Immunocytochemical demonstration of the peroxisomal ATPase of yeasts

pH regulation in glycosomes of procyclic form Trypanosoma brucei

Here we report the use of a fluorescein-tagged peroxisomal targeting sequence peptide (F-PTS1, acetyl-C{K(FITC)}GGAKL) for investigating pH regulation of glycosomes in live procyclic form Trypanosoma brucei When added to cells, this fluorescent peptide is internalized within vesicular structures, including glycosomes, and can be visualized after 30-60 min. Using F-PTS1 we are able to observe the pH conditions inside glycosomes in response to starvation conditions. Previous studies have shown that in the absence of glucose, the glycosome exhibits mild acidification from pH 7.4 ± 0.2 to 6.8 ± 0.2. Our results suggest that this response occurs under proline starvation as well. This pH regulation is found to be independent from cytosolic pH and requires a source of Na⁺ ions. Glycosomes were also observed to be more resistant to external pH changes than the cytosol; placement of cells in acidic buffers (pH 5) reduced the pH of the cytosol by 0.8 ± 0.1 pH units, whereas glycosomal pH decreases by 0.5 ± 0.1 pH units. This observation suggests that regulation of glycosomal pH is different and independent from cytosolic pH regulation. Furthermore, pH regulation is likely to work by an active process, because cells depleted of ATP with 2-deoxyglucose and sodium azide were unable to properly regulate pH. Finally, inhibitor studies with bafilomycin and EIPA suggest that both V-ATPases and Na⁺/H⁺ exchangers are required for glycosomal pH regulation.

Alcohol oxidase (AOX1) from Pichia pastoris is a novel inhibitor of prion propagation and a potential ATPase

Previous results suggest that methylotrophic yeasts may contain factors that modulate prion stability. Alcohol oxidase (AOX), a key enzyme in methanol metabolism, is an abundant protein that is specific to methylotrophic yeasts. We examined the effect of Pichia pastoris AOX1 on prion phenotypes in Saccharomyces cerevisiae. The S. cerevisiae prion states [PSI(+)] and [URE3] arise from aggregation of the proteins Sup35p and Ure2p respectively, and correlate with the ability of Sup35p and Ure2p to form amyloid-like fibrils in vitro. We found that expression of P. pastoris AOX1 in S. cerevisiae had no effect on propagation of the [PSI(+)] prion, but inhibited propagation of [URE3]. Addition of AOX1 early in the time-course of fibril formation inhibits Ure2p fibril formation in vitro. AOX1 has not previously been identified as an ATPase. However, we discovered that in addition to its flavin adenine dinucleotide-dependent AOX activity, AOX1 possesses ATPase activity. This study identifies AOX1 as a novel prion inhibitory factor and a potential ATPase.

Peroxisomes as novel players in cell calcium homeostasis

Ca2+ concentration in peroxisomal matrix ([Ca2+](perox)) has been monitored dynamically in mammalian cells expressing variants of Ca2+-sensitive aequorin specifically targeted to peroxisomes. Upon stimulation with agonists that induce Ca2+ release from intracellular stores, peroxisomes transiently take up Ca2+ reaching peak values in the lumen as high as 50-100 microm, depending on cell types. Also in resting cells, peroxisomes sustain a Ca2+ gradient, [Ca2+](perox) being approximately 20-fold higher than [Ca2+] in the cytosol ([Ca2+](cyt)). The properties of Ca2+ traffic across the peroxisomal membrane are different from those reported for other subcellular organelles. The sensitivity of peroxisomal Ca2+ uptake to agents dissipating H+ and Na+ gradients unravels the existence of a complex bioenergetic framework including V-ATPase, Ca2+/H+, and Ca2+/Na+ activities whose components are yet to be identified at a molecular level. The different [Ca2+](perox) of resting and stimulated cells suggest that Ca2+ could play an important role in the regulation of peroxisomal metabolism.

