The success of Mycobacterium species as pathogens depends on their ability to maintain an infection inside the phagocytic vacuole of the macrophage. Although the bacteria are reported to modulate maturation of their intracellular vacuoles, the nature of such modifications is unknown. In this study, vacuoles formed around Mycobacterium avium failed to acidify below pH 6.3 to 6.5. Immunoelectron microscopy of infected macrophages and immunoblotting of isolated phagosomes showed that Mycobacterium vacuoles acquire the lysosomal membrane protein LAMP-1, but not the vesicular proton-adenosine triphosphatase (ATPase) responsible for phagosomal acidification. This suggests either a selective inhibition of fusion with proton-ATPase-containing vesicles or a rapid removal of the complex from Mycobacterium phagosomes.

Bone resorption depends on the formation, by osteoclasts, of an acidic extracellular compartment wherein matrix is degraded. The mechanism by which osteoclasts transport protons into that resorptive microenvironment was identified by means of adenosine triphosphate-dependent weak base accumulation in isolated osteoclast membrane vesicles, which exhibited substrate and inhibition properties characteristic of the vacuolar, electrogenic H+-transporting adenosine triphosphatase (H+-ATPase). Identify of the proton pump was confirmed by immunoblot of osteoclast membrane proteins probed with antibody to vacuolar H+-ATPase isolated from bovine kidney. The osteoclast's H+-ATPase was immunocytochemically localized to the cell-bone attachment site. Immunoelectron microscopy showed that the H+-ATPase was present in the ruffled membrane, the resorptive organ of the cell.

The cellular distributions of the kidney form of the erythrocyte band 3 chloride/bicarbonate exchanger and the kidney vacuolar H.transporting ATPase were examined in rat kidney collecting duct by immunoCytochemical stining of adjacent semithin sections. Polyclonal anti-peptide antibodies directed against two regions of murine erythroid band 3 gave a pattern of basolateral labeling similar to that seen with antibodies directed against the entire protein. In the medullary

Vacuolar Hϩ -ATPases (V-ATPases) are a family of electrogenic ATP-driven proton pumps which function in almost every eukaryotic cell. V-ATPases are required to maintain proton gradients between intracellular compartments and for proton secretion from the plasma membrane of certain specialized cells (reviewed in references 32, 42, and 43). In the intracellular compartments, V-ATPases acidify early and late endosomes, lysosomes, and Golgi-derived secretory vesicles, providing the motive force and optimal pH for internalization and dissociation of ligand-receptor complexes, pH-driven secretion, and activation of lysosomal enzymes for protein processing and degradation (43, 48). Plasma membrane VATPases are essential for acid secretion and bicarbonate transport in the proximal tubule and collecting duct of the kidney, pH homeostasis in macrophages and neutrophils, acidification of the extracellular environment by certain tumor cells, and K ϩ secretion in insect midgut cells, (24, 43). V-ATPase-dependent acidification of the extracellular compartment between the osteoclast ruffled membrane and the bone surface is crucial for bone remodeling (23).V-ATPase is a large, multisubunit protein. It is composed of at least 13 subunits with a total molecular mass of about 900 kDa and has two domains, the V 1 domain (catalytic, membrane-associated, 640 kDa) and V o domain (membrane spanning, 260 kDa). The V 1 domain is responsible primarily for ATP hydrolysis. It consists of eight different subunits (A to H) in a stoichiometry of A 3 B 3 CDEFG 2 H 1-2 . V 1 attaches to the proton-translocating V o domain, which is composed of five subunits (a, b, c, cЈ, and cЉ) in a stoichiometry of abcЈcЉc 4 (20, 32, 43). Regulation of V-ATPase function in response to physiological stimuli is thought to be a multilevel process. It includes control of the expression of V-ATPase subunit genes (34, 62), intracellular targeting and translocation from vesicles to the plasma membrane (2, 9, 10), and reversible dissociation of the V o and V 1 domains, entailing inactivation of the pump (30,31,55).Several extrinsic regulatory factors have been reported to control V-ATPase-mediated proton transport. They include bicarbonate concentration and the closely related parameters pH and pCO 2 , mineralocorticoid hormones, endothelin, angiotensin II (reviewed in references 23 and 24), and cytokines, such as interleukin-1 (IL-1) (7) and IL-4 and -13 (59). The underlying intracellular signaling mechanisms remain largely unknown. Involvement of G proteins and protein kinases A and C has been implicated in several studies (23,24). The E subunit of V-ATPase is able to interact directly with the Dbl homology domain of the guanine nucleotide exchange factor