Prelysosomal acidic vacuoles in Dictyostelium discoideum

A PKC that controls polyphosphate levels, pinocytosis and exocytosis, regulates stationary phase onset in Dictyostelium

Many cells can pause their growth cycle, a topic much enriched by studies of the stationary phase (SP) of model microorganisms. While several kinases are implicated in SP onset, a possible role for protein kinase C remains unknown. We show that Dictyostelium discoideum cells lacking pkcA entered SP at a reduced cell density, but only in shaking conditions. Precocious SP entry occurs because extracellular polyphosphate (polyP) levels reach a threshold at the lower cell density; adding exopolyphosphatase to pkcA- cells reverses the effect and mimics wild type growth. PkcA's regulation of polyP depended on inositol hexakisphosphate kinase and phospholipase D. PkcA- mutants also had higher actin levels, higher rates of exocytosis and lower pinocytosis rates. Postlysosomes were smaller and present in fewer pkcA- cells, compared to the wildtype. Overall, the results suggest that a reduced PkcA level triggers SP primarily because cells do not acquire or retain nutrients as efficiently, thus mimicking, or amplifying, the conditions of actual starvation.

Alveolar macrophages and type I IFN in airway homeostasis and immunity

Globally, respiratory infections cause more than 4 million deaths per year, with influenza and tuberculosis (TB) in particular being major causes of mortality and morbidity. Although immune cell activation is critical for killing respiratory pathogens, this response must be tightly regulated to effectively control and eliminate invading microorganisms while minimizing immunopathology and maintaining pulmonary function. The distinct microenvironment of the lung is constantly patrolled by alveolar macrophages (Mφ), which are essential for tissue homeostasis, early pathogen recognition, initiation of the local immune response, and resolution of inflammation. Here, we focus on recent advances that have provided insight into the relation between pulmonary Mφ, type I interferon (IFN) signaling, and the delicate balance between protective and pathological immune responses in the lung.

Phagocytosis and host-pathogen interactions in Dictyostelium with a look at macrophages

Research into phagocytosis and host-pathogen interactions in the lower eukaryote Dictyostelium discoideum has flourished in recent years. This chapter presents a glimpse of where this research stands, with emphasis on the cell biology of the phagocytic process and on the wealth of molecular genetic data that have been gathered. The basic mechanistic machinery and most of the underlying genes appear to be evolutionarily conserved, reflecting the fact that phagocytosis arose as an efficient way to ingest food in single protozoan cells devoid of a rigid cell wall. In spite of some differences, the signal transduction pathways regulating phagosome biogenesis are also emerging as ultimately similar between Dictyostelium and macrophages. Both cell types are hosts for many pathogenic invasive bacteria, which exploit phagocytosis to grow intracellularly. We present an overwiew, based on the analysis of mutants, on how Dictyostelium contributes as a genetic model system to decipher the complexity of host-pathogen interactions.

Thioredoxin reductase is required for growth and regulates entry into culmination of Dictyostelium discoideum

The thioredoxin system, consisting of thioredoxin, thioredoxin reductase and NADPH, has been well established to be critical for the redox regulation of protein function and signalling. To investigate the role of thioredoxin reductase (Trr) in Dictyostelium discoideum, we generated mutant cells that underexpress or overexpress Trr. Trr-underexpressing cells exhibited severe defects in axenic growth and development. Trr-overexpressing (TrrOE) cells formed very tiny plaques on a bacterial lawn and had a lower rate of bacterial uptake. When developed in the dark, TrrOE cells exhibited a slugger phenotype, defined by a prolonged migrating slug stage. Like other slugger mutants, they were hypersensitive to ammonia, which has been known to inhibit culmination by raising the pH of intracellular acidic compartments. Interestingly, TrrOE cells showed defective acidification of intracellular compartments and decreased activity of vacuolar H+-ATPase which functions in the acidification of intracellular compartments. Moreover, biochemical studies revealed that the thioredoxin system can directly reduce the catalytic subunit of vacuolar H+-ATPase whose activity is regulated by reversible disulphide bond formation. Taken together, these results suggest that Dictyostelium Trr may be essential for growth and play a role in regulation of phagocytosis and culmination, possibly through the modulation of vacuolar H+-ATPase activity.

