Reconstitution of the recombinant 70-kDa subunit of the clathrin-coated vesicle H+ ATPase

Tissue distribution and subcellular localization of the ClC-5 chloride channel in rat intestinal cells

ClC-5 is the Cl- channel that is mutated in Dent's disease, an X-chromosome-linked disease characterized by low molecular weight proteinuria, hypercalciuria, and kidney stones. It is predominantly expressed in endocytically active renal proximal cells. We investigated whether this Cl- channel could also be expressed in intestinal tissues that have endocytotic machinery. ClC-5 mRNA was detected in the rat duodenum, jejunum, ileum, and colon. Western blot analyses revealed the presence of the 83-kDa ClC-5 protein in these tissues. Indirect immunofluorescence studies showed that ClC-5 was mainly concentrated in the cytoplasm above the nuclei of enterocytes and colon cells. ClC-5 partially colocalized with the transcytosed polymeric immunoglobulin receptor but was not detectable together with the brush-border-anchored sucrase isomaltase. A subfractionation of vesicles obtained by differential centrifugation showed that ClC-5 is associated with the vacuolar 70-kDa H+-ATPase and the small GTPases rab4 and rab5a, two markers of early endosomes. Thus these results indicate that ClC-5 is present in the small intestine and colon of rats and suggest that it plays a role in the endocytotic pathways of intestinal cells.

Properties of three isoforms of the 116-kDa subunit of vacuolar H+-ATPase from a single vertebrate species. Cloning, gene expression and protein characterization of functionally distinct isoforms in Gallus gallus

Vacuolar H+-ATPases (V-ATPases) are involved in a wide variety of essential cellular processes. An unresolved question is how the cell regulates the activity of these proton pumps and their targeting to distinct cellular compartments. There is growing evidence for the presence of subunit diversity amongst V-pumps, particularly regarding the 116-kDa subunit (called the a subunit). We have cloned and characterized three isoforms (a1, a2 and a3) of this subunit from chicken. The amino-acid sequences of these homologues are approximately 50% similar and their nucleotide differences indicate that they are products of distinct genes. The levels of mRNA expression of these isoforms was quantified by ribonuclease protection analysis. The a1 and a2 isoforms have a similar tissue distribution, with the highest level of mRNA expression in brain, an intermediate level in kidney and relatively low levels in liver and bone. In contrast, the highest level of expression of the a3 isoform is in bone and liver, with a moderate level in kidney, and the lowest level in brain. An antibody against the a1 isoform reacted with a 116 kDa protein in a brain V-ATPase preparation that was not detected in bone or liver V-ATPase preparations, whereas an antibody against the a3 isoform reacted with a 116-kDa peptide in bone and liver, but not brain V-ATPases preparations. The bone and brain V-ATPases showed differential sensitivity to the inhibitors bafilomycin and (2Z,4E)-5-(5,6-dichloro-2-indolyl)-2-methoxy-N-[4-(2, 2,6,6-tetramethyl)piperidinyl]-2,4-pentadienamide. Thus, this work demonstrates the presence of structurally and functionally distinct V-ATPases in a single vertebrate species.

Identification and reconstitution of an isoform of the 116-kDa subunit of the vacuolar proton translocating ATPase

We have identified a cDNA encoding an isoform of the 116-kDa subunit of the bovine vacuolar proton translocating ATPase. The predicted protein sequence of the new isoform, designated a2, consists of 854 amino acids with a calculated molecular mass of 98,010 Da; it has approximately 50% identity to the original isoform (a1) we described (Peng, S.-B., Crider, B. P., Xie, X.-S., and Stone, D.K. (1994) J. Biol. Chem. 269, 17262-17266). Sequence comparison indicates that the a2 isoform is the bovine homologue of a 116-kDa polypeptide identified in mouse as an immune regulatory factor (Lee, C.-K., Ghoshal, K., and Beaman, K.D. (1990) Mol. Immunol. 27, 1137-1144). The bovine a1 and a2 isoforms share strikingly similar structures with hydrophilic amino-terminal halves that are composed of more than 30% charged residues and hydrophobic carboxyl-terminal halves that contain 6-8 transmembrane regions. Northern blot analysis demonstrates that isoform a2 is highly expressed in lung, kidney, and spleen. To determine the possible role of the a2 isoform in vacuolar proton pump function, we purified from bovine lung a vacuolar pump proton channel (VO) containing isoform a2. This VO conducts bafilomycin-sensitive proton flow after reconstitution and acid activation, and supports proton pumping activity after assembly with the catalytic sector (V1) of vacuolar-type proton translocating ATPase (V-ATPase) and sub-58-kDa doublet, a 50-57-kDa polypeptide heterodimer required for V-ATPase function. These data indicate that the a2 isoform of the 116-kDa polypeptide functions as part of the proton channel of V-ATPases.

