Sodium ion-Pumping V-Type ATPase in E. hirae

Enterococcus hirae is resistant against high salt and high pH!

Enterococcus hirae is a gram-positive bacterium. It can grow at high salt and alkaline pH condition. We found that sodium ion-pumping ATPase in the cytoplasmic membrane is responsible for this property. Furthermore, this ATPase is the first described V-type ATPase in an eubacterium.

What is V-type ATPase?

Homeostasis of ion concentrations in a cell is essential for all living cells, including microorganisms, animals and plants. The homeostasis is maintained by several ion transporting proteins, especially ion-pumping ATPases in cell membranes. There are three types of ion-pumping ATPases, P-type ATPase in cytoplasmic membranes, F-type ATPase in mitochondria, chloroplasts or cytoplasmic membranes of bacteria, and V-type ATPase in organellar membranes. F-type and V-type ATPases are the relatives. F-ATPase synthesizes ATP by using energy produced by oxidation of substrates in mitochondria or by light absorption in chloroplasts. V-ATPase utilizes ATP to pump out ions, usually protons in organella in eukaryotic cells.

V-ATPase is important in the pH maintenance (acidification) of inside of organella or outside of osteoclasts. The acidification is profoundly related to cholesterol metabolism, storage of metabolites, autophagy, etc in organella and also to osteoclasis. And these functions are related to many diseases, such as arterial sclerosis, osteoporosis and so on. The acidification mechanism and its regulation, especially those of V-ATPase are studied by many researchers.

We found a bacterial V-ATPase. Bacterial V-type ATPase is produced in a large amount and easily purified. Therefore, we expect that the biochemical and molecular biological studies of our ATPase should give us a fundamental understanding of the properties of V-type ATPases.

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What we have done

• Cloning and sequencing of the sodium ion-pumping ATPase operon
  　　Takase et al., J. Biol. Chem. 269, 11037-11044 (1994)
• Regulation of gene expression of the operon
  (induction by a high concentration of sodium ion)
  　　Murata et al., J. Biol. Chem. 271, 23661-23666 (1996)
• Physiological role of the ATPase
  (Maintenance of sodium and potassium ion concentrations in a cell)
  　　Murata et al., J. Biol. Chem. 271, 10042-10047 (1996)
• Purification and reconstitution of the sodium ion-pumping ATPase function
  　　Murata et al., J. Biol. Chem. 272, 24885-24890 (1997)
  　　Murata et al., J. Biochem (Tokyo) 125, 414-421 (1999)
  
• Structure and function of the ATPase
  　　Takase et al., in preparation
• Study of energy transducing mechanism of the ATPase
  (The coupling ion of our ATPase is sodium ion, which makes our system advantageous.)
  　　Murata et al., in preparation

What we are going to do

Study of structure and function of the ATPase (including X-ray crystallography)

Biochemical properties of the ATPase
(In view of searching for inhibitors as candidates for general drugs against V-ATPases and the elucidation of their inhibition mechanisms.)

reference

F-type ATPase in mitochondria is a relative of V-type ATPase. The three dimensional structure of the catalytic part of F-ATPase has been determined by Walker's group (Abrahams, J. P. et al., Nature 370, 621-628 (1994)). And a collaboration of Yoshida's and Kinoshita's groups in Japan (Noji et al., Nature 386, 299-302 (1997)) has succeeded in showing that the catalytic part of F-ATPase is the smallest propeller motor in the world. We believe that our V-type ATPase has the similar properties, which we would like to show in near future.

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