Autophagy Gene Disruption Reveals a Non-vacuolar Cell Death Pathway in Dictyostelium

Types of cell death include apoptosis, necrosis, and autophagic cell death. The latter can be defined as death of cells containing autophagosomes, autophagic bodies, and/or vacuoles. Are autophagy and vacuolization causes, consequences, or side effects in cell death with autophagy? Would control of autophagy suffice to control this type of cell death? We disrupted the atg1 autophagy gene in Dictyostelium discoideum, a genetically tractable model for developmental autophagic vacuolar cell death. The procedure that induced autophagy, vacuolization, and death in wild-type cells led in atg1 mutant cells to impaired autophagy and to no vacuolization, demonstrating that atg1 is required for vacuolization. Unexpectedly, however, cell death still took place, with a non-vacuolar and centrally condensed morphology. Thus, a cell death mechanism that does not require vacuolization can operate in this cell death model showing conspicuous vacuolization. The revelation of non-vacuolar cell death in this protist by autophagy gene disruption is reminiscent of caspase inhibition revealing necrotic cell death in animal cells. Thus, hidden alternative cell death pathways may be found across kingdoms and for diverse types of cell death.

Dynamics of the vacuolar H+-ATPase in the contractile vacuole complex and the endosomal pathway ofDictyosteliumcells

The vacuolar H+-ATPase (V-ATPase) is a multi-subunit enzyme that plays important roles in eukaryotic cells. In Dictyostelium, it is found primarily in membranes of the contractile vacuole complex, where it energizes fluid accumulation by this osmoregulatory organelle and also in membranes of endolysosomes, where it serves to acidify the endosomal lumen. In the present study, a fusion was created between vatM, the gene encoding the 100 kDa transmembrane subunit of the V-ATPase, and the gene encoding Green Fluorescent Protein (GFP). When expressed in Dictyostelium cells, this fusion protein, VatM-GFP, was correctly targeted to contractile vacuole and endolysosomal membranes and was competent to direct assembly of the V-ATPase enzyme complex. Protease treatment of isolated endosomes indicated that the GFP moiety, located on the C-terminus of VatM, was exposed to the cytoplasmic side of the endosomal membrane rather than to the lumenal side. VatM-GFP labeling of the contractile vacuole complex revealed clearly the dynamics of this pleiomorphic vesiculotubular organelle. VatM-GFP labeling of endosomes allowed direct visualization of the trafficking of vacuolar proton pumps in this pathway, which appeared to be entirely independent from the contractile vacuole membrane system. In cells whose endosomes were pre-labeled with TRITC-dextran and then fed yeast particles,VatM-GFP was delivered to newly formed yeast phagosomes with the same time course as TRITC-dextran, consistent with transfer via a direct fusion of endosomes with phagosomes. Several minutes were required before the intensity of the VatM-GFP labeling of new phagosomes reached the level observed in older phagosomes, suggesting that this fusion process was progressive and continuous. VatM-GFP was retrieved from the phagosome membrane prior to exocytosis of the indigestible remnants of the yeast particle. These data suggest that vacuolar proton pumps are recycled by fusion of advanced with newly formed endosomes.

Role of esterase gp70 and its influence on growth and development of Dictyostelium discoideum

Gp70 is an esterase originally called crystal protein because of its presence in crystalline structures in aggregation-competent Dictyostelium discoideum cells. Although postulated to break down spore coats, the function of gp70 in vivo was incompletely investigated. Our immunolocalization and biochemical studies of vegetative D. discoideum amoebae show that gp70 was recruited to phagosomes and found in lysosomes. Purified gp70 was effective at hydrolyzing naphthyl substrates with acyl chains typical of lipids and lipopolysaccharides, indicating that the gp70 was involved in digesting endocytosed molecules. The activity of purified gp70 was inhibited by reductants that retarded its electrophoretic mobility and verified the presence of intramolecular disulfide bonds predicted by its amino acid sequence. Compared to wild-type cells, cells overexpressing gp70 were more phagocytically active, had shorter generation times, and produced more fruiting bodies per unit area, while cells lacking gp70 were phagocytically less active with longer doubling times, developed more slowly, and had significantly fewer fruiting bodies per unit area. Consistent with the phenotype of a disrupted metabolism, one-third of the gp70-minus cells were large and multinucleated. Together, these results indicated that despite its crystalline appearance, gp70 was an active esterase involved in both the growth and the development of D. discoideum.

