Structural properties of the proton translocating complex of the clathrin-coated vesicle

The clathrin-coated vesicle proton pump is a representative member of the new class of endomembrane proton ATPases that share an inhibitor profile which distinguishes them from classic F1F0 and E1E2-type proton pumps. The coated vesicle proton pump is a large (530 kDa) heteroligomer composed of eight polypeptides with molecular masses of 116, 70, 58, 40, 38, 34, 33 and 17 kDa. The 200-fold purified enzyme catalyses ATP-generated proton pumping when reconstituted in liposomes composed of pure lipids. Subunit function has been determined by partial reaction analysis of subunit and subcomplex activities. The isolated 17 kDa subunit, when co-reconstituted with bacteriorhodopsin, forms a dicyclohexylcarbodiimide-inhibitable proton channel. Selective removal of the 116 kDa subunit transforms the proton ATPase from a Mg2+-activatable to a Ca2+-activatable ATPase. Subsequent dissociation and reconstitution of subunits reveals that the 70, 58, 40 and 33 kDa components are required, in composite, to form a functional ATP-hydrolytic core, and that no single subunit or subcomplex deficient in these subunits can catalyse ATP hydrolysis.

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.

Recombinant SFD isoforms activate vacuolar proton pumps

The vacuolar proton pump of clathrin-coated vesicles is composed of two general sectors, a cytosolic, ATP hydrolytic domain (V1) and an intramembranous proton channel, V0. V1 is comprised of 8-9 subunits including polypeptides of 50 and 57 kDa, termed SFD (Sub Fifty-eight-kDa Doublet). Although SFD is essential to the activation of ATPase and proton pumping activities catalyzed by holoenzyme, its constituent polypeptides have not been separated to determine their respective roles in ATPase functions. Recent molecular characterization of these subunits revealed that they are isoforms that arise through an alternative splicing mechanism (Zhou, Z., Peng, S.-B., Crider, B.P., Slaughter, C., Xie, X.S., and Stone, D.K. (1998) J. Biol. Chem. 273, 5878-5884). To determine the functional characteristics of the 57-kDa (SFDalpha)1 and 50-kDa (SFDbeta) isoforms, we expressed these proteins in Escherichia coli. We determined that purified recombinant proteins, rSFDalpha and rSFDbeta, when reassembled with SFD-depleted holoenzyme, are functionally interchangeable in restoration of ATPase and proton pumping activities. In addition, we determined that the V-pump of chromaffin granules has only the SFDalpha isoform in its native state and that rSFDalpha and rSFDbeta are equally effective in restoring ATPase and proton pumping activities to SFD-depleted enzyme. Finally, we found that SFDalpha and SFDbeta structurally interact not only with V1, but also withV0, indicating that these activator subunits may play both structural and functional roles in coupling ATP hydrolysis to proton flow.

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.

Molecular characterization of the 50- and 57-kDa subunits of the bovine vacuolar proton pump

The vacuolar type proton-translocating ATPase of clathrin-coated vesicles is composed of two large domains: an extramembranous catalytic sector and a transmembranous proton channel. In addition, two polypeptides of 50 and 57 kDa have been found to co-purify with the pump. These proteins, termed SFD (sub-fifty-eight-kDa dimer) activate ATPase activity of the enzyme and couple ATPase activity to proton flow (Xie, X.-S., Crider, B.P., Ma, Y.-M., and Stone, D. K. (1994) J. Biol. Chem. 269, 28509-25815). It has also been reported that the clathrin-coated vesicle proton pump contains AP50, a 50-kDa component of the AP-2 complex responsible for the assembly of clathrin-coated pits, and that AP50 is essential for function of the proton pump (Liu, Q., Feng, Y., and Forgac, M. (1994) J. Biol. Chem. 269, 31592-31597). We demonstrate through the use of anti-AP50 antibody, identical to that of the latter study, that hydroxylapatite chromatography removes AP50 from impure proton pump preparations and that purified proton pump, devoid of AP50, is fully functional. To determine the true molecular identity of SFD, both the 50- and 57-kDa polypeptides were directly sequenced. A polymerase chain reaction-based strategy was used to screen a bovine brain cDNA library, yielding independent full-length clones (SFD-4A and SFD-21); these were identical in their open reading frames and encoded a protein with a predicted mass of 54,187 Da. The SFD-21 clone was then used in a reverse transcription-polymerase chain reaction-based strategy to isolate a related, but distinct, transcript present in bovine brain mRNA. The nucleotide and predicted amino acid sequences of this isolate are identical to SFD-21 except that the isolate contains a 54-base pair insert in the open reading frame, resulting in a protein with a predicted mass of 55,933 Da. Both clones had 16% identity to VMA13 of Saccharomyces cerevisiae. No sequence homology between the SFD clones and AP50 was detectable. Anti-peptide antibodies were generated against an epitope common to the two proteins and to the unique 18-amino acid insert of the larger protein. The former reacted with both components of native SFD, whereas the latter reacted only with the 57-kDa component. We term the 57- and 50-kDa polypeptides SFDalpha and SFDbeta, respectively.

