A morphological view on mitochondrial protein targeting

Mitochondrial protein targeting includes both intramitochondrial sorting of proteins encoded by the organellar genome and import and subsequent sorting of nuclear encoded precursor proteins. Only a few proteins are encoded by the mitochondrial genome and synthesized in the organellar matrix. These include predominantly inner membrane proteins that are perhaps co-translationally inserted into this membrane. Biochemical data suggest that insertion into the inner membrane may be confined to the inner boundary membrane. Ultrastructurally, however, a preferential association of ribosomes with either inner boundary or cristae membranes has not been established. The majority of the mitochondrial proteins are nuclear encoded and synthesized as precursors in the cytosol. Electron microscopic studies revealed that import of precursor proteins is generally confined to sites where both mitochondrial envelope membranes are closely apposed. In line with these observations, biochemical studies indicated that precursor proteins destined for the inner membrane or matrix have to interact with the energized inner membrane to allow complete passage of the precursor through the outer membrane. As a consequence, the mitochondrial envelope membranes have to be in close proximity at protein import sites. In isolated mitochondria distinct sites (designated as contact sites) exist where both envelope membranes are closely apposed and presumably stably associated. In situ, however, mitochondrial boundary membranes are in close proximity over large areas that cover almost the entire mitochondrial periphery. Consequently, the relative area of the mitochondrial surface, where both boundary membranes are in sufficient proximity for allowing protein translocation, is generally larger in situ compared to that in isolated organelles. Immunocytochemical localization studies showed a rather random distribution of components of the mitochondrial protein translocation machinery over the entire mitochondrial surface and not confined to contact sites. Based on these ultrastructural data and recent biochemical findings we propose that mitochondrial protein import sites are dynamic in nature and include relatively labile regions of close association of the boundary membranes. In vitro, however, mitochondrial protein import may preferentially take place at or near the presumably stable contact sites.

Investigations of mannan and glucan in the cell wall of Candida boidinii cultivated on methanol

The polysaccharide components (mannan and glucan) in the cell wall of Candida boidinii M 363 grown on methanol and glucose as control were investigated using electron microscopy, cytochemical and biochemical methods. An ultrastructural rearrangement of the polymers in the cell wall of yeasts cultivated on methanol in comparison to those cultivated on glucose was established. The morphological changes correlate to the quantitative changes in the polysaccharide constituents of the cell wall. The forming and the role of thiosemihydrocarbazide (TSHC)--negative zones in the Candida boidinii cell wall cultivated on methanol media are discussed.

The fungicidal effect of human lactoferrin on Candida albicans and Candida krusei

Five oral isolates each of Candida albicans and Candida krusei were studied for their sensitivity to the fungicidal effect of human lactoferrin. Significant inter- and intraspecies variations were observed and with most isolates the sensitivity of C. krusei to lactoferrin was greater than that of C. albicans. Fungicidal activity of lactoferrin was dose-dependent and observable only with the iron-free form of the molecule (apo-lactoferrin). Iron-saturated lactoferrin was ineffective against all isolates. Supernatant protein assays and scanning electron microscopy indicated cell surface alterations--leakage of proteins and formation of surface blebs--only in those Candida isolates that were sensitive to apo-lactoferrin. As lactoferrin is a common, non-immune, mucosal defence protein, its varying mode of action against C. albicans and C. krusei may be related to their different oral carriage rates.

A rapid method for microbial sample preparation for the scanning electron microscope

A synthetic aromatic polymer has been used for preparing replicas of different microorganisms. This method of preparing highly concentrated (9.6 k) microbiological samples for scanning electron microscopy was compared with a standard method. The micrographs of the replicated samples are satisfactory. This method is rapid, cost effective and produces good results, especially in the case of spore-forming mycelial microorganisms.

[The interaction of pathogenic microorganisms with the sorbent polymethylsiloxane]

The method of electron microscopy has been used to study adhesion of the microbic cells of standard strains of Staphylococcus aureus, Escherichia coli and fungi of genus Candida on the organosilicon sorbent polymethylsiloxane (PMS) and medicamentous complex containing it. This complex contains furazolidone and metronidazole immobilized on silver ions-modified PMS. It is shown that the adhesion of microorganisms is accompanied by their destruction whose rate on pure PMS and medicamentous complex is different. Using experimental data the assumptions are advanced concerning the mechanism of the PMS interaction with Gram-positive and Gram-negative microorganisms as well as with fungi of genus Candida.