Phagosomal proteins of Dictyostelium discoideum

In recognizing food particles. Dictyostelium cell-surface molecules initiate cytoskeletal rearrangements that result in phagosome formation. After feeding D. discoideum cells latex beads, early phagosomes were isolated on sucrose step gradients. Protein analyses of these vesicles showed that they contained glycoproteins and surface-labeled species corresponding to integral plasma membrane proteins. Cytoskeletal proteins also were associated with phagosomes, including myosin II, actin and a 30 kDa-actin bundling protein. As seen by the acridine orange fluorescence of vesicles containing bacteria, phagosomes were acidified rapidly by a vacuolar H(+)-ATPase that was detected by immunoblotting. Except for the loss of cytoskeletal proteins, few other changes over time were noted in the protein profiles of phagosomes, suggesting that phagosome maturation was incomplete. The indigestibility of the beads possibly inhibited further endocytic processing, which has been observed by others. Since nascent phagosomes contained molecules of both the cytoskeleton and plasma membrane, they will be useful in studies aimed at identifying specific protein associations occurring between membrane proteins and the cytoskeleton during phagocytosis.

A voltage- and K+-dependent K+ channel from a membrane fraction enriched in contractile vacuole of Dictyostelium discoideum

We obtained a membrane fraction enriched in the contractile vacuole by aqueous-polymer two-phase partitioning and its channel activities were analysed by incorporating it into artificial planar lipid bilayers. In asymmetrical KCl solutions (cis, 300 mM/100 mM, trans), we observed single-channel currents of a highly K(+)-selective channel with slope conductance of 102 pS and reversal potential of -20.4 mV, which corresponded to PK+/PCl- = 7. They showed bursts separated by infrequent quiescent periods. At 0 mV the mean open time was 2.0 ms. Among monovalent cations, Na+ and Li+ were impermeable, whereas Rb+ showed permeability equivalent to that of K+, although the unitary conductance was apparently reduced when the current flowed from the Rb+ containing side, suggesting that Rb+ is a permeant blocking ion. The open probability within bursts remained constant at approx.0.6 as long as the holding potential was positive on the cis side with respect to the trans side, but it decreased to 0 at negative potential. This channel was blocked by submillimolar concentrations of quinine and 30 mM TEA+. The open probability-voltage relationship showed a striking dependency on the KCl concentration on either side. This channel may play a role in water transport in this organelle.

Biosynthesis, processing, and intracellular transport of GM2 activator protein in human epidermal keratinocytes. The lysosomal targeting of the GM2 activator is independent of a mannose-6-phosphate signal

The processing, intracellular transport, and endocytosis of the GM2 activator protein (GM2AP), an essential cofactor of beta-hexosaminidase A for the degradation of ganglioside GM2, was investigated in human epidermal keratinocytes. The GM2AP precursor is synthesized as an 18-kDa peptide, which is singly glycosylated, resulting in 22-kDa high mannose and 24-27-kDa complex glycoforms. A small portion of the 22-kDa form bears phosphomannosyl residues. About 30% of the GM2AP precursor is secreted during 12 h after synthesis, consisting almost exclusively of complex glycoforms. In a post-Golgi compartment, the intracellular remainder is converted to a 20-kDa mature form within 24 h, bearing a heavily trimmed N-glycan on a 17-kDa backbone. Interestingly, even nonglycosylated GM2AP is delivered to the lysosome, as shown by tunicamycin treatment and subcellular fractionation. Also, its endocytosis is independent of carbohydrate-linked signals and is even more effective for nonglycosylated GM2AP. We conclude that a mannose-6-phosphate-independent pathway for the lysosomal delivery of GM2AP exists in cultured human keratinocytes.

Acid-activatable cysteine proteinases in the cellular slime mold Dictyostelium discoideum

Studies of the cysteine proteinases of the cellular slime mold Dictyostelium discoideum have been aided by a simple acid treatment step that was incorporated into the standard one-dimensional gelatin-sodium dodecyl sulfate-polyacrylamide gel electrophoresis assay procedure. The step involved immersing the separating gel in 10% (v/v) glacial acetic acid for 30-60 s immediately after electrophoresis. This modified approach revealed the presence of acid-activatable forms of some enzymes with noticeable increases in their ability to hydrolyze gelatin, a substrate present in the sodium dodecyl sulfate-polyacrylamide gels, and peptidyl amidomethylcoumarins. The activation has been analyzed using extracts of dormant spores from which cysteine proteinase activity had previously appeared low or virtually absent. The major acid-activatable proteinase had an apparent molecular mass of 48 kDa. Its activation was not due to autocatalysis as it was not prevented by mercuric chloride, an inhibitor of the enzyme, and was not accompanied by a significant change in electrophoretic mobility. It was most likely due to a conformational change and/or the removal of a low molecular weight inhibitor. The acid treatment has also revealed the presence of acid-activatable cysteine proteinases in vegetative cells, in which cysteine proteinase activity is present at high levels, as well as among enzymes from the developmental cells which have much lower cysteine proteinase activity. Indeed novel developmental forms were detected at some stages. These results provide additional insight concerning cysteine proteinase expression at various stages during development in the slime molds. A developmental model is presented which suggests that the crypticity of the cysteine proteinases in dormant spores may be governed by proton pumps and endogenous lysosomotropic agents.