In situ measurements of the pH of mammalian peroxisomes using the fluorescent protein pHluorin

Peroxisomes are metabolically active organelles that participate in the oxidation of long-chain fatty acids and in the biosynthesis of bile acids, cholesterol, and ether phospholipids. Even though maintenance of a stable acid-base milieu is essential for proper peroxisomal function, the determination of the peroxisomal pH (pH(p)) remains inconclusive, and little is known about its regulation. To measure the pH of intact peroxisomes in situ, we used the peroxisome-specific carboxyl-terminal targeting sequence, SKL, to deliver a pH-sensitive mutant of the green fluorescent protein (pHluorin-SKL) selectively into peroxisomes. Proper targeting was verified by colocalization with the peroxisomal marker catalase. Peroxisomes were visualized by imaging fluorescence microscopy, and ratiometric measurements were combined with calibration using ionophores or a null-point method to estimate pH(p). The pH(p) was between 6.9 and 7.1, resembling the cytosolic pH. Manipulation of the cytosolic pH in intact cells or after permeabilization of the plasmalemma with streptolysin O revealed that pH(p) changed in parallel, suggesting that the peroxisomal membrane is highly permeable to H(+) (equivalents). We conclude that peroxisomes do not regulate their pH independently, but instead their large H(+) permeability effectively connects them with the buffer reservoir of the cytoplasm and with the homeostatic mechanisms that control cytosolic pH.

The in vitro permeability of yeast peroxisomal membranes is caused by a 31 kDa integral membrane protein

A major 31 kDa integral peroxisomal membrane protein (PMP31) of Hansenula polymorpha was purified to homogeneity from isolated peroxisomal membranes by FPLC after solubilization by Triton X-100. Biochemical analysis indicated that this protein, which showed cross-reactivity with antibodies against the 31 kDa porin of the mitochondrial outer membrane of Saccharomyces cerevisiae, had pore-forming properties. Firstly, proteoliposomes composed of asolectin and purified PMP31 showed selective permeability, determined as the [14C]sucrose/[3H]dextran leakage ratios. Furthermore, the generation of a delta psi by potassium diffusion gradients was negatively affected by the presence of PMP31 in asolectin liposomes. A similar effect was observed in proteoliposomes containing purified cytochrome c oxidase as a delta psi generating system. Control experiments confirmed that the observed leakage is significant and introduced by the incorporation of PMP31 protein. Selective sucrose leakage was abolished in samples pretreated with glutaraldehyde; an identical effect of glutaraldehyde was, however, not observed for the membrane potential measurements.

Import of proteins into peroxisomes and other microbodies

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ATPase activities in peroxisome-proliferating yeast

Preliminary studies on yeast peroxisomes have suggested that the membrane of these organelles may contain a proton-pumping ATPase. It has been reported that peroxisome-associated activity is similar to the F0-F1 mitochondrial type ATPase in its sensitivity to azide at pH 9.0, but characteristics of the plasma membrane type ATPase are also evident in peroxisomal preparations in that they exhibit pH 6.5 activity that is sensitive to vanadate. A comparative study of the prominent organellar ATPase activities was undertaken as a probe into the existence of an enzyme that is unique to the peroxisome, and biochemical properties of yeast mitochondrial, plasma membrane, together with peroxisomally-associated H(+)-ATPases are presented. Enzyme marker analysis of sucrose gradient fractions revealed a high degree of correlation between the amount of azide-sensitive pH 9.0 ATPase activity and that of the mitochondrial membrane marker, cytochrome c oxidase, in peroxisomal preparations. Purified mitochondrial and peroxisomally-associated activities were highly sensitive to the presence of sodium azide, N,N' -dicyclohexylcarbodiimide (DCCD) and venturicidin when measured at pH 9.0. Comparisons of peroxisomal activities with those of the purified plasma membrane at pH 6.0 in the presence of azide showed similar sensitivity profiles with respect to inhibitors of yeast plasma membrane ATPases such as vanadate and p-chloromercuriphenyl-sulfonic acid (CMP). Purified peroxisomal membranes, furthermore, reacted with antibody to the mitochondrial F1 subunit (as revealed by Western blot analysis), and [35S] methionine-labeled, glucose-grown cells processed with unlabeled methanol-grown cells, yielded sucrose gradient fractions that were radioactive in bands that were also recognized by F1 antibody. Isolated fractions in these experiments had similar ratios of cpm:pH 9.0 ATPase activities, suggesting that this activity is mitochondrial in origin. The data presented for the characteristics of the peroxisomally-associated activity strongly suggest that the majority of the ATPase activity found in peroxisomal preparations is derived from other organelles.