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.

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.

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

Vacuolar-type proton pumps are complex heterooligomers. When dissociated into subcomplexes and subunits, the partial reactions of ATP hydrolysis and transmembranous proton flow can be assigned to isolated domains. Data suggest that the molecular site of ATP hydrolysis resides within the 70-kDa subunit but that ATPase activity likely requires at least three additional subunits of 58, 40, and 33 kDa (Xie, X.-S., and Stone, D. K. (1988) J. Biol. Chem. 263, 9859-9867). We have now cloned and sequenced the 70-kDa subunit from bovine brain and have expressed the protein in insect Sf9 (Spodoptera frugiperda) cells with a recombinant baculovirus. When purified, the protein has no significant ATPase activity but can be photoaffinity labeled with [alpha 32P]ATP and UV irradiation with an apparent Kd of 35 microM. When reconstituted with biochemically prepared 58-, 40-, and 33-kDa polypeptides, the recombinant 70-kDa subunit restores Ca(2+)-activated ATP hydrolysis to a specific activity of 0.6 mumol P(i).mg protein-1.min-1, thus demonstrating that ATP hydrolysis in vacuolar-type proton pumps is dependent upon both the 70-kDa subunit as well as multi-subunit interactions.

Role of a 50-57-kDa polypeptide heterodimer in the function of the clathrin-coated vesicle proton pump

The vacuolar-type proton-translocating ATPase of clathrin-coated vesicles is composed of an integral membrane proton channel (VB) and a peripheral catalytic sector (VC). Native enzyme can catalyze the hydrolysis of both MgATP and CaATP and support proton pumping when reconstituted into liposomes. In contrast, isolated VC catalyzes only Ca(2+)-activated ATP hydrolysis and cannot support proton pumping when reconstituted into liposomes (Xie, X.-S., and Stone, D. K. (1988) J. Biol. Chem. 263, 9859-9867). We now report that solubilized isolated VC can be reassembled with purified VB to restore properties of native enzyme, including Mg(2+)-activated ATP hydrolysis and proton-pumping capability. Investigation of this reassembly revealed that a heterodimer, composed of polypeptides of 50 and 57 kDa, stimulates Ca(2+)-activated ATPase activity of isolated VC 2-fold and Mg(2+)-activated ATPase activity catalyzed by the reassembled pump 9-fold. Moreover, this heterodimer stimulated proton transport by the reassembled pump > 20-fold. When separated from the proton pump, the dimer has no detectable kinase activity. Maximal stimulation occurs at a molar ratio of heterodimer to reassembled pump of 3, implying a structural, nonenzymatic mechanism. These data indicate that the 50-kDa and/or the 57-kDa polypeptide likely plays an essential and potentially regulatory role in the function of the proton-translocating ATPase of clathrin-coated vesicles.

Isolation and reconstitution of a vacuolar-type proton pump of osteoclast membranes

A vacuolar-type proton-translocating ATPase was extracted from ruffled membranes of chicken osteoclasts with 1% polyoxyethylene 9-lauryl ether (C12E9) and was purified 13-fold by glycerol gradient centrifugation. The isolated pump appears by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to have a subunit composition similar to that of the clathrin-coated vesicle proton pump, in that subunits of apparent molecular masses of 116, 71, 57, 40, 39, 33, and 17 kDa are present in the osteoclast pump preparation. In addition, the 116-, 71-, 57-, and 40-kDa components were shown to cross-react with specific antisera generated against the homologous subunits of the clathrin-coated vesicle proton pump. The isolated osteoclast H(+)-ATPase was reconstituted into liposomes prepared from purified lipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cholesterol) by a cholate-dilution, freeze-thaw method. Proton transport catalyzed by the reconstituted pump was inhibited by bafilomycin A1 (10 nM) and N-ethylmaleimide (1 mM) but was insensitive to vanadate. We propose that osteoclast-mediated bone resorption is effected by a vacuolar-type proton pump with functional and structural similarities to that isolated from clathrin-coated vesicles.

