Distinct expression patterns of different subunit isoforms of the V-ATPase in the rat epididymis.

In the epididymis and vas deferens, the vacuolar H(+)ATPase (V-ATPase), located in the apical pole of narrow and clear cells, is required to establish an acidic luminal pH. Low pH is important for the maturation of sperm and their storage in a quiescent state. The V-ATPase also participates in the acidification of intracellular organelles. The V-ATPase contains many subunits, and several of these subunits have multiple isoforms. So far, only subunits ATP6V1B1, ATP6V1B2, and ATP6V1E2, previously identified as B1, B2, and E subunits, have been described in the rat epididymis. Here, we report the localization of V-ATPase subunit isoforms ATP6V1A, ATP6V1C1, ATP6V1C2, ATP6V1G1, ATP6V1G3, ATP6V0A1, ATP6V0A2, ATP6V0A4, ATP6V0D1, and ATP6V0D2, previously labeled A, C1, C2, G1, G3, a1, a2, a4, d1, and d2, in epithelial cells of the rat epididymis and vas deferens. Narrow and clear cells showed a strong apical staining for all subunits, except the ATP6V0A2 isoform. Subunits ATP6V0A2 and ATP6V1A were detected in intracellular structures closely associated but not identical to the TGN of principal cells and narrow/clear cells, and subunit ATP6V0D1 was strongly expressed in the apical membrane of principal cells in the apparent absence of other V-ATPase subunits. In conclusion, more than one isoform of subunits ATP6V1C, ATP6V1G, ATP6V0A, and ATP6V0D of the V-ATPase are present in the epididymal and vas deferens epithelium. Our results confirm that narrow and clear cells are well fit for active proton secretion. In addition, the diverse functions of the V-ATPase may be established through the utilization of specific subunit isoforms. In principal cells, the ATP6V0D1 isoform may have a physiological function that is distinct from its role in proton transport via the V-ATPase complex.

Association of mouse sorting nexin 1 with early endosomes.

Sorting nexin 1 (SNX1) is a protein that binds to the cytoplasmic domain of plasma membrane receptors. We found that mouse sorting nexin 1 (SNX1) (521 amino acid residues) could partially rescue a yeast vam3 mutant defective in docking/fusion of vacuolar membranes. In mammalian cells, SNX1 is peripherally associated with membrane structures and localized immunochemically with EEA1, a marker protein of early endosomes. These results suggest that SNX1 regulates endocytic trafficking of plasma membrane proteins in early endosomes. Gel filtration of cell lysates and the purified recombinant protein, together with two-hybrid analysis, indicated that SNX1 self-assembles into a complex of approximately 300 kDa.

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

The 23-kDa proteolipid subunit of mouse vacuolar-type proton-translocating ATPase (V-ATPase) was predicted to be a hydrophobic polypeptide of 205 amino acid residues with five putative transmembrane segments. It exhibits sequence similarity to Vma16p of Saccharomyces cerevisiae and vha-4 of Caenorhabdittis elegans (83 and 84%, respectively). Southern blot analysis indicated that the proteolipid is encoded by a single gene, Atp6f, in the mouse genome. Atp6f was mapped to approximately 55 cM on chromosome 4, and its genomic organization is similar to that of the human gene: 8 exons separated by 7 introns, with boundaries matching the GT-AG rule. RNA blotting demonstrated that Atp6f is transcribed as 1.0- and 1.8-kb mRNAs in multiple tissues to varying degrees. The major transcription initiation sites are at -13 and -58 bp upstream of the translation initiation codon. The epitope-tagged 23-kDa protoelipid was localized in endomembrane organelles in CHO cells, as expected for a component of a vacuolar-type proton pump.

ATP synthase F(1) sector rotation. Defective torque generation in the beta subunit Ser-174 to Phe mutant and its suppression by second mutations.

Subunit gamma of the ATP synthase F(1) sector is located at the center of the alpha(3)beta(3) hexamer and rotates unidirectionally during ATP hydrolysis, generating the rotational torque of approximately 45 pN.nm. A mutant F(1) with the betaSer-174 to Phe substitution (betaS174F) in the beta subunit generated lower torque ( approximately 17 pN.nm), indicating that betaS174F is mechanically defective, the first such mutant reported. The defective rotation of betaS174F was suppressed by a second-site mutation, betaGly-149 to Ala, betaIle-163 to Ala, or betaIle-166 to Ala in the same subunit, but not by betaLeu-238 to Ala. These results suggest that the region between betaGly-149 and betaSer-174 plays an important role in the coupling between ATP hydrolysis and mechanical work.

a4, a unique kidney-specific isoform of mouse vacuolar H+-ATPase subunit a.

