Structure and function of the yeast vacuolar membrane proton ATPase

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Yeast Saccharomyces cerevisiae possesses two known types of proton ATPases: mitochondrial F0F1-type ATP synthase and plasma membrane E1E2-type H+-ATPase.
• Vacuolar membrane proton ATPases are essential for maintaining cellular acidity and ionic homeostasis.

Purpose of the Study:

• To characterize a novel H+-translocating ATPase found in the vacuolar membrane of Saccharomyces cerevisiae.
• To elucidate the subunit composition, enzymological properties, and functional characteristics of this newly identified ATPase.

Main Methods:

• Purification of the vacuolar membrane H+-ATPase to near homogeneity.
• Enzymological assays to determine substrate specificities and inhibitor sensitivities.
• Determination of functional molecular masses under different kinetic conditions.
• Cloning and sequencing of structural genes (VMA1, VMA2, VMA3).

Main Results:

• A three-subunit H+-ATPase (subunits a, b, and c) was purified from yeast vacuolar membranes.
• The enzyme exhibits distinct properties differentiating it from mitochondrial and plasma membrane ATPases.
• Subunit a contains the catalytic site, and subunit c facilitates proton translocation.
• Functional molecular masses were determined for single- and multi-cycle ATP hydrolysis.
• Gene cloning provided tools for further structural and biogenetic studies.

Conclusions:

• The vacuolar membrane H+-ATPase represents a distinct third class of H+-translocating ATPase in yeast.
• This enzyme is critical for acidifying the vacuole, a key compartment for cellular ionic homeostasis.
• Further molecular studies are facilitated by the cloning of its structural genes.