Mouse Atp6f, the gene encoding the 23-kDa proteolipid of vacuolar proton translocating ATPase

Our research on proton pumping ATPases over three decades: their biochemistry, molecular biology and cell biology

ATP is synthesized by F-type proton-translocating ATPases (F-ATPases) coupled with an electrochemical proton gradient established by an electron transfer chain. This mechanism is ubiquitously found in mitochondria, chloroplasts and bacteria. Vacuolar-type ATPases (V-ATPases) are found in endomembrane organelles, including lysosomes, endosomes, synaptic vesicles, etc., of animal and plant cells. These two physiologically different proton pumps exhibit similarities in subunit assembly, catalysis and the coupling mechanism from chemistry to proton transport through subunit rotation. We mostly discuss our own studies on the two proton pumps over the last three decades, including ones on purification, kinetic analysis, rotational catalysis and the diverse roles of acidic luminal organelles. The diversity of organellar proton pumps and their stochastic fluctuation are the important concepts derived recently from our studies.

The a3 isoform of V-ATPase regulates insulin secretion from pancreatic beta-cells

Vacuolar-type H(+)-ATPase (V-ATPase) is a multi-subunit enzyme that has important roles in the acidification of a variety of intracellular compartments and some extracellular milieus. Four isoforms for the membrane-intrinsic subunit (subunit a) of the V-ATPase have been identified in mammals, and they confer distinct cellular localizations and activities on the proton pump. We found that V-ATPase with the a3 isoform is highly expressed in pancreatic islets, and is localized to membranes of insulin-containing secretory granules in beta-cells. oc/oc mice, which have a null mutation at the a3 locus, exhibited a reduced level of insulin in the blood, even with high glucose administration. However, islet lysates contained mature insulin, and the ratio of the amount of insulin to proinsulin in oc/oc islets was similar to that of wild-type islets, indicating that processing of insulin was normal even in the absence of the a3 function. The insulin contents of oc/oc islets were reduced slightly, but this was not significant enough to explain the reduced levels of the blood insulin. The secretion of insulin from isolated islets in response to glucose or depolarizing stimulation was impaired. These results suggest that the a3 isoform of V-ATPase has a regulatory function in the exocytosis of insulin secretion.

Proton pumping ATPases and diverse inside-acidic compartments

Proton-translocating ATPases are essential cellular energy converters that transduce the chemical energy of ATP hydrolysis into transmembrane proton electrochemical potential differences. The structures, catalytic mechanism, and cellular functions of three major classes of ATPases including the F-type, V-type, and P-type ATPase are discussed in this review. Physiological roles of the acidic organelles and compartments contained are also discussed.

Identification of differentially expressed genes representing dendritic cell precursors and their progeny

The development of dendritic cells (DCs) from hematopoietic progenitors is not well understood. Using a spleen-derived long-term culture (LTC) system, it has been possible to continuously generate DCs from progenitors maintained in culture. The nonadherent LTC-DC population is composed of 2 major subsets. These are the small LTC-DC or DC precursors and their progeny, the large LTC-DCs that phenotypically resemble immature DCs. In this study, subtracted cDNA libraries were generated containing sequences differentially expressed in small or large LTC-DCs. Differential screening was then used on plated library clones to select genes expressed in either the small or the large cell population. Real-time polymerase chain reaction (PCR) has been used to verify the selection procedure for several genes of particular interest. Known genes isolated from subtracted libraries were related to stages in DC development and supported previous findings regarding the function of small and large LTC-DCs. Large LTC-DCs expressed a number of immunologically important genes encoding CD86, CCR1, osteopontin, and lysozyme. Small LTC-DCs resembled progenitor DCs expressing genes related to the organization of the cytoskeleton, the regulation of antigen processing, and a number of mitochondrial and ribosomal proteins. Novel transcripts were isolated from small and large LTC-DC-subtracted libraries that could encode novel proteins important in DC development. This study describes changes in gene expression related to the development of CD11c+CD11b+ major histocompatibility complex 2 low (MHC2lo) CD8alpha- DCs from precursors in a stroma-dependent culture system in the absence of exogenous cytokines.

From lysosomes to the plasma membrane: localization of vacuolar-type H+ -ATPase with the a3 isoform during osteoclast differentiation

Osteoclasts generate a massive acid flux to mobilize bone calcium. Local extracellular acidification is carried out by vacuolar type H+-ATPase (V-ATPase) localized in the plasma membrane. We have shown that a3, one of the four subunit a isoforms (a1, a2, a3, and a4), is a component of the plasma membrane V-ATPase (Toyomura, T., Oka, T., Yamaguchi, C., Wada, Y., and Futai, M. (2000) J. Biol. Chem. 275, 8760-8765). To establish the unique localization of V-ATPase, we have used a murine macrophage cell line, RAW 264.7, that can differentiate into multinuclear osteoclast-like cells on stimulation with RANKL (receptor activator of nuclear factor kappaB ligand). The V-ATPase with the a3 isoform was localized to late endosomes and lysosomes, whereas those with the a1 and a2 isoforms were localized to organelles other than lysosomes. After stimulation, the V-ATPase with the a3 isoform was immunochemically colocalized with lysosome marker lamp2 and was detected in acidic organelles. These organelles were also colocalized with microtubules, and the signals of lamp2 and a3 were dispersed by nocodazole, a microtubule depolymerizer. In RAW-derived osteoclasts cultured on mouse skull pieces, the a3 isoform was transported to the plasma membrane facing the bone and accumulated inside podosome rings. These findings indicate that V-ATPases with the a3 isoform localized in late endosomes/lysosomes are transported to the cell periphery during differentiation and finally assembled into the plasma membrane of mature osteoclasts.

Evidence that there are two copies of subunit c" in V0 complexes in the vacuolar H+-ATPase

The proton-translocating core of eukaryotic vacuolar H(+)-ATPase (V-ATPase), V(0) consists of a hexameric arrangement of transmembrane alpha-helices formed from the related polypeptides, subunit c and subunit c". The former is comprised of four transmembrane alpha-helices, whilst the latter has an extra transmembrane domain at its N-terminus. In addition, the fungal form of V(0) contains a minor subunit c-related polypeptide, subunit c'. All three are required for activity of the proton pump in Saccharomyces cerevisiae. We have introduced cysteine residues in the N-terminal extension of subunit c" in a cysteine-free form. All mutant forms are active in the V-ATPase from S. cerevisiae. Oxidation of vacuolar membranes containing the cysteine-replaced forms gave a cross-linked product of 42000Da. Analysis of this species showed it to be a dimeric form of subunit c", and further studies confirmed there are two copies of subunit c" in the V-ATPases in which it is present. Co-expression of double cysteine-replaced forms of both subunit c and c" gave rise to only homotypic cross-linked forms. Also, subunit c oligomeric complexes are present in vacuolar membranes in the absence of subunit c", consistent with previous observations showing hexameric arrangements of subunit c in gap-junction-like membranes. In vitro studies showed subunit c" can bind to subunit c and itself. The extent of binding can be increased by removal of the N-terminal domain of subunit c". This domain may therefore function to limit the copy number of subunit c" in V(0). A deletion study shows that the domain is essential for the activity of subunit c". The results can be combined into a model of V(0) which contains two subunit c" protomers with the extra transmembrane domain located toward the central pore. Thus the predicted stoichiometry of V(0) in which subunit c" is present is subunit c(3):subunit c'(1):subunit c"(2). On the basis of the mutational and binding studies, it seems likely that two copies of subunit c" are next to each other.