The vacuolar-type H+ -ATPase (V-ATPase) translocates protons across membranes. Here, we have identified a mouse cDNA coding for a fourth isoform (a4) of the membrane sector subunit a of V-ATPase. This isoform was specifically expressed in kidney, but not in the heart, brain, spleen, lung, liver, muscle, or testis. Immunoprecipitation experiments, together with sequence similarities for other isoforms (a1, a2, and a3), indicate that the a4 isoform is a component of V-ATPase. Moreover, histochemical studies show that a4 is localized in the apical and basolateral plasma membranes of cortical alpha- and beta-intercalated cells, respectively. These results suggest that the V-ATPase, with the a4 isoform, is important for renal acid/base homeostasis.

Four subunit a isoforms of Caenorhabditis elegans vacuolar H+-ATPase. Cell-specific expression during development.

We have identified four genes (vha-5, vha-6, vha-7, and unc-32) coding for vacuolar-type proton-translocating ATPase (V-ATPase) subunit a in Caenorhabditis elegans, the first example of four distinct isoforms in eukaryotes. Their products had nine putative transmembrane regions, exhibited 43-60% identity and 62-84% similarity with the bovine subunit a1 isoform, and retained 11 amino acid residues essential for yeast V-ATPase activity (Leng, X. H., Manolson, M. F., and Forgac, M. (1998) J. Biol. Chem. 273, 6717-6723). The similarities, together with the results of immunoprecipitation, suggest that these isoforms are components of V-ATPase. Transgenic and immunofluorescence analyses revealed that these genes were strongly expressed in distinct cells; vha-5 was strongly expressed in an H-shaped excretory cell, vha-6 was strongly expressed in intestine, vha-7 was strongly expressed in hypodermis, and unc-32 was strongly expressed in nerve cells. Furthermore, the vha-7 and unc-32 genes were also expressed in the uteri of hermaphrodites. RNA interference analysis showed that the double-stranded RNA for unc-32 caused embryonic lethality similar to that seen with other subunit genes (vha-1, vha-4, and vha-11) (Oka, T., and Futai, M. (2000) J. Biol. Chem. 275, 29556-29561). The progenies of worms injected with the vha-5 or vha-6 double-stranded RNA became died at a specific larval stage, whereas the vha-7 double-stranded RNA showed no effect on development. These results suggest that V-ATPases with these isoforms generate acidic compartments essential for worm development in a cell-specific manner.

Rotation of a complex of the gamma subunit and c ring of Escherichia coli ATP synthase. The rotor and stator are interchangeable.

ATP synthase (F0F1) transforms an electrochemical proton gradient into chemical energy (ATP) through the rotation of a subunit assembly. It has been suggested that a complex of the gamma subunit and c ring (c(10-14)) of F0F1 could rotate together during ATP hydrolysis and synthesis (Sambongi, Y., Iko, Y., Tanabe, M., Omote, H., Iwamoto-Kihara, A., Ueda, I., Yanagida, T., Wada, Y., and Futai, M. (1999) Science 286, 1722-1724). We observed that the rotation of the c ring with the cI28T mutation (c subunit cIle-28 replaced by Thr) was less sensitive to venturicidin than that of the wild type, consistent with the antibiotic effect on the cI28T mutant and wild-type ATPase activities (Fillingame, R. H., Oldenburg, M., and Fraga, D. (1991) J. Biol. Chem. 266, 20934-20939). Furthermore, we engineered F0F1 to see the alpha(3)beta(3) hexamer rotation; a biotin tag was introduced into the alpha or beta subunit, and a His tag was introduced into the c subunit. The engineered enzymes could be purified by metal affinity chromatography and density gradient centrifugation. They were immobilized on a glass surface through the c subunit, and an actin filament was connected to the alpha or beta subunit. The filament rotated upon the addition of ATP and generated essentially the same frictional torque as one connected to the c ring. These results indicate that the gammaepsilonc(10-14) complex is a mechanical unit of the enzyme and that it can be used as a rotor or a stator experimentally, depending on the subunit immobilized.

Acidic endomembrane organelles are required for mouse postimplantation development.