Electron microprobe study of the effect of long-term vacuum storage of freeze-dried cryosections on distribution of elements in cells

The distribution of elements in yeast cells was measured on freshly prepared, freeze-dried cryosections and compared with the distribution obtained on the same sections after storage for 20 months in a vacuum below 2.6 kPa. The average concentration of phosphorus remained unchanged but was equalized throughout the cells, i.e. it migrated from vacuoles into the cytoplasm and mitochondria. Zinc remained preferentially localized in the vacuoles, but the ratio between vacuolar and cytoplasmic zinc concentrations decreased about three-fold. Cytoplasmic and mitochondrial concentrations of potassium remained unchanged while partial release from the vacuoles and subsequently from the cells was observed. This resulted in a homogeneous distribution of potassium in the cells after 20 months and some of the vacuolar potassium appeared in spectra of formvar film measured several micrometres from the cells. A large increase in the sodium (from 160 to 360% more than in fresh sections), magnesium (from 110 to 200% more) and sulphur (from 70 to 350% more) contents was observed in all cellular compartments (except for vacuoles, where only a 20% increase in the magnesium content was observed), while chlorine was almost completely released from the cells. The limitations of the use of long-term vacuum-stored cryosections for electron microprobe analysis of cells are discussed.

Thrombin-induced rabbit platelet microbicidal protein is fungicidal in vitro

Platelet microbicidal protein (PMP) is released from platelets in response to thrombin stimulation. PMP is known to possess in vitro bactericidal activity against Staphylococcus aureus and viridans group streptococci. To determine whether PMP is active against other intravascular pathogens, we evaluated its potential fungicidal activity against strains of Candida species and Cryptococcus neoformans. Anionic resin adsorption and gel electrophoresis confirmed that the fungicidal activity of PMP resided in a small (approximately 8.5-kDa), cationic protein, identical to previous studies of PMP-induced bacterial killing (M.R. Yeaman, S.M. Puentes, D.C. Norman, and A.S. Bayer, Infect. Immun. 60:1202-1209, 1992). When assayed over a 180-min period in vitro, the susceptibilities of these fungi to PMP varied considerably. Generally, Candida albicans strains (mean survival, 33.5% +/- 6.9% [n = 6]) as well as isolates of Candida glabrata (mean survival, 50.8% +/- 2.9% [n = 2]) were the most susceptible to killing by PMP, while Candida guillermondii and Candida parapsilosis were relatively resistant to PMP-induced killing. Compared with C. albicans, C. neoformans was relatively resistant to the fungicidal activity of PMP, with a mean survival among the isolates studied of 77.4% +/- 12.4% (n = 6). Against C. albicans, PMP-induced fungicidal activity was time dependent (range, 0 to 180 min), PMP concentration dependent (range, 10 to 150 U/ml), and inversely related to the fungal inoculum (range, 5 x 10(3) to 1 x 10(5) CFU/ml). Scanning electron microscopy of PMP-exposed C. albicans and C. neoformans cells revealed extensive surface damage and collapse, suggesting that the site of PMP fungicidal action may directly or indirectly involve the fungal cell envelope.

Development of multipurpose peroxisomes in Candida boidinii grown in oleic acid-methanol limited continuous cultures

We have studied the development and metabolic significance of peroxisomes in the yeast Candida boidinii following adaptation of the organism to cultivation conditions which require the simultaneous presence and activity of two independent peroxisome-mediated pathways for growth. After the addition of methanol to oleic acid-grown cells at late exponentional growth, a number of new small peroxisomes developed which, apart from the presence of beta-oxidation enzymes, were characterized by the presence of enzymes involved in methanol metabolism (alcohol oxidase and dihydroxyacetone synthase). The latter proteins, however, were absent in the larger organelles which were originally present in the oleic acid-grown cells prior to the addition of methanol and which contained only enzymes of the beta-oxidation pathway. Subsequent experiments on cells from continuous cultures grown on a mixture of oleic acid and methanol at steady-state conditions revealed that both the enzymes of the beta-oxidation pathway and those involved in methanol metabolism were found in one and the same compartment. Thus, under these conditions the cells contained peroxisomes which were concurrently involved in the metabolism of two different carbon sources simultaneously used for growth. Our results indicated that the heterogeneity in the peroxisomal population of a single cell, observed in the transient state following the addition of methanol, is only temporary and due to heterogeneity among these organelles with respect to their capacity to incorporate newly synthesized matrix proteins.