Localization of cyclic-AMP receptors with acidosomes in Dictyostelium discoideum

Earlier studies have shown that in Dictyostelium discoideum, a buoyant membrane fraction contained approximately 90% of the vacuolar proton pump (V-H(+)-ATPase) activity, leading to its designation acidosomes. It was proposed that acidosomes may be involved in endocytosis, specially in the acidification of endosomes. In this study we further investigated the putative function(s) of acidosomes. The findings suggest that acidosomes contain abundant receptors for cyclic AMP (CAR1) and that it may be the site for recycling of internalized receptors. Acidosomes also contain an abundance of Rab4 (Bush et al. 1994), a marker for early endosomes. By these criteria, we suggest that the acidosomes are analogous to early or recycling endosome present in mammalian cells. These findings suggest that the structure earlier defined biochemically, morphologically and immunologically as acidosomes may represent early and/or recycling endosomes in this protist.

Ca2+/H+ exchange in acidic vacuoles of Trypanosoma brucei

The use of digitonin to permeabilize the plasma membrane of Trypanosoma brucei procyclic and bloodstream trypomastigotes allowed the identification of a non-mitochondrial nigericin-sensitive Ca2+ compartment. The proton ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was able to cause Ca2+ release from this compartment, which was also sensitive to sodium orthovanadate. Preincubation of the cells with the vacuolar H(+)-ATPase inhibitor bafilomycin A1 greatly reduced the nigericin-sensitive Ca2+ compartment. Bafilomycin A1 inhibited the initial rate of ATP-dependent non-mitochondrial Ca2+ uptake and stimulated the initial rate of nigericin-induced Ca2+ release by permeabilized procyclic trypomastigotes. ATP-dependent and bafilomycin A1- and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl)-sensitive Acridine Orange uptake was demonstrated in permeabilized cells. Under these conditions Acridine Orange was concentrated in abundant cytoplasmic round vacuoles by a process inhibited by bafilomycin A1, NBD-Cl, nigericin, and Ca2+. Vanadate or EGTA significantly increased Acridine Orange uptake, while Ca2+ released Acridine Orange from these preparations, thus suggesting that the dye and Ca2+ were being accumulated in the same acidic vacuole. Acridine Orange uptake was reversed by nigericin, bafilomycin A1 and NH4Cl. The results are consistent with the presence of a Ca2+/H(+)-ATPase system pumping Ca2+ into an acidic vacuole, that we tentatively named the acidocalcisome.

Analysis of successive endocytic compartments isolated from Dictyostelium discoideum by magnetic fractionation

A colloidal iron probe was fed to the amoeba Dictyostelium discoideum and chased for different intervals. Successive segments of the endocytic pathway were then isolated magnetically at high yield and purity. There were approx. 500 endocytic vacuoles per cell; their diameters increased from approx. 0.1-0.2 microns after 3 min of feeding to approx. 2 microns after 15 min of feeding and 60 min of chase. The wave-like progression of ingested probes along the endocytic pathway suggested that the transfer of cargo involved a maturation mechanism rather than the shuttling of cargo between stable compartments. The lifetime of primary pinosomes was calculated to be approx. 1 s. Multivesicular bodies were common in the 3 min fraction and abundant in 15 min lysosomes. alpha- and beta-adaptins of molecular masses of approx. 89 and 83 kDa were richer in the 3 min vesicles than in plasma membranes and later endocytic vacuoles. Acid phosphatase, intrinsic vacuole acidity, the vacuolar proton pump protein and pump activity were present at all endocytic stages but rose between the 3 min and 15 min vacuoles and declined thereafter. Bis(monoacyglycero)phosphate or BMP, a lipid characteristic of lysosomes, followed a similar time course; it contributed up to half of the total lipid in lysosomal vacuoles. We conclude that there is both continuity and differentiation along this endocytic pathway.

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.