Vacuolar-type H(+)-ATPase (V-ATPase) plays a major role in endomembrane and plasma membrane proton transport in eukaryotes. We found that the acidic compartments generated by V-ATPase are present from the one-cell stage of mouse preimplantation embryos. Upon differentiation of trophoblasts and the inner cell mass at the blastocyst stage, these compartments exhibited a polarized perinuclear distribution. PL16(-/-) embryos, lacking the V-ATPase 16-kDa proteolipid (c subunit), developed to the blastocyst stage and were implanted in the uterine epithelium, but died shortly thereafter. This mutant showed severe defects in development of the embryonic and extraembryonic tissues at a stage that coincided with rapid cell proliferation. When cultured in vitro, PL16(-/-) blastocysts could hatch and become attached to the surface of a culture dish, but the inner cell mass grew significantly slower and most cells failed to survive for more than 4 days. PL16(-/-) cells showed impaired endocytosis as well as organellar acidification. The Golgi complex became swollen and vacuolated, possibly due to the absence of the luminal acidic pH. These results clearly indicate that acidic compartments are essential for development after implantation.

Biological nano motor, ATP synthase F(o)F(1): from catalysis to gammaepsilonc(10-12) subunit assembly rotation.

Proton translocating ATPase (ATP synthase), a chemiosmotic enzyme, synthesizes ATP from ADP and phosphate coupling with the electrochemical ion gradient across the membrane. This enzyme has been studied extensively by combined genetic, biochemical and biophysical approaches. Such studies revealed a unique mechanism which transforms an electrochemical ion gradient into chemical energy through the rotation of a subunit assembly. Thus, this enzyme can be defined as a nano motor capable of coupling a chemical reaction and ion translocation, or more simply, as a protein complex carrying out rotational catalysis. In this article, we briefly discuss our recent work, emphasizing the rotation of subunit assembly (gammaepsilonc(10-12)) which is formed from peripheral and intrinsic membrane subunits.

Caenorhabditis elegans senses protons through amphid chemosensory neurons: proton signals elicit avoidance behavior.

Acidic pH is known to cause pain sensation through nociceptive neurons as well as taste transduction in mammals. Caenorhabditis elegans avoids an acidic environment (pH lower than approximately 4.0) formed by organic or inorganic acids. This avoidance behavior was dependent on multiple amphid chemosensory neurons, and inhibited by a mutation of capsaicin receptor homologue, and by the addition of amiloride and ruthenium red (inhibitors of proton-gated Na+ channels and capsaicin receptors, respectively). These results indicate that C. elegans recognizes protons as a nociceptive stimulus, through multiple neurons, which elicits avoidance behavior. It is of special interest that a system similar to that of mammalian signal transduction is responsible for this nematode's acid avoidance.

Requirement of V-ATPase for ovulation and embryogenesis in Caenorhabditis elegans.

Immunofluorescence analysis indicated that VHA-11, the C subunit of Caenorhabditis elegans V-ATPase, was localized in dot-like structures around the nuclei of early embryonic cells and was also detected in embryonic intestinal cells after comma stage. Vital staining with acridine orange showed that the intestinal cells had acidic compartments generated by V-ATPase, consistent with the intracellular localization of VHA-11. RNA interference could efficiently silence vha-11 gene expression: introduction of vha-11 double strand RNA led to embryonic lethality. Worms injected with the vha-11 double strand RNA produced embryos that became lethal. The development of embryos was arrested at various stages. However, their numbers gradually decreased, and the worms eventually became sterile due to the failure of ovulation. Similar results were obtained for RNA interference of the V-ATPase proteolipid genes. These results suggest that V-ATPases, and thus inside-acidic organelles, are required for ovulation and embryogenesis.

Synthase (H(+) ATPase): coupling between catalysis, mechanical work, and proton translocation.

Coupling with electrochemical proton gradient, ATP synthase (F(0)F(1)) synthesizes ATP from ADP and phosphate. Mutational studies on high-resolution structure have been useful in understanding this complicated membrane enzyme. We discuss mainly the mechanism of catalysis in the beta subunit of F(1) sector and roles of the gamma subunit in energy coupling. The gamma-subunit rotation during catalysis is also discussed.

Cycloaromatization and DNA cleavage of novel non-conjugated aromatic enetetrayne systems.

The novel, non-conjugated aromatic enetetrayne (2) underwent thermal cycloaromatization reaction to give polyphenylene derivative 6, forming a methyl cation as an active intermediate, and showed DNA-cleaving activity.

Upstream regions directing heart-specific expression of the GATA6 gene during mouse early development.

The expression of murine transcription factor GATA6 is restricted to tissues including the heart and gastrointestinal systems during embryogenesis, and is maintained throughout postnatal life. We have characterized the 5' upstream region (6.4 kb) of the mouse GATA6 gene, and identified two closely spaced transcription initiation sites. The flanking sequence lacks a typical TATA-box, and is rich in guanine and cytosine. The role of the 5' upstream region was examined using the lacZ reporter gene in transgenic mice. A construct containing the 5' flanking sequence (4.9 kb), untranslated exon 1 and 1.3 kb intron 1 could drive the gene expression in the embryonic and adult heart regions. Weak expression was also observed in the stomach, liver, and bronchial arch in addition to the cardiac region. Deletion of the 5' upstream region ( approximately 1.2 kb) or intron 1 abolished all this expression, indicating that at least two cis-acting control elements are necessary for heart-specific expression of GATA6 in vivo.