Beta-oxidation of butyrate, the short-chain-length fatty acid, occurs in peroxisomes in the yeast Candida tropicalis

When an n-alkane-utilizable yeast, Candida tropicalis pK233, was cultivated on butyrate, the fatty acid of shortest chain-length for beta-oxidation, as the sole source of carbon and energy, catalase and the enzymes of the fatty acid beta-oxidation system were inducibly synthesized at high levels. As in the alkane-grown cells, the proliferation of peroxisomes was harmonized with the induction of peroxisomal enzymes. The results of subcellular fractionation and immunoelectronmicroscopy indicated the localization of these enzymes in peroxisomes, not in mitochondria. It was suggested that only peroxisomes have a role in fatty acid beta-oxidation in the yeast cells, unlike in mammalian cells, in which cooperation between peroxisomes and mitochondria is essential.

Immunocytochemical localization of alkane-inducible cytochrome P-450 and its NADPH-dependent reductase in the yeast Candida maltosa

Antibodies directed against cytochrome P-450Cm1 and the NADPH-cytochrome P-450 reductase were used to study the induction and intracellular localization of these components of the alkane monooxygenase system in the yeast Candida maltosa. Transition from glucose to n-hexadecane utilization resulted in an about 100-fold increase of the immunodetectable P-450 form whereas the reductase was only moderately induced by a factor of about 5. P-450 but not the reductase was further increased by oxygen limitation during cultivation on n-hexadecane. Using an immunogold technique on ultrathin cryosections, P-450 was found to be concentrated in the nuclear envelope during the early phase of the induction process. However, after maximal induction, the highest labeling was observed in membranes of the endoplasmic reticulum closely associated with the peroxisomes and the plasma membrane. Double-labeling experiments revealed that P-450 and its reductase were distributed in the same regions of the endoplasmic reticulum.

Net phosphate transport in phosphate-starved Candida utilis: relationships with pH and K+

Phosphate transport was studied in phosphate-starved Candida utilis using 31P-NMR and in situ pH and K(+)-specific electrodes. A transient efflux of K+ was associated with the initial net influx of orthophosphate across the plasma membrane and decrease of both the plasma membrane pH gradient and the cytosol pH. Recovery of cytosol pH, and the plasma membrane pH gradient after phosphate uptake, was slow when extracellular K+ was depleted, but rapid when K+ was added to the suspension either with orthophosphate or after phosphate uptake. Net phosphate transport into the vacuole occurs against its concentration gradient and is associated with an increase of the tonoplast pH gradient. It is proposed that transport of orthophosphate into the vacuole is mediated by an electrical uniport and driven by the tonoplast membrane potential.

Evidence of peroxisomes and peroxisomal enzyme activities in the oleaginous yeast Apiotrichum curvatum

The presence of peroxisomes and peroxisomal enzyme activities were investigated in the oleaginous yeast Apiotrichum curvatum ATCC 20509 (formerly Candida curvata D.) Catalase, a marker enzyme for peroxisomes, was measured in cell-free extracts prepared by sonication. The nature of the carbon and nitrogen sources in the growth medium greatly affected catalase activity. Cells grown on corn oil had high specific activity of catalase, but those grown on glucose, sucrose, or maltose had low specific activity. High specific activity of catalase was measured in cultures grown on media that supported poor growth (with soluble starch as carbon source or with methylamine, urea, or asparagine as nitrogen source). Peroxisomes from cells grown on corn oil were separated from other subcellular fractions in a discontinuous sucrose gradient. Major peaks of activity of fatty acid beta-oxidation and of two key enzymes in the glyoxylate cycle were found in fractions containing peroxisomes, but not in fractions corresponding to the mitochondria. Peroxisomal beta-oxidation showed equivalent activity with palmitoyl CoA or n-octanoyl CoA as substrate. Mitochondria did not seem to contain NAD-linked glutamate dehydrogenase. Peroxisomes with a homogeneous matrix and core surrounded by a single-layer membrane were observed with an electron microscope in cells grown on corn oil, but not in those grown on glucose. Staining with 3,3'-diaminobenzidine revealed that catalase activity was located in peroxisomes. Peroxisomes in this oleaginous yeast play important roles in lipid metabolism.

Candida mannan: chemistry, suppression of cell-mediated immunity, and possible mechanisms of action

The ability of Candida albicans to establish an infection involves multiple components of this fungal pathogen, but its ability to persist in host tissue may involve primarily the immunosuppressive property of a major cell wall glycoprotein, mannan. Mannan and oligosaccharide fragments of mannan are potent inhibitors of cell-mediated immunity and appear to reproduce the immune deficit of patients with the mucocutaneous form of candidiasis. However, neither the exact structures of these inhibitory species nor their mechanisms of action have yet been clearly defined. Different investigators have proposed that mannan or mannan catabolites act upon monocytes or suppressor T lymphocytes, but research from unrelated areas has provided still other possibilities for consideration. These include interference with cytokine activities, lymphocyte-monocyte interactions, and leukocyte homing. To stimulate further research of the immunosuppressive property of C. albicans mannan, we have reviewed (i) the relationship of mannan to other antigens and virulence factors of the fungus; (ii) the chemistry of mannan, together with methods for preparation of mannan and mannan fragments; and (iii) the historical evidence for immunosuppression by Candida mannan and the mechanisms currently proposed for this property; and (iv) we have speculated upon still other mechanisms by which mannan might influence host defense functions. It is possible that understanding the immunosuppressive effects of mannan will provide clues to novel therapies for candidiasis that will enhance the efficacy of both available and future anti-Candida agents.