Bafilomycin inhibits proton flow through the H+ channel of vacuolar proton pumps

Vacuolar-type proton-translocating ATPases are complex heterooligomers that are characterized by a specific inhibition by bafilomycin A1. These enzymes have a peripheral ATP hydrolytic domain as well as a transmembranous sector. The transmembranous sector has been isolated by glycerol gradient centrifugation, and this subcomplex is composed of polypeptides of 116, 39, and 17 kDa. Both this sector and native holoenzyme were reconstituted into potassium-loaded (150 mM KCl) liposomes prepared from pure lipids. When diluted into potassium-free buffer, a valinomycin-induced membrane potential did not drive proton uptake, as assessed by acridine orange quenching. In contrast, pretreatment of both the reconstituted proton pump and isolated transmembranous sector at pH 4.2 activated a latent proton conductance. Bafilomycin A1 (1 nM) inhibited ATP-energized proton pumping catalyzed by the proton pump, as well as membrane potential-driven proton flow through both the acid-activated proton pump and the isolated proton pore. Thus bafilomycin A1 inhibits vacuolar proton pumps by blocking proton conduction through the proton pore, which we term VB.

Alternative mRNA splicing generates tissue-specific isoforms of 116-kDa polypeptide of vacuolar proton pump

The cDNA encoding the 116-kDa polypeptide of the bovine brain vacuolar-type proton translocating ATPases has been cloned and sequenced. One of five clones differed from all others in that it contained an 18-base pair deletion within the coding region, whereas it was identical to the other clones in overlapping coding and noncoding regions, indicating that this heterogeneity arises through an alternative splicing mechanism. By conventional Northern analysis, only one 4.1-kilobase mRNA was identified in bovine brain, heart, kidney, liver, and spleen. However, a polymerase chain reaction-based analysis revealed two species of mRNA with a tissue-specific distribution. Type I, containing the 18-base pair insert, was found in brain, whereas the truncated (Type II) form was found in all tissues examined. Similar tissue distributions of rat mRNA were observed. The deletion site accounting for this variability occurs within a predicted protease sensitivity motif (PEST site), suggesting that differences in the biological half-life of the two 116-kDa isoforms may exist.

Reconstitution of recombinant 33-kDa subunit of the clathrin-coated vesicle H(+)-ATPase

Evidence suggests that the ATP hydrolytic sector of the clathrin-coated vesicle proton-translocating ATPase is composed of four subunits of molecular masses of 70, 58, 40, and 33 kDa (Xie, X. S., and Stone, D. K. (1988) J. Biol. Chem. 263, 9859-9867). We have now expressed recombinant 33-kDa polypeptide in Escherichia coli and in Spodoptera frugiperda (Sf9) cells. This subunit, renatured and purified from both sources, lacks intrinsic ATPase activity. Co-reconstitution of these recombinant 33-kDa polypeptides and recombinant 40-kDa subunit to a biochemically prepared 70-58-kDa subcomplex results in a 6-fold stimulation of calcium-activated, N-ethyl-maleimide-sensitive ATPase activity, documenting the essential role of the 33- and 40-kDa components in vacuolar type proton pump function and furthering the aim of reconstitution of a purely recombinant hydrolytic core.

Ultrastructural localization of H+ATPase in rabbit cortical collecting duct

In contrast to results obtained in the rat kidney, studies of H+ATPase localization in the rabbit kidney have failed to demonstrate basolateral plasma membrane H+ATPase immunoreactivity in intercalated cells in the cortical collecting duct (CCD). Previous studies have relied on light microscopic immunofluorescence techniques, which have limited resolution. Therefore, the immunogold procedure was used to localize H+ATPase in rabbit collecting ducts at the ultrastructural level. Rabbit kidneys were preserved in vivo with periodate-lysine-paraformaldehyde or glutaraldehyde solutions, and samples of cortex were embedded in Lowicryl K4M. Thin sections were labeled for H+ATPase by the immunogold procedure with a rabbit polyclonal antibody against the 70-kd subunit of bovine brain H+ATPase. Three patterns of localization of H+ATPase were observed. The majority of intercalated cells in the CCD exhibited label over cytoplasmic vesicles only. In these cells, no label was associated with either the apical or basolateral plasma membranes. In a second group of cells, label for H+ATPase was observed along the basolateral plasma membrane and over cytoplasmic vesicles throughout the cell. Rarely, intercalated cells with H+ATPase label along the apical plasma membrane and over the apical cytoplasmic vesicles were observed in the CCD. In the initial collecting tubule and connecting segment, intercalated cells with either pronounced apical or basolateral plasma membrane label prevailed, whereas few cells exhibited label restricted to the cytoplasmic vesicles. In summary, in the rabbit CCD, three patterns of H+ATPase distribution exist in intercalated cells, two of which conform to published models of type A and type B intercalated cells.