Subunit G of the vacuolar proton pump. Molecular characterization and functional expression

The vacuolar type proton pump of clathrin-coated vesicles has a multisubunit ATP hydrolytic center that is peripheral to the membrane. Polypeptides present in this domain include the well characterized subunits A, B, C, D, E, and F; SFD, a dimer composed of 50- and 57-kDa polypeptides; and polypeptides termed G and H. Of these, subunits A, B, C, and E have been shown to be necessary but not sufficient for significant ATPase activity; in addition, either polypeptide G or H is also required for ATP hydrolysis (Xie, X.-S. (1996) J. Biol. Chem. 271, 30980-30985). In this study, the polypeptides G and H were purified and directly sequenced. Subsequent molecular analysis has revealed that these proteins are isoforms, which we designate G1 and G2. The cDNAs encoding the rat and bovine brain and chicken osteoclast forms of G1 have been cloned. The open reading frames of the rat and bovine clones encode hydrophilic proteins of 118 amino acids that differ at only five residues; bovine G1 has 36% identity with VMA10, a component of the proton channel of yeast. Northern blot analysis revealed a 1. 0-kilobase pair transcript encoding G1 in bovine brain, kidney, heart, and spleen. The cDNA encoding bovine polypeptide H was cloned and sequenced, revealing this protein to be 64% identical to G1, constituting isoform G2. In Northern blot analysis, a single 1. 7-kilobase pair transcript hybridized with a probe to G2 in brain, but not in heart, kidney, or spleen. An antibody against a bovine G1-specific domain reacts with V pump from bovine brain, kidney, and chromaffin granule, whereas an anti-G2 antibody reacts only with proton pump from brain. The bovine forms of G1 and G2 were subsequently expressed in Escherichia coli and Sf9 cells, respectively, and purified to homogeneity. Reconstitution of ATP hydrolysis was achieved by combination of recombinant subunits A, B, C, and E with either recombinant G1 or G2, demonstrating the role of these isoforms in pump function.

Vacuolar H(+)-ATPase: from mammals to yeast and back

Vacuolar H(+)-adenosine triphosphatase (V-ATPase) is composed of distinct catalytic (V1) and membrane (V0) sectors containing several subunits. The biochemistry of the enzyme was mainly studied in organelles from mammalian cells such as chromaffin granules and clathrin-coated vesicles. Subsequently, mammalian cDNAs and yeast genes encoding subunits of V-ATPase were cloned and sequenced. The sequence information revealed the relation between V- and F-ATPase that evolved from a common ancestor. The isolation of yeast genes encoding subunits of V-ATPase opened an avenue for molecular biology studies of the enzyme. Because V-ATPase is present in every known eukaryotic cell and provides energy for vital transport systems, it was anticipated that disruption of genes encoding V-ATPase subunits would be lethal. Fortunately, yeast cells can survive the absence of V-ATPase by 'drinking' the acidic medium. So far only yeast cells have been shown to be viable without an active V-ATPase. In contrast to yeast, mammalian cells may have more than one gene encoding each of the subunits of the enzyme. Some of these genes encode tissue- and/or organelle-specific subunits. Expression of these specific cDNAs in yeast cells may reveal their unique functions in mammalian cells. Following the route from mammals to yeast and back may prove useful in the study of many other complicated processes.

