MOLECULAR STRUCTURE AND PHYSIOLOGICAL CHANGES OF MEMBRANE TRANSPORT SYSTEMS OF PLANT VACUOLE.

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We have investigated the membrane transport systems of plant vacuole. The vacuole performs numerous functions vital to cellular homeostasis, including the accumulation of metabolites and ions. These vacuolar functions are supported by the primary and secondary active transport systems. Two proton pumps, namely H^+-ATPase and H^+-pyrophosphatase, acidify the vacuole and create a membrane potential, which is utilized as an energy source for the secondary transport systems. In addition to the two proton pumps, we had analyzed a very hydrophobic protein, tentatively named VM23, which is thought to be a water channel of vacuolar membrane.In this study, we obtained the following findings. (1) H^+-pyrophosphatase widely distributes among green plants including fern and moss. Thus, H^+-pyrophosphatase is a common enzyme of green plant. However, VM23 were not found in the vacuolar membranes from Acetabularia, Chara, Conocephalum, or Kalanchoe. (2) We got a cDNA for barley H^+-pyrophosphatase. In the primary amino acid sequence estimated from the cDNA,we found two important parts, a catalytic part for substrate hydrolysis and a proton-channel forming domain. This information supports our proposal that a single polypeptide of 73 kilodaltons not only hydrolyses inorganic pyrophosphate but also transports protons across the membrane. (3) It is known that protein body transforms to central vacuole in pumpkin cotyledon during germination. Our investigation revealed that in the germinating cotyledons the proton pumps and VM23 are newly synthesized and accumulated in the protein-body membrane as functional forms.