Proliferation and metabolic significance of peroxisomes in Candida boidinii during growth on D-alanine or oleic acid as the sole carbon source

We have studied the induction of peroxisomes in the methylotrophic yeast Candida boidinii by D-alanine and oleic acid. The organism was able to utilize each of these compounds as the sole carbon source and grew with growth rates of mu = 0.20 h-1 (on D-alanine) or mu = 0.43 h-1 (on oleic acid). Growth was associated with the development of many peroxisomes in the cells. On D-alanine a cluster of tightly interwoven organelles was observed which made up 6.3% of the cytoplasmic volume and were characterized by the presence of D-amino acid oxidase and catalase. On oleic acid rounded to elongated peroxisomes were dominant which were scattered throughout the cytoplasm. These organelles contained increased levels of beta-oxidation enzymes; their relative volume fraction amounted 12.8% of the cytoplasmic volume.

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)

Isolation and characterization of membranes from oleic acid-induced peroxisomes of Candida tropicalis

We report a methodology for the isolation of peroxisome membranes from the yeast Candida tropicalis pK233 grown on oleic acid, and the characterization of the polypeptide and lipid compositions of these membranes. Peroxisomes purified in either sucrose or Nycodenz gradients are treated with Tris-HCl (pH 8.5) and then with sodium carbonate (pH 11.5) to yield a final peroxisome membrane preparation (hereafter called ‘peroxisome membranes’). Electron microscopy revealed peroxisome membranes that are approximately 8.1 nm thick, have a typical trilaminar appearance, and form either flattened sheets or whorled structures. Peroxisome membranes contain 3.1% and 2.2% of the total protein of sucrose- and Nycodenz-gradient-purified peroxisomes, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed three predominant polypeptide bands of 34 (PMP 34), 29 (PMP 29), and 24 (PMP 24) × 10(3) Mr in peroxisome membranes. Immunoblotting with an antiserum to PMP 24 showed that PMP 24 segregates with the peroxisome membrane fractions and is induced by growth of Candida tropicalis on oleic acid. Peroxisome membranes contain neutral lipids and phospholipids. The principal phospholipids are phosphatidyl choline and phosphatidyl ethanolamine. The phospholipid/protein ratio of peroxisome membranes is approximately 430 nmol mg-1.

Occurrence of peroxisomal membrane proteins in methylotrophic yeasts grown under different conditions

We have studied the substructure and polypeptide composition of the peroxisomal membranes in two methylotrophic yeasts in relation to different growth conditions. The results obtained indicated that no significant ultrastructural differences existed between the membranes of variously grown cells. The presence of specific peroxisomal membrane proteins (PMPs) was studied biochemically. On sodium dodecyl sulphate-polyacrylamide gels of purified microbody membranes isolated from methanol-grown Hansenula polymorpha, prominent protein bands were observed at 22, 31, 35, 42, 49 and 51 kD. These proteins were also present when the cells were grown in media containing ethanol and/or ethylamine. Apart from these, several other PMPs were specifically induced under these conditions, namely 24, 29, 37 and 62 kD proteins. The polypeptide composition of peroxisomal membranes from H. polymorpha was compared with that of another methylotroph, Candida biodinii. In the latter organism a specific PMP with a molecular weight of 23 kD was induced during growth on D-alanine instead of ammonium sulphate as the nitrogen source.

Immunochemical and structural analysis of the cell wall mannan as the basis of the taxonomic reidentification of a yeast strain

The immunochemical properties and structural features of D-mannans from two Candida parapsilosis strains were studied. Weak cross-reactivity of D-mannans with antisera produced to C. parapsilosis strains was observed, as well as significant differences in the mannans structures were found by means of methylation analysis, acetolysis and 1H-NMR spectroscopy. In view of the discrepancies found, new taxonomic identification of one strain was carried out. On the basis of assimilation of potassium nitrate and formation of hat-shaped spores as well as other differences found by investigation of physiological characteristics, one strain of C. parapsilosis was reidentified as Hansenula anomala. This work demonstrates that immunochemical and structural investigations of cell-wall polysaccharide components can serve as a basis for taxonomic identification of yeast strains.