Isolation of a protein activator of the clathrin-coated vesicle proton pump

An activator of the clathrin-coated vesicle proton translocating ATPase has been purified 1600-fold from bovine brain. The activator, which requires detergent (polyoxyethylene 9-lauryl ether) for release from clathrin-coated vesicles, is heat-stable, trypsin-sensitive, and has an apparent molecular mass of about 6 kDa as determined by high performance liquid chromatography. The activator stimulates the purified H(+)-ATPase of coated vesicles over 50-fold under acidic conditions. Similarly, the activator stimulates proton pumping catalyzed by the reconstituted proton pump. Importantly, this stimulation of proton pumping is observed only when the activator is reconstituted into the interior of the proteoliposomes. Moreover, the activator protein is demonstrated to protect, and co-sediment with, purified proton pump during glycerol gradient centrifugation performed in the presence of ATP. These observations support the notion that this activator serves to determine the pH set point of acidic endomembranes through interactions with the transmembranous sectors of the proton pump.

Reconstitution of recombinant 40-kDa subunit of the clathrin-coated vesicle H(+)-ATPase

We have proposed a model of the ATP hydrolytic sector of the clathrin-coated vesicle H(+)-ATPase wherein significant catalysis requires four subunits of molecular masses of 70, 58, 40, and 33 kDa (Xie, X.-S., and Stone, D. K. (1988) J. Biol. Chem. 263, 9859-9867). We have cloned and expressed the 40-kDa component in Escherichia coli and have purified the recombinant protein to homogeneity. This subunit lacks ATP hydrolytic capacity, but when reconstituted to a 40 kDa-depleted hydrolytic sector, there is a greater than 20-fold increase in calcium-activated, N-ethylmaleimide-sensitive ATP hydrolysis, indicating that this subunit is required for vacuolar-type proton pump function.

Increased number and microtubule-associated dispersal of acidic intracellular compartments accompany differentiation of cultured human keratinocytes

Intracellular acidic compartments serve several functions, including uptake of nutrients, processing and sorting of secreted and membrane-bound proteins, and even entry of viruses into cells. In this study, we examined the distribution of acidic compartments in normal human keratinocytes cultured in serum-free medium. Acridine orange was used to stain acidic organelles (red fluorescence), and adherent cells were evaluated by fluorescence microscopy and by interactive laser cytometry (ILC). Keratinocytes cultured in low [Ca++] (0.15 mM) exhibited morphologic characteristics associated with basal cells; red acidic vesicles in these cells were aggregated around the nucleus, sparing the peripheral cytoplasm. After 24 h of culture in high [Ca++] (1.5 mM) keratinocytes showed morphologic changes associated with differentiated cells, including increased number and dispersal of red vesicles to the periphery of the cytoplasm. Keratinocytes cultured in 0.15 mM [Ca++], but treated with phorbol 12-myristate 13-acetate (PMA, 5-100 ng/ml) to induce terminal differentiation, developed similar features. Incubation in media with either high [Ca++] or PMA also induced radial extension of the microtubule network, suggesting that the distribution of acidic organelles occurs along this network. Finally, crude keratinocyte membranes were evaluated by radioactive assay for the presence of three ion-translocating ATPase activities, plasma membrane Na/K ATPase, mitochondrial ATPase, and vacuolar H+ pump ATPase, the latter being the activity responsible for acidification of intracellular compartments. Both basaloid and differentiated keratinocytes exhibited similar vacuolar H+ pump ATPase activity, as measured by its sensitivity to bafilomycin.

Vacuolar acidification and bafilomycin-sensitive proton translocating ATPase in human epidermal Langerhans cells

Langerhans cells (LC) are the principal antigen-presenting cells (APC) of squamous epithelia. We have previously shown that freshly isolated LC (fLC) are able to deliver endocytosed membrane MHC class II molecules into acidic environments, and that this capacity is lost when LC are placed in culture (cLC). Inasmuch as processing of antigens requires their passage through acidic compartments, we undertook the present study to examine the ability of fLC and cLC to take up acridine orange, and to identify proton-translocating ATPases in these cells. Using flow cytometry and fluorescence microscopy, acridine orange was observed to accumulate in acidic compartments in both fLC and cLC. Using a radioactive ATPase assay, crude membrane preparations from both fLC and cLC were shown to possess three types of ion-translocating ATPase, based on sensitivity to the following inhibitors: ouabain (Na+, K+ ATPase), oligomycin (mitochondrial F1F0 ATPase), and bafilomycin (vacuolar-type proton pump ATPase); the last type is responsible for acidification in vacuolar compartments. cLC displayed markedly less (less than 50%) total ATPase activity compared to fLC; however, the relative proportions of specific ATPases were similar in fLC and cLC. Combined use of the three inhibitors resulted in abrogation of only 25-40% of the total ATPase activity. Finally, treatment of LC with bafilomycin inhibited both acridine orange uptake and acidification of internalized HLA-DR molecules. These results confirm the ability of both fLC and cLC to acidify vacuolar compartments, thereby suggesting that lack of acidification of endocytosed membrane class II molecules in cultured cells is due to alternative routing to non-acidic organelles.