Biosynthesis and assembly of alcohol oxidase, a peroxisomal matrix protein in methylotrophic yeasts: a review

Alcohol oxidase (AO) catalyses the first step of methanol metabolism in yeasts. In vivo the enzyme is compartmentalized in special cell compartments, called peroxisomes. The enzyme along with the organelles are induced during growth of methylotrophic yeasts on methanol as the sole carbon source. Like all other peroxisomal matrix proteins, AO is encoded by a nuclear gene. Expression of the protein is regulated by a repression/derepression mechanism, but also by induction. Inactive monomeric precursor protein is synthesized in the cytosol and subsequently imported post-translationally into peroxisomes without further processing. Assembly into the active homo-octameric enzyme and binding of the prosthetic group flavin adenine dinucleotide occurs inside the organelle. When enhanced concentration of octameric alcohol oxidase are present in the organelles, the enzyme may form a crystalloid. Oligomerization is not dependent on translocation of AO precursors into their target organelle since octameric, active AO is detected in the cytosol and nucleus of peroxisome-deficient mutants of Hansenula polymorpha: at high expression rates large cytosolic AO crystalloids are formed, which occasionally are also encountered inside the nucleus of such mutants. This paper summarizes recent findings and views on the mechanisms involved in synthesis, import, assembly and crystallization of this important peroxisomal enzyme.

Properties of ATPase activity associated with peroxisomes of rat and bovine liver

1. Peroxisomes were isolated from bovine and rat liver by use of differential and density gradient centrifugations. 2. In the final density gradient (Nycodenz) a distinct peak of ATPase activity codistributed with the peroxisome marker catalase and was well separated from the bulk of the ATPase activity and from markers for other subcellular organelles. 3. The peroxisome-associated ATPase had a pH optimum of 7.5 and was inhibited by N-ethylmaleimide, by N,N'-dicyclohexylcarbodiimide and by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, but was unaffected by up to 30 microM n-tributyltin chloride. 4. Prolonged incubation with oligomycin at high concentrations indicated that 50% of peroxisomal ATPase was resistant to this inhibitor. The oligomycin-sensitive ATPase activity required at least a four-fold higher ratio of inhibitor to protein for inhibition than mitochondrial ATPase did. It was concluded that oligomycin-sensitive and oligomycin-resistant ATPase may be associated with liver peroxisomes.

Structure/function relationships in methylotrophic yeasts

This symposium marks the 15th anniversary of the discovery of microbodies in methylotrophic yeasts. In the intervening years much has been learned about the structure, function and biogenesis of these organelles and these advances are described. As our endeavours continued, unexpected results have confused commonly held views. This was for instance the case when microbody-minus mutants of yeasts became available which showed that some microbody matrix enzymes may be functional when present in the cytosol while others are not. At the molecular level, our understanding of structure/function relationships is also expanding. Examples are structural elements which relate to protein topogenesis and function of enzymes in different cell compartments. Other, perhaps more unusual, adaptations have also been encountered; some involve protein-protein interactions or even modified cofactors which possibly have helped methylotrophic yeasts to establish and/or maintain themselves in natural ecosystems.

Permeability properties of peroxisomal membranes from yeasts

We have studied the permeability properties of intact peroxisomes and purified peroxisomal membranes from two methylotrophic yeasts. After incorporation of sucrose and dextran in proteoliposomes composed of asolectin and peroxisomal membranes isolated from the yeasts Hansenula polymorpha and Candida boidinii a selective leakage of sucrose occurred indicating that the peroxisomal membranes were permeable to small molecules. Since the permeability of yeast peroxisomal membranes in vitro may be due to the isolation procedure employed, the osmotic stability of peroxisomes was tested during incubations of intact protoplasts in hypotonic media. Mild osmotic swelling of the protoplasts also resulted in swelling of the peroxisomes present in these cells but not in a release of their matrix proteins. The latter was only observed when the integrity of the cells was disturbed due to disruption of the cell membrane during further lowering of the concentration of the osmotic stabilizer. Stability tests with purified peroxisomes indicated that this leak of matrix proteins was not associated with the permeability to sucrose. Various attempts to mimic the in vivo situation and generate a proton motive force across the peroxisomal membranes in order to influence the permeability properties failed. Two different proton pumps were used for this purpose namely bacteriorhodopsin (BR) and reaction center-light-harvesting complex I (RCLH1 complex). After introduction of BR into the membrane of intact peroxisomes generation of a pH-gradient was not or barely detectable. Since this pump readily generated a pH-gradient in pure liposomes, these results strengthened the initial observations on the leakiness of the peroxisomal membrane fragments.(ABSTRACT TRUNCATED AT 250 WORDS)