plant growth - plant cells enlarge primarily by uptake of water into vacuoles
  6. vacuolar H+-pyrophosphatase
    • works in parallel with the V-ATPase
    • H+ ion gains energy from hydrolysis of PPi
    • induced by chilling or drop in O2 level when ATP levels are depleted
    • -> backup system
  7. characteristics of ABC transporters
    • transport large organic molecules into the vacuole such as flavonoids and anthocyanins
    • use the energy of ATP hydrolysis directly - independent of H+ gradient
    • form phosphorylated intermediates
    • found in vacuolar membrane mostly
  8. classes of ABC transporters
    • multidrug resistance proteins:
    • first identified in human cancer cells
    • helps confer resistance to anticancer drugs
    • expression of P-Glycoprotein 1 - efflux pump

    • multidrug resistance-associated proteins:
    • differed in sequence homology
  9. Plant MRPs
    • most are glutathione-conjugated pumps (GS-X)
    • detoxify herbicides
    • protect against oxidative damage
    • pigment accumulation
    • store anitmicrobial compounds
    • store phytochelatin-Cd2+ complexes
  10. glutathione transferases
    GSH (cytosol) + X (cytosol) ->-> (GSH transferase) GS-X (cytosol) ->-> (GS-X pump) GS-X (vacuole)
  11. ABC transporters genes cloned
    • only MRPs have been defined functionally - GS-X pumps
    • determined by expressing the plant genes in yeast then testing the yeast
    • ex. at MRP2 has been shown to transport GSH-conjugated Chl catabolites
Card Set
6. Vacuolar H+ ATPase and Pyrophosphatase
plant physiology and biochemistry exam 5