Structure of the 116-kDa polypeptide of the clathrin-coated vesicle/synaptic vesicle proton pump

A 116-kDa polypeptide has recently been found to be a common component of vacuolar proton pumps isolated from a variety of sources. The 116-kDa subunit of the proton pump was purified from clathrin-coated vesicles of bovine brain, and internal sequences were obtained from proteolytic peptides. Oligonucleotide probes designed from these peptide sequences were utilized in polymerase chain reactions to isolate partial bovine cDNA clones for the protein. Sequences from these were then utilized to isolate rat brain cDNA clones containing the full-length coding region. RNA blots indicate the presence of an abundant 3.9-kilobase message for the 116-kDa subunit in brain, and primer extension analysis demonstrates that the cloned sequence is full-length. The rat cDNA sequences predict synthesis of a protein of 96,267 Da. Analysis of the deduced amino acid sequence of the 116-kDa subunit suggests that it consists of two fundamental domains: a hydrophilic amino-terminal half that is composed of greater than 30% charged residues, and a hydrophobic carboxyl-terminal half that contains at least six transmembrane regions. The structural properties of the 116-kDa proton pump polypeptide agree well with its proposed function in coupling ATP hydrolysis by the cytoplasmic subunits to proton translocation by the intramembranous components of the pump.

Structure of vacuolar proton pumps

At the present time, knowledge of the structure of the proton-translocating ATPase responsible for urinary acidification is far from complete. Key issues awaiting resolution are full definition of subunit structure and an understanding of the interactions among functional domains of this complex hetero-oligomer. Such a detailed analysis is required to begin study of the biogenesis of the vacuolar proton pump and to elucidate the complexities of its molecular regulation.

Isolation and reconstitution of the chloride transporter of clathrin-coated vesicles

Clathrin-coated vesicle acidification is mediated by an endomembrane proton translocating ATPase. This pump is electrogenic, and significant pH gradient formation requires the parallel movement of chloride through a chloride transporter in order to maintain net electroneutrality. We have solubilized, isolated and achieved 270-fold purification of this chloride transporter by means of selective detergent solubilization with cholate and polyoxyethelene 9-lauryl ether (C12E9), hydroxylapatite chromatography, and glycerol gradient centrifugation. Stabilization of the solubilized transporter requires 5 mM dithiothreitol. The partially purified transporter was co-reconstituted with the purified clathrin-coated vesicle proton translocating complex to yield preparations of proteoliposomes capable of valinomycin-independent proton pumping, as assessed by ATP-generated acridine orange quenching. In addition, the chloride transporter was independently reconstituted and was shown to catalyze diisothiocyano-disulfonic acid stilbene-sensitive 36Cl uptake. The anionic conductive selectivity of the reconstituted transporter (chloride = bromide greater than nitrate) exactly matched that of the transporter of native clathrin-coated vesicles. These studies demonstrate that the chloride transporter of vacuolar acidification systems is structurally and functionally dissociable from co-existing proton pumps and allow for investigations of pump-transporter interactions in a resolved system.

Vacuolar proton pumps

Recently a new class of proton-translocating ATPases has been localized to endomembrane compartments in plant, fungal, and mammalian cells. These proton pumps are large hetero-oligomers which have an ATP hydrolytic sector that is functionally and structurally distinct from a transmembranous proton pore. Enzymatic characteristics of these proton pumps are discussed as well as the current state of knowledge regarding subunit composition and function. In addition, recent primary sequence data are discussed which indicate that these proton pumps share a common ancestor with F1F0-type proton pumps of mitochondria.

Human endomembrane H+ pump strongly resembles the ATP-synthetase of Archaebacteria

Preparations of mammalian H+ pumps that acidify intracellular vesicles contain eight or nine polypeptides, ranging in size from 116 to 17 kDa. Biochemical analysis indicates that the 70- and 58-kDa polypeptides are subunits critical for ATP hydrolysis. The amino acid sequences of the major catalytic subunits (58 and 70 kDa) of the endomembrane H+ pump are unknown from animal cells. We report here the complete sequence of the 58-kDa subunit derived from a human kidney cDNA clone and partial sequences of the 70- and 58-kDa subunits purified from clathrin-coated vesicles of bovine brain. The amino acid sequences of both proteins strongly resemble the sequences of the corresponding subunits of the vacuolar H+ pumps of Archaebacteria, plants, and fungi. The archaebacterial enzyme is believed to use a H+ gradient to synthesize ATP. Thus, a common ancestral protein has given rise to a H+ pump that synthesizes ATP in one organism and hydrolyzes it in another and is highly conserved from prokaryotes to humans. The same pump appears to mediate the acidification of intracellular organelles, including coated vesicles, lysosomes, and secretory granules, as well as extracellular fluids such as urine.