Clathrin heavy chain is required for pinocytosis, the presence of large vacuoles, and development in Dictyostelium

To investigate the intracellular role of the clathrin heavy chain in living cells, we have used "antisense" RNA to engineer mutant Dictyostelium discoideum cells that are severely deficient in clathrin heavy chain expression. Immunoblots stained with an anti-clathrin heavy chain antiserum revealed that mutant cells contained undetectable amounts of clathrin heavy chain protein. Similarly, Northern blots showed an absence of clathrin heavy chain mRNA. Clathrin heavy chain-deficient Dictyostelium cells were viable, but exhibited growth rates twofold slower than parental cells. Whereas many morphological features of the mutant cells were normal, mutant cells lacked coated pits and coated vesicles. Clathrin-deficient cells were also missing large translucent vacuoles that serve as endosomes and contractile vacuoles. In the absence of clathrin heavy chain, mutant cells displayed three distinct functional defects: (a) impairment in endocytosis of fluid phase markers, but competence in another endocytic pathway, the phagocytosis of solid particles; (b) defects in osmoregulation; and (c) inability to complete the starvation-induced development cycle.

ATP-driven Ca2+/H+ antiport in acid vesicles from Dictyostelium

Amoebae of the cellular slime mold Dictyostelium discoideum possess an extensive and dynamic endomembrane system that includes many types of acidic vacuoles. A light membrane fraction from Dictyostelium, rich in vacuolar-type H(+)-ATPase, has been described [Padh, H., Lavasa, M. & Steck, T.L. (1989) J. Cell Biol. 108, 865-874]. Here, we show that this "acidosomal" fraction also contains a high-affinity vanadate-sensitive Ca2+ uptake activity that is stimulated by the pH gradient formed by the H(+)-ATPase. We attribute this Ca2+ uptake to the presence of a H(+)-countertransporting Ca(2+)-ATPase, pumping Ca2+ into an acidic compartment.

Association of calmodulin and an unconventional myosin with the contractile vacuole complex of Dictyostelium discoideum

mAbs specific for calmodulin were used to examine the distribution of calmodulin in vegetative Dictyostelium cells. Indirect immunofluorescence indicated that calmodulin was greatly enriched at the periphery of phase lucent vacuoles. The presence of these vacuoles in newly germinated (non-feeding) as well as growing cells, and the response of the vacuoles to changes in the osmotic environment, identified them as contractile vacuoles, osmoregulatory organelles. No evidence was found for an association of calmodulin with endosomes or lysosomes, nor was calmodulin enriched along cytoskeletal filaments. When membranes from Dictyostelium cells were fractionated on equilibrium sucrose density gradients, calmodulin cofractionated with alkaline phosphatase, a cytochemical marker for contractile vacuole membranes, at a density of 1.156 g/ml. Several high molecular weight calmodulin-binding proteins were enriched in the same region of the gradient. One of the calmodulin-binding polypeptides (molecular mass approximately 150 kD) cross-reacted with an antiserum specific for Acanthamoeba myosin IC. By indirect immunofluorescence, this protein was also enriched on contractile vacuole membranes. These results suggest that a calmodulin-binding unconventional myosin is associated with contractile vacuoles in Dictyostelium; similar proteins in yeast and mammalian cells have been implicated in vesicle movement.

Induced pinocytosis and endosomal pathways in Amoeba proteus

The mode of endosome formation and processing was studied in Amoeba proteus by using bovine serum albumin (BSA) either coupled to the fluorochrome TRITC or to 12 nm gold particles (Au(12)) as a pinocytosis inducer. The intraendosomal modifications of BSA-TRITC and BSA-Au(12) start with the separation of the ligand complex from receptor sites of the endosomal membrane obviously caused by acidification (1-15 min). Subsequently, the delivery with lysosomal enzymes and the disintegration of the ligand complex into individual components (BSA and Au(12)) occurs (15-30 min). The intracytotic transport of the ligand components to distinct cellular compartments leads to separate endosome populations with a common fate, i.e. fusion with the cell membrane and exocytosis of the lysosomal content. However, a certain amount of vesicles containing the hydrolyzable ligand BSA escapes exocytosis and is delivered to the digestive apparatus thus contributing to the nutrient pool of the cell (30-40 min). The application of antibodies against human or Tetrahymena digestive enzymes resulted in the detection of a 22 kD protein which shows a distinct cross-reactivity with anti-cathepsin D. The protein is localized in the Golgi apparatus, in small vesicles and in secondary lysosomes containing ingested material. In general, induced pinocytosis can mainly be viewed as a survival mechanism developed by A. proteus to overcome extracellular stress situations. The consecutive endocytotic pathways involve the lysosomal system as well as some other intracellular compartments and are comparable to the situation in higher organisms.

