Structure and function of the proton-conducting sector of the vacuolar H(+)-ATPase.

Abstract

Introduction: the vacuolar H +-ATPase Acidification of intracellular compartments, which in many cases is essential for their function, is maintained in a variety of eukaryotic systems by the activity of the vacuolar (V-type) H+-ATPases. These endomembrane-bound proton pumps are responsible for generating the low internal pH of components of the endocytic and secretory pathways, such as clathrin-coated vesicles, Golgi and secretory vesicles, lysosomes and endosomes generated during receptor-mediated endocytosis [l-31. V-ATPases are also found in the vacuolar membranes of plants and fungi, in which the proton motive force generated by the hydrolysis of cytosolic ATP is coupled to movement of ions and solutes into the vacuole [4]. The enzyme therefore performs a central role in ionic homeostasis within the cell. V-type ATPases may also be associated with the plasma membrane. For example, a plasma membrane-bound V-type ATPase is an essential component in the mechanism of bone resorption in osteoclast cells [S]. In recent years it has become more apparent that members of this class of multiprotein complex are of fundamental importance to a great variety of physiological processes, suggesting that they will, in future, be of increasing pharmacological significance [ 61. Although analogous in organization to the F,-F,, ATP synthases of mitochondria and chloroplasts, the V-ATPases operate exclusively as ATPdriven proton pumps. The holocomplex may have a total mass of 650 kDa and comprises a soluble V, sector associated with an integral membrane V, sector [ 1 -3,7]. The V, sector, which is responsible for substrate binding and regulation of activity, may consist of as many as six types of subunit [7 ,8] . The V, sector, which contains the elements responsible