Organellar proton-ATPases

Functional properties of a hybrid vacuolar H(+)-ATPase in Saccharomyces cells expressing the Nephrops 16-kDa proteolipid

The hydrophobic 16-kDa polypeptide which forms gap-junction-like structures in the crustacean Nephrops norvegicus is a member of a highly conserved family of proteolipids involved in a variety of membrane transport functions in eukaryotic cells. This family also includes the product of the Saccharomyces cerevisiae VMA3 gene which encodes an integral membrane component of the vacuolar membrane H(+)-ATPase. The cDNA for the Nephrops proteolipid complements a mutation in the yeast VMA3 gene, resulting in assembly of a hybrid H(+)-ATPase comprising yeast catalytic subunits and Nephrops integral membrane components. The hybrid vacuolar ATPase was capable of ATP hydrolysis which was coupled to proton translocation and showed inhibitor binding and enzymological properties similar to those of wild-type V-ATPases (Km for ATP, 0.4 mM), suggesting that both yeast and crustacean proteolipids share conserved structure at regions of protein interaction. To facilitate isolation of the Nephrops proteolipid by affinity chromatography on a Ni(2+)-binding support, six C-terminal histidine residues were added to the proteolipid. This modification did not prohibit assembly into the hybrid H(+)-ATPase, although the resultant enzyme did have a markedly elevated Km (1.8 mM). The membrane-bound Vo sector of the ATPase was isolated by the affinity-chromatography procedure and reconstituted into synthetic vesicles. This complex was found to be impermeable to small cations in the absence of catalytic ATPase subunits either in situ in the vacuolar membrane or in the reconstituted system. The functional significance of this impermeability and the structure/function relationships between proteolipids from different sources are discussed.

Lack of acidification in Mycobacterium phagosomes produced by exclusion of the vesicular proton-ATPase

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.

An immunocytochemical analysis of the vacuolar proton pump in Dictyostelium discoideum

Antisera were generated in rabbits against the vacuolar proton pump (V-H(+)-ATPase) purified from Dictyostelium discoideum. The antisera inhibited V-H(+)-ATPase but not F1-ATPase activity and immunoprecipitated and immunoblotted only the polypeptide subunits of the V-H(+)-ATPase from cell homogenates. Immunocytochemical analysis of intact cells and subcellular fractions showed that the predominant immunoreactive organelles were clusters of empty, irregular vacuoles of various sizes and shapes, which corresponded to the acidosomes. The cytoplasmic surfaces of lysosomes, phagosomes and the tubular spongiome of the contractile vacuole also bore the pump antigen. The lumina of multivesicular bodies were often stained intensely; the internalized antigen may have been derived from acidosomes by autophagy. Antibodies against V-H(+)-ATPases from plant and animal cells cross-reacted with the proton pumps of Dictyostelium. Antisera directed against the V-H(+)-ATPase of Dictyostelium decorated a profusion of small vacuoles scattered throughout the cytoplasm of hepatocytes, epithelial cells, macrophages and fibroblasts. The pattern paralleled that of the endocytic and acidic spaces; there was no clear indication of discrete acidosomes in these mammalian cells. We conclude that the V-H(+)-ATPase in Dictyostelium is distributed among diverse endomembrane organelles and is immunologically cross-reactive with the proton pumps on endocytic vacuoles in mammalian cells.

Presynaptic events involved in neurotransmission

Vacuolar H(+)-ATPase (V-ATPase) plays a key role in neurotransmission. It provides the energy for the uptake and storage of neurotransmitters in synaptic vesicles and granules. It also may play a role in the biogenesis of synaptic vesicles as well as in neurosecretion. This is one of the most conserved fundamental enzymes in nature, but functions in a wide variety or organelles and membranes. Its structure, function, molecular biology and biogenesis is discussed in relation to its role in neurotransmission. Termination of neurotransmission is carried out by neurotransmitter transporters that function in the reuptake of the neurotransmitters into the presynaptic cells. We cloned, sequenced and expressed several cDNAs encoding neurotransmitter transporters. Their specificity and site of synthesis revealed some new aspects of neurotransmission.