Purification of a vanadate-sensitive ATPase from clathrin-coated vesicles of bovine brain

Clathrin-coated vesicle acidification is mediated by an N-ethylmaleimide-sensitive, vanadate-resistant proton-translocating ATPase. This enzyme is a 530-kDa hetero-oligomer which catalyzes ATP-dependent proton pumping when reconstituted (Xie, X. S., and Stone, D. K. (1986) J. Biol. Chem. 261, 2492-2495). We now report the purification of a second ATPase from bovine brain clathrin-coated vesicles which is inhibited by both N-ethylmaleimide (1 mM) and vanadate (10 microM). Localization of the ATPase to clathrin-coated vesicles was demonstrated by the precipitation of ouabain-resistant, vanadate-sensitive ATPase activity with anti-clathrin antibodies. The enzyme was solubilized with 0.1% polyoxyethylene 9-lauryl ether and has been purified 700-fold to a specific activity of 42 mumol of Pi.mg of protein-1.min-1. A molecular mass of 116 kDa was determined by centrifugation in sucrose gradients prepared in H2O and D2O, by high performance liquid chromatography using gel filtration, and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis performed under reducing conditions. The ATPase is unlike any known mammalian E1E2-type ATPase in that it is not inhibited by ouabain or [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) and it is not activated by Na+, K+, or Ca2+.

Partial resolution and reconstitution of the subunits of the clathrin-coated vesicle proton ATPase responsible for Ca2+-activated ATP hydrolysis

The clathrin-coated vesicle proton-translocating complex is composed of a maximum of eight major polypeptides. Of these potential subunits, only the 17-kDa component, which is a proton pore, has been defined functionally (Sun, S.Z., Xie, X. S., and Stone, D. K. (1987) J. Biol. Chem. 262, 14790-14794). ATPase-and proton-pumping activities of the 200-fold purified proton-translocating complex are supported by Mg2+, whereas Ca2+ will only activate ATP hydrolysis. Like Mg2+-activated ATPase activity, Ca2+-supported ATP hydrolysis is inhibited by N-ethylmaleimide, NO3-, and an inhibitory antibody and is stimulated by Cl- and phosphatidylserine. Thus, Ca2+ prevents coupling of ATPase activity to vectoral proton movement, and Ca2+-activated ATPase activity is a partial reaction useful for analyzing the subunit structure required for ATP hydrolysis. The 530-kDa holoenzyme was dissociated with 3 M urea and subcomplexes, and isolated subunits were partially resolved by glycerol gradient centrifugation. No combination of these components yielded Mg2+-activated ATPase or proton pumping. Ca2+-activated ATP hydrolysis was not catalyzed by a subcomplex containing the 70- and 58-kDa subunits but was restored by recombination of the 70-, 58-, 40-, and 33-kDa polypeptides, indicating that these are subunits of the clathrin-coated vesicle proton pump which are necessary for ATP hydrolysis.

Isolation and reconstitution of the dicyclohexylcarbodiimide-sensitive proton pore of the clathrin-coated vesicle proton translocating complex

The clathrin-coated vesicle proton translocating complex is composed of a maximum of eight polypeptides. The function of the components of this system have not been defined. Proton pumping catalyzed by the reconstituted, 200-fold purified proton translocating complex of clathrin-coated vesicles is inhibited 50% at a dicyclohexylcarbodiimide (DCCD)/protein ratio of 0.66 mumol of DCCD/mg of protein. At an identical DCCD/protein ratio, the 17-kDa component of the proton pump is labeled by [14C]DCCD. Through toluene extraction, the 17-kDa subunit has been isolated from the holoenzyme. The 17-kDa polypeptide diminished proteoliposome acidification when coreconstituted with either bacteriorhodopsin or the intact clathrin-coated vesicle proton translocating ATPase. In both instances, treatment of the 17-kDa polypeptide with DCCD restored proteoliposome acidification. Moreover, the proton-conducting activity of the 17-kDa polypeptide is abolished by trypsin digestion. These results demonstrate that the 17-kDa polypeptide present in the isolated proton ATPase of clathrin-coated vesicles is a subunit which functions as a transmembranous proton pore.