Respiratory capacities of mitochondria of Saccharomyces cerevisiae CBS 8066 and Candida utilis CBS 621 grown under glucose limitation

A comparative study was made of the in vitro respiratory capacity of mitochondria isolated from Saccharomyces cerevisiae and Candida utilis grown in glucose-limited chemostat cultures. An electron-microscopic analysis of whole cells revealed that the volume density of mitochondria was the same in both yeasts. Mitochondria from both organisms exhibited respiratory control with NADH, pyruvate + malate, 2-oxoglutarate + acetate or malate, and ethanol. The rate of oxidation of these compounds by isolated mitochondria was the same in both yeasts. The rate of oxidation of NADPH by mitochondria from S. cerevisiae was 10 times lower than by those from C. utilis. However, this low rate probably has no influence on the overall in vivo respiratory capacity of S. cerevisiae. The results are discussed in relation to the differences in metabolic behaviour between S. cerevisiae and C. utilis upon transition of cultures from glucose limitation to glucose excess. It is concluded that the occurrence of alcoholic fermentation in S. cerevisiae under these conditions does not result from a bottleneck in the respiratory capacity of the mitochondria.

[Isolation of yeast nuclei and characteristics of their lipid composition]

Intact nuclei of a high purity degree were isolated from Candida utilis protoplasts, and their lipid content and composition were determined.

Scanning electron microscope study of Torulopsis ethanolitolerans prepared by glutaraldehyde--thiosemicarbohydrazide procedure

Torulopsis ethanolitolerans subject to both the sparing and coarse heat treatment were studied in the scanning electron microscope. The reduction of adhesivity, increased permeability and higher rigidity of the yeast wall was achieved by an original glutaraldehyde-paraformaldehyde fixation, low osmolarity in vacuo and subsequent thiosemicarbohydrazide incubation, followed by addition of metal salt. The impregnation of the metal throughout the specimen due to the reaction of the thiosemicarbohydrazide with glutaraldehyde allowed viewing of small or intricate surface details of the yeast. Structural differences of the yeast processed by sparing and coarse heat treatment were shown to be better from the thiosemicarbohydrazide incubated samples compared to those that were prepared with osmium tetroxide.

Validity of Candida natalensis van der Walt et Tscheuschner

In order to verify whether Candida natalensis is a synonym of Candida sake or not, we investigated DNA base composition, DNA-DNA hybridization, the electrophoretic patterns of enzymes, and proton magnetic resonance (PMR) spectra of cell wall mannans of representative strains of C. natalensis and C. sake. No significant differences in DNA base compositions were found among strains of C. natalensis and C. sake. Very low DNA relatedness was detected between the type strain of C. natalensis and the type strain of C. sake, and little DNA relatedness was demonstrated between a strain of C. natalensis and the type strain of C. sake. Furthermore, little DNA relatedness was found between the type strain and a strain of C. natalensis. These results well correlated with those of the electrophoretic patterns of seven enzymes and PMR spectra of cell wall mannans. C. natalensis is a distinct species from C. sake. Also, one strain, which was previously identified as C. natalensis, is a different species from C. natalensis and C. sake.

Evidence for functional heterogeneity among microbodies in yeasts

We have studied the biogenesis and enzymic composition of microbodies in different yeasts during adaptation of cells to a new growth environment. After a shift of cells of Candida boidinii and Hansenula polymorpha from glucose to methanol/methylamine-containing media, newly synthesized alcohol oxidase and amine oxidase are imported in one and the same organelle together with catalase; as a consequence the cells contain one class of morphologically and enzymatically identical microbodies. Similar results were obtained when Candida utilis cells were transferred from glucose to ethanol/ethylamine-containing media upon which all cells formed microbodies containing amine oxidase and catalase. However, when methanol-limited cells of H. polymorpha were transferred from media containing ammonium sulphate to those with methylamine as the nitrogen source, newly synthesized amine oxidase was incorporated only in part of the microbodies present in these cells. This uptake was confined to the few smaller organelles generally present at the perimeter of the cells, which were considered not fully developed (immature) as judged by their size. Essentially similar results were obtained when stationary phase cells of C. boidinii or C. utilis - grown on methanol and ethanol plus ammonium sulphate, respectively - were shifted to media containing (m)ethylamine as the nitrogen source. These results indicate that mature microbodies may exist in yeasts which no longer are involved in the uptake of matrix proteins. Therefore, these yeasts may display heterogeneities in their microbody population.