Escherichia coli ATP synthase alpha subunit Arg-376: the catalytic site arginine does not participate in the hydrolysis/synthesis reaction but is required for promotion to the steady state.

The three catalytic sites of the F(O)F(1) ATP synthase interact through a cooperative mechanism that is required for the promotion of catalysis. Replacement of the conserved alpha subunit Arg-376 in the Escherichia coli F(1) catalytic site with Ala or Lys resulted in turnover rates of ATP hydrolysis that were 2 x 10(3)-fold lower than that of the wild type. Mutant enzymes catalyzed hydrolysis at a single site with kinetics similar to that of the wild type; however, addition of excess ATP did not chase bound ATP, ADP, or Pi from the catalytic site, indicating that binding of ATP to the second and third sites failed to promote release of products from the first site. Direct monitoring of nucleotide binding in the alphaR376A and alphaR376K mutant F(1) by a tryptophan in place of betaTyr-331 (Weber et al. (1993) J. Biol. Chem. 268, 20126-20133) showed that the catalytic sites of the mutant enzymes, like the wild type, have different affinities and therefore, are structurally asymmetric. These results indicate that alphaArg-376, which is close to the beta- or gamma-phosphate group of bound ADP or ATP, respectively, does not make a significant contribution to the catalytic reaction, but coordination of the arginine to nucleotide filling the low-affinity sites is essential for promotion of rotational catalysis to steady-state turnover.

Three subunit a isoforms of mouse vacuolar H(+)-ATPase. Preferential expression of the a3 isoform during osteoclast differentiation.

Vacuolar H(+)-ATPase (V-ATPase) is a multi-subunit enzyme with a membrane peripheral catalytic (V(1)) and an intrinsic (V(o)) sector. We have identified three cDNA clones coding for isoforms of mouse V(o) subunit a (a1, a2, and a3). They exhibit 48-52% identity with each other and high similarity to subunit a of other species. The a1 isoform was mainly expressed in brain and liver. The a2 isoform was observed in heart and kidney in addition to brain and liver. Transcripts for the a3 isoform were strongly expressed in heart and liver. The a3 isoform was induced during osteoclast differentiation, and localized in the plasma membrane and cytoplasmic filamentous structures. In contrast to a3, the a1 isoform was constitutively expressed and localized in the cytoplasmic endomembrane compartments of the same cells. These findings suggest that the a3 isoform is a component of the plasma membrane V-ATPase essential for bone resorption.

Syntaxin 7 mediates endocytic trafficking to late endosomes.

The lysosome functions are ensured by accurate membrane trafficking in the cell. We found that mouse syntaxin 7 could complement yeast vam3 and pep12 mutants defective in docking/fusion to vacuolar and prevacuolar membranes, respectively. Immunohistochemical studies showed that syntaxin 7 is localized to late endosomes, but not to early endosomes. Induced expression of mutant syntaxin 7 blocked endocytic transport from early to late endosomes but did not block the transport of cathepsin D and lamp-2 from the trans-Golgi network to lysosomes. Thus, syntaxin 7 mediates the endocytic trafficking from early endosomes to late endosomes and lysosomes. These results also suggest that the biosynthetic pathway utilizes a different machinery from that of the endocytic pathway in the docking/fusion to late endosomes.

Regulation and reversibility of vacuolar H(+)-ATPase.

Arabidopsis thaliana vacuolar H(+)-translocating pyrophosphatase (V-PPase) was expressed functionally in yeast vacuoles with endogenous vacuolar H(+)-ATPase (V-ATPase), and the regulation and reversibility of V-ATPase were studied using these vacuoles. Analysis of electrochemical proton gradient (DeltamuH) formation with ATP and pyrophosphate indicated that the proton transport by V-ATPase or V-PPase is not regulated strictly by the proton chemical gradient (DeltapH). On the other hand, vacuolar membranes may have a regulatory mechanism for maintaining a constant membrane potential (DeltaPsi). Chimeric vacuolar membranes showed ATP synthesis coupled with DeltamuH established by V-PPase. The ATP synthesis was sensitive to bafilomycin A(1) and exhibited two apparent K(m) values for ADP. These results indicate that V-ATPase is a reversible enzyme. The ATP synthesis was not observed in the presence of nigericin, which dissipates DeltapH but not DeltaPsi, suggesting that DeltapH is essential for ATP synthesis.