Partial purification and characterization of dynein adenosine triphosphatase from bovine sperm

Outer dynein arm polypeptides that possess Mg+2-adenosine triphosphatase (ATPase) activity have been extracted from the flagellar axonemes of demembranated bovine sperm. Electron microscopy of intact and salt-extracted sperm demonstrates a relatively selective removal of the outer dynein arms. The salt extract contains a specific ATPase activity of 55 nmoles inorganic phosphate (Pi)/min/mg protein. Sucrose density gradient centrifugation of this extract results in a 6-fold increase in specific activity of ATPase (333 nmole/Pi/min/mg protein), which sediments as a single 13S peak. Concomitant with the increase in specific activity, there is enrichment of three high molecular weight polypeptides (Mr greater than 300,000) characteristic of dynein heavy chains. ATPase activities in the initial extract and in the 13S peak are inhibited by concentrations of vanadate and erythro-9-[3-2-(hydroxynonyl)]adenine similar to those that inhibit ATPase activity in sea urchin sperm dynein. These findings indicate that outer arm dynein ATPase can be extracted and partially purified from bovine sperm.

Thermolabile proton translocating ATPase and pump activities in a clathrin-coated vesicle fraction from an acidification defective Chinese hamster cell line

We have recently described a mutant of Chinese hamster ovary cells, termed G.7.1, that contains a temperature-sensitive, conditionally lethal mutation resulting in defective vacuolar acidification (Marnell, M. H., Mathis, L. S., Stookey, M., Shia, S.-P., Stone, D.K., and Draper, R. K. (1984) J. Cell Biol. 99, 1907-1916). To further characterize the lesion, clathrin-coated vesicles were partially purified from wild type and G.7.1 cells, and the thermolabilities of vanadate and oligomycin-insensitive, N-ethylmaleimide-sensitive, H+-ATPase activity, 32Pi-ATPase exchange activity, and proton pumping were compared. All three parameters of H+ pump activity were markedly diminished by preincubation at 44 degrees C for vesicles harvested from the G.7.1 cells, but not for those from wild type cells. Phosphatidylserine did not protect against heat inactivation in vesicle fractions prepared from G.7.1 cells. The results suggest that the mutation responsible for defective acidification in G.7.1 cells is expressed at the level of the proton pump of organelles present in our clathrin-coated vesicle-enriched preparation.

Natural history of borderline patients treated by intensive hospitalization

The authors have the largest follow-up sample of long-term treatment ever done. They have studied almost 500 patients, 20 years after discharge from the long-term treatment unit at Psychiatric Institute. This article reviews some of their findings and identifies the patient population that requires and can benefit from long-term treatment.

High-level expression of the 32.5-kilodalton calelectrin in ductal epithelia as revealed by immunocytochemistry

Calelectrins are a family of antigenically related Ca2+-binding proteins that have only recently been described. They have the important property of binding to membranes only in the presence of Ca2+. We systematically studied the tissue localization of one calelectrin, the 32.5-kilodalton species, in rats using immunocytochemistry. We found that high levels were exclusively present in the epithelial cells of bile and pancreatic ducts, renal collecting ducts, bronchial epithelia, and brain ependyma. In all of these organs, the other cells were not immunoreactive. In addition, strong immunoreactivity was found in the intercalated disks of myocardial cells, and mild immunoreactivity was observed in several endocrine tissues. In contrast, the cellular distribution of the 67-kilodalton calelectrin was more diffuse, involving most parenchymal cells in addition to the already-mentioned cells. Due to the presence of high levels of 32.5-kilodalton calelectrin in some cell types, this protein may be used as a histochemical marker for differentiated ductal epithelial cells, some specialized epithelia, myocardial cells, and Paneth cells.

Lipid requirements for reconstitution of the proton-translocating complex of clathrin-coated vesicles

We have recently reported the reconstitution of the clathrin-coated vesicle proton-translocating complex with liposomes prepared from ethanol-extracted crude bovine brain lipids. Reconstitution of proton pumping by the isolated proton ATPase has now been achieved with liposomes prepared from pure lipids. Optimal proton pumping, as assessed by ATP-generated acridine orange quenching and 32Pi-ATP exchange, is achieved with liposomes prepared from phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cholesterol at a weight ratio of 40:26.5:7.5:26, and a lipid-to-protein weight ratio of 200:1. Under such conditions, the extent of decrease of ATP-generated acridine orange quenching is 150-fold greater than that of native clathrin-coated vesicles. Cholesterol is required to stabilize the proteoliposome. Phosphatidylserine, which is most effective in activating ATP-hydrolytic activity of the solubilized enzyme, is not obligatory for reconstitution of proton pumping.