Spatial variation of sequestered calcium in the multicellular stage of Dictyostelium discoideum as assayed by chlortetracycline fluorescence

We have used chlortetracycline (CTC) as a fluorescent probe to detect the distribution of sequestered calcium in multicellular stages of Dictyostelium discoideum. Tips of late aggregates, slugs and early culminating masses fluoresce very strongly. Most of the fluorescence is intracellular in origin and emanates from a small number of intense punctate sources. The sources correspond in part to autophagic vacuoles vis. neutral-red staining, acidic digestive vesicles, and may also include intracellular organelles; cytoplasmic fluorescence is much weaker in comparison. The level of fluorescence drops in the middle portion of slugs and rises again in the posteriormost region, though not to as high a level as in the tip. This holds good irrespective of whether CTC is applied only in the neighbourhood of the aggregate centre, only in the aggregate periphery, or to the whole aggregate. We infer that there must be a good deal of mixing in the stages leading from aggregation to slug formation; thus the serial order in which cells enter an aggregate does not bear any relation to their ultimate fates. The other implication of our study is that calcium sequestration is much more extensive in prestalk and anterior-like cells than in prespore cells. These findings are discussed with regard to possible implications for pattern formation.

Developing Dictyostelium cells contain the lysosomal enzyme alpha-mannosidase in a secretory granule

The prespore vesicle (PSV) is an organelle which secretes spore coat proteins and gal/galNAc polysaccharides from prespore cells of Dictyostelium. By combining the techniques of protein A-gold immunocytochemistry and ricin-gold affinity cytochemistry we have demonstrated colocalization of the lysosomal enzyme alpha-mannosidase with gal/galNAc polysaccharides in prespore vesicles and the spore coat. To determine the origin of prespore vesicles a series of pulse-chase experiments were performed. Cells were labeled with [35S]methionine or [35S]sulfate at different times during development and allowed to differentiate in the presence of unlabeled methionine or sulfate for various periods of time. The cells were homogenized and intracellular organelles were separated using Percoll density gradient centrifugation. The distribution of [35S]methionine-labeled alpha-mannosidase and [35S]sulfate-labeled glycoproteins in the Percoll gradients was determined. It was found that prespore vesicles contained protein which was previously found in lysosomes. Newly labeled protein also entered these vesicles. The data suggest that developing Dictyostelium cells either restructure preexisting lysosomes into prespore vesicles or transport protein between these two organelles. We propose that secretory granules and lysosomes may have a common biosynthetic origin and may be evolutionarily related.

Characterization of a vacuolar proton ATPase in Dictyostelium discoideum

Of the total ATPase activity in homogenates of the ameba, Dictyostelium discoideum, approximately one-third was inhibited at pH 7 by 25 microM 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl). Upon isopycnic sucrose density gradient centrifugation, the bulk of the NBD-CI-sensitive ATPase activity was recovered in a major membrane fraction with a broad peak at 1.16 g/ml, well-resolved from markers for plasma membranes, mitochondria, lysosomes and contractile vacuoles. The gradient peak had a specific activity of 0.5 mumol/min per mg protein. The activity was half-inhibited by 1 microM silicotungstate, 2 microM diisothiocyanatostilbene disulfonate (DIDS), 2.5 microM dicyclohexylcarbodiimide (DCCD), 4 microM NBD-CI and 20 microM N-ethylmaleimide (NEM) but was resistant to conventional inhibitors of mitochondrial and plasma membrane ATPase. That this ATPase activity constituted a proton pump was shown by the MgATP-dependent uptake and quenching of Acridine orange fluorescence by partially purified vacuoles. The Acridine orange uptake was specifically blocked by the aforementioned inhibitors. The generation of proton electrochemical gradients was suggested by the stimulation of enzyme activity by protonophores (fatty acids) and cation exchangers (nigericin). Uncoupling stimulated the ATPase activity as much as 20-fold, revealing an unusually high impermeability of the membranes to protons. ATPase activity was also stimulated by halide ions, apparently through a parallel conductance pathway. Under a variety of sensitive test conditions, the reverse enzyme reaction (i.e., incorporation of 32Pi into ATP) was not detected. We conclude that this major H+-ATPase serves to acidify the abundant prelysosomal vacuoles found in D. discoideum (Padh et al. (1989) J. Cell Biol. 108, 865-874). The finding of a vacuolar H+-ATPase in a protist suggests the ubiquity of this enzyme among the eukaryotic kingdoms.