Purification and properties of a cytosolic V1-ATPase

The native V1 complex of the tobacco hornworm vacuolar type ATPase (V-ATPase) was purified from cytosolic extracts of molting larval midgut. It consisted of the established V-ATPase subunits A, B, and E along with the 14-kDa subunit F and the novel 13-kDa subunit G. The final amount of purified V1 complex made up an unexpectedly high 2% of the total cytosolic protein, with a yield of approximately 0.4 mg/g of tissue. An equally high amount of cytosolic V1 complex was obtained from starving intermolt larvae. By contrast, the cytosolic V1 pool was reduced drastically in feeding intermolt larvae or in larvae that had been refed after starvation. The activity of the membrane-bound V-ATPase holoenzyme was inversely related to the size of the cytosolic V1 pool, suggesting that the insect plasma membrane V-ATPase is regulated by reversible disassembly of the V1 complex as a function of the feeding condition of the larvae. Like F1-ATPases, the purified V1 complex exhibited Ca2+-dependent ATPase activity and, in the presence of 25% methanol, exhibited Mg2+-dependent ATPase activity. Therefore, we designate the native V1 complex, V1-ATPase. Both enzyme activities were completely inhibited by micromolar N-ethylmaleimide. In contrast to the Ca2+-dependent V1-ATPase activity, the Mg2+/methanol-dependent V1-ATPase activity did not decrease with the incubation time and thus was not inhibited by ADP. Methanol appears to induce a conformational change of the V1 complex, leading to enzymatic properties of the V1-ATPase that are similar to those of the membrane-bound V-ATPase holoenzyme. This is the first time that a native and enzymatically active V1 complex has been purified from the cytosol.

Identification of a 14-kDa subunit associated with the catalytic sector of clathrin-coated vesicle H+-ATPase

The clathrin-coated vesicle H+-ATPase is composed of a peripheral catalytic sector (VC) and an integral membrane proton channel (VB), both of which are multiple subunit complexes. This study was conducted to determine if subunit F, previously identified in vacuolar proton pumps of tobacco hornworm and yeast, was present in mammalian pumps. Using a polymerase chain reaction-based strategy, we have isolated and sequenced cDNA clones from bovine and rat brain cDNA libraries. A full-length clone from rat brain encodes a 119-amino acid polypeptide with a predicted molecular mass of 13, 370 Da and with approximately 72 and 49% identity to subunit F of tobacco hornworm and yeast, respectively. Southern and Northern blot analyses indicate that the protein is encoded by a single gene. An anti-peptide antibody, directed against deduced protein sequence, was affinity-purified and shown to react with a 14-kDa polypeptide that is present in a highly purified pump prepared from clathrin-coated vesicles and also isolated VC. When stripped clathrin-coated vacuolars and purified chromaffin granule membranes were treated with KI in the presence of ATP, the 14-kDa subunit was released from both membranes, further indicating that it is part of the peripheral catalytic sector. In addition, direct sequencing of this 14-kDa component of the coated vacuolar proton pump confirmed its identity as a subunit F homologue.

Acidification of lysosomes and endosomes

Lysosomes, endosomes, and a variety of other intracellular organelles are acidified by a family of unique proton pumps, termed the vacuolar H(+)-ATPases, that are evolutionarily related to bacterial membrane proton pumps and the F1-F0 H(+)-ATPases that catalyze ATP synthesis in mitochondria and chloroplasts. The electrogenic vacuolar H(+)-ATPase is responsible for generating electrical and chemical gradients across organelle membranes with the magnitude of these gradients ultimately determined by both proton pump regulatory mechanisms and, more importantly, associated ion and organic solute transporters located in vesicle membranes. Analogous to Na+, K(+)-ATPase on the cell membrane, the vacuolar proton pump not only acidifies the vesicle interior but provides a potential energy source for driving a variety of coupled transporters, many of them unique to specific organelles. Although the basic mechanism for organelle acidification is now well understood, it is already apparent that there are many differences in both the function of the proton pump and the associated transporters in different organelles and different cell types. These differences and their physiologic and pathophysiologic implications are exciting areas for future investigation.