Stimulation of haematopoiesis in primates by continuous infusion of recombinant human GM-CSF

Certain proteins are known to play an important part in the proliferation, differentiation and functional activation of haematopoietic progenitor cells in vitro. These proteins include erythropoietin and various colony-stimulating factors (CSFs), one of which is granulocyte-macrophage colony-stimulating factor (GM-CSF). Recently, both murine and human GM-CSF have been purified to homogeneity and complementary DNAs encoding them have been cloned. Although the in vitro activity of recombinant human GM-CSF has been investigated intensively, little is known about the functional activity of this protein in vivo. There is strong evidence that colony-stimulating activities produced by various human and murine tumour tissues and cell lines can stimulate granulopoiesis in mice, as can human urinary extracts. A partially purified preparation of human urinary colony-stimulating factor, however, proved only marginally effective in stimulating granulopoiesis in humans. All these studies suffer from the lack of a homogeneous preparation of colony-stimulating factor. It has recently been shown that recombinant murine multi-CSF or interleukin-3 can stimulate haematopoiesis in mice in vivo. Large-scale production of recombinant human GM-CSF now permits us to examine its effects in vivo using a primate model. We find that the continuous infusion of GM-CSF in healthy monkeys rapidly elicits a dramatic leukocytosis and a substantial reticulocytosis. A similar effect has been observed in one pancytopenic, immunodeficient rhesus macaque. These results suggest that GM-CSF could prove useful in several clinical situations.

Isolation and reconstitution of the clathrin-coated vesicle proton translocating complex

Clathrin-coated vesicles contain a proton translocating ATPase which is insensitive to azide but inhibited by N-ethylmaleimide. The ATP hydrolytic subunit of this proton pump has been solubilized, partially purified, and reconstituted into H+-ATPase-depleted coated vesicle membranes (Xie, X.-S., Stone, D.K., and Racker, E. (1984) J. Biol. Chem. 259, 11676-11678). In this communication we report that the entire proton transporting complex has been solubilized and purified 200-fold. The complex, when reconstituted into brain lipid liposomes, catalyzes azide-resistant, N-ethylmaleimide-sensitive H+ transport manifested as both generation of a pH gradient and an electrical gradient. The complex has an apparent molecular mass of 530 kDa.

A Chinese hamster ovary cell mutant with a heat-sensitive, conditional-lethal defect in vacuolar function

We describe a mutant derived from Chinese hamster ovary cells that is offt-sensitive for viability and for resistance to certain protein toxins. This mutant, termed G.7.1, grows normally at 34 degrees C but does not grow in Dulbecco's modified Eagle's medium at 39.5 degrees C. However, when this medium is supplemented with FeSO4, the mutant cells will grow at the elevated temperature. At 39.5 degrees C, G.7.1 cells acquire resistance to diphtheria toxin, modeccin, and Pseudomonas aeruginosa exotoxin A, all of which are protein toxins that require endocytosis and exposure to a low pH within vesicles before they can invade the cytosol and kill cells. The properties of mutant G.7.1 could result from a heat-sensitive lesion that impairs vacuolar acidification. We assayed the ATP-stimulated generation of pH gradients across the membrane of vesicles in cell-free preparations from mutant and parental cells by the partitioning of acridine orange into acidic compartments and found that the acidification response of the mutant cells was heat-labile. Altogether the evidence suggests that G.7.1 cells contain a heat-sensitive lesion that impairs vacuolar acidification and that they fail to grow in normal medium at 39.5 degrees C because they cannot extract Fe+3 from transferrin, a process that normally requires exposing transferrin to a low pH within endosomal vesicles.

Activation and partial purification of the ATPase of clathrin-coated vesicles and reconstitution of the proton pump

A N-ethylmaleimide-sensitive ATPase was extracted and partially purified from clathrin-coated vesicles of bovine brain. During purification the enzyme lost activity which was restored by a purified phospholipid fraction from brain. Phosphatidylserine, but no other commercial phospholipids tested, replaced the brain lipid fraction as activator. Particles depleted of the ATPase exhibited no H+ pump activity when reconstituted with brain phospholipids by the cholate dilution procedure. H+ pump activity was restored by incubating the reconstituted vesicles with the partially purified ATPase.

Inhibition of clathrin-coated vesicle acidification by duramycin

Clathrin-coated vesicles contain a proton translocating ATPase which operates in parallel with a chloride transporter (Xie, X.-S., Stone, D.K., and Racker, E. (1983) J. Biol. Chem. 258, 14834-14838). The polypeptide antibiotic, duramycin, has a dual inhibitory effect on clathrin-coated vesicle acidification. Low amounts of duramycin (5 micrograms/100 micrograms of protein) inhibit by 50% the proton translocation facilitated by chloride translocation. Under these conditions duramycin inhibits also 36Cl uptake when driven by either the electrogenic proton pump or by inward directed K+ movement in the presence of valinomycin. Higher amounts of duramycin (20 micrograms/100 micrograms of protein) are needed to inhibit by 50% the proton pump itself, as evidenced by reduced proton translocation facilitated by an outward potassium movement in the presence of valinomycin. In addition, the amount of duramycin needed to inhibit the proton pump corresponded well with the amount needed to inhibit the ouabain-insensitive, N-ethylmaleimide-sensitive ATPase activity of clathrin-coated vesicles.