The putative electrogenic nitrate-proton symport of the yeast Candida utilis. Comparison with the systems absorbing glucose or lactate

Expanding the Knowledge on the Skillful Yeast Cyberlindnera jadinii

Cyberlindnera jadinii is widely used as a source of single-cell protein and is known for its ability to synthesize a great variety of valuable compounds for the food and pharmaceutical industries. Its capacity to produce compounds such as food additives, supplements, and organic acids, among other fine chemicals, has turned it into an attractive microorganism in the biotechnology field. In this review, we performed a robust phylogenetic analysis using the core proteome of C. jadinii and other fungal species, from Asco- to Basidiomycota, to elucidate the evolutionary roots of this species. In addition, we report the evolution of this species nomenclature over-time and the existence of a teleomorph (C. jadinii) and anamorph state (Candida utilis) and summarize the current nomenclature of most common strains. Finally, we highlight relevant traits of its physiology, the solute membrane transporters so far characterized, as well as the molecular tools currently available for its genomic manipulation. The emerging applications of this yeast reinforce its potential in the white biotechnology sector. Nonetheless, it is necessary to expand the knowledge on its metabolism, regulatory networks, and transport mechanisms, as well as to develop more robust genetic manipulation systems and synthetic biology tools to promote the full exploitation of C. jadinii.

Energy depletion protects Candida albicans against antimicrobial peptides by rigidifying its cell membrane

Inhibitors of the energy metabolism, such as sodium azide and valinomycin, render yeast cells completely resistant against the killing action of a number of cationic antimicrobial peptides, including the salivary antimicrobial peptide Histatin 5. In this study the Histatin 5-mediated killing of the opportunistic yeast Candida albicans was used as a model system to comprehensively investigate the molecular basis underlying this phenomenon. Using confocal and electron microscopy it was demonstrated that the energy poison azide reversibly blocked the entry of Histatin 5 at the level of the yeast cell wall. Azide treatment hardly induced depolarization of the yeast cell membrane potential, excluding it as a cause of the lowered sensitivity. In contrast, the diminished sensitivity to Histatin 5 of energy-depleted C. albicans was restored by increasing the fluidity of the membrane using the membrane fluidizer benzyl alcohol. Furthermore, rigidification of the membrane by incubation at low temperature or in the presence of the membrane rigidifier Me(2)SO increased the resistance against Histatin 5, while not affecting the energy charge of the cell. In line, azide induced alterations in the physical state of the interior of the lipid bilayer. These data demonstrate that changes in the physical state of the membrane underlie the increased resistance to antimicrobial peptides.

Energy coupling to nitrate uptake into the denitrifying cells of Paracoccus denitrificans

This study deals with the effects of the agents that dissipate the individual components of the proton motive force (short-chain fatty acids, nigericin, and valinomycin) upon the methyl viologen-coupled nitrate reductase activity in intact cells. Substitution of butyrate or acetate for chloride in Tris-buffered assay media resulted in a marked inhibition at pH 7. In a Tris--chloride buffer of neutral pH, the reaction was almost fully inhibitable by nigericin. Alkalinisation increased the IC(50) value for nigericin and decreased the maximal inhibition attained. Both types of inhibitions could be reversed by the permeabilisation of cells or by the addition of nitrite, and that caused by nigericin disappeared at high extracellular concentrations of potassium. These data indicate that nitrate transport step relies heavily on the pH gradient at neutral pH. Since the affinity of cells for nitrate was strongly diminished by imposing an inside-positive potassium (or lithium) diffusion potential at alkaline external pH, a potential dependent step may be of significance in the transporter cycle under these conditions. Experiments with sodium-depleted media provided no hints for Na(+) as a possible H(+) substitute.

Assimilation of nitrate by yeasts

Nitrate assimilation has received much attention in filamentous fungi and plants but not so much in yeasts. Recently the availability of classical genetic and molecular biology tools for the yeast Hansenula polymorpha has allowed the advance of the study of this metabolic pathway in yeasts. The genes YNT1, YNR1 and YNI1, encoding respectively nitrate transport, nitrate reductase and nitrite reductase, have been cloned, as well as two other genes encoding transcriptional regulatory factors. All these genes lie closely together in a cluster. Transcriptional regulation is the main regulatory mechanism that controls the levels of the enzymes involved in nitrate metabolism although other mechanisms may also be operative. The process involved in the sensing and signalling of the presence of nitrate in the medium is not well understood. In this article the current state of the studies of nitrate assimilation in yeasts as well as possible venues for future research are reviewed.

Effects of histatin 5 and derived peptides on Candida albicans

Three anti-microbial peptides were compared with respect to their killing activity against Candida albicans and their ability to disturb its cellular and internal membranes. Histatin 5 is an anti-fungal peptide occurring naturally in human saliva, while dhvar4 and dhvar5 are variants of its active domain, with increased anti-microbial activity. dhvar4 has increased amphipathicity compared with histatin 5, whereas dhvar5 has amphipathicity comparable with that of histatin 5. All three peptides caused depolarization of the cytoplasmic and/or mitochondrial membrane, indicating membranolytic activity. For the variant peptides both depolarization and killing occurred at a faster rate. With FITC-labelled peptides, no association with the cytoplasmic membrane was observed, contradicting the formation of permanent transmembrane multimeric peptide pores. Instead, the peptides were internalized and act on internal membranes, as demonstrated with mitochondrion- and vacuole-specific markers. In comparison with histatin 5, the variant peptides showed a more destructive effect on mitochondria. Entry of the peptides and subsequent killing were dependent on the metabolic state of the cells. Blocking of the mitochondrial activity led to complete protection against histatin 5 activity, whereas that of dhvar4 was hardly affected and that of dhvar5 was affected only intermediately.

Characterization of histatin 5 with respect to amphipathicity, hydrophobicity, and effects on cell and mitochondrial membrane integrity excludes a candidacidal mechanism of pore formation

Histatin 5 is a 24-residue peptide from human saliva with antifungal properties. We recently demonstrated that histatin 5 translocates across the yeast membrane and targets to the mitochondria, suggesting an unusual antifungal mechanism (Helmerhorst, E. J., Breeuwer, P., van't Hof, W., Walgreen-Weterings, E., Oomen, L. C. J. M., Veerman, E. C. I., Nieuw Amerongen, A. V., and Abee, T. (1999) J. Biol. Chem. 274, 7286-7291). The present study used specifically designed synthetic analogs of histatin 5 to elucidate the role of peptide amphipathicity, hydrophobicity, and the propensity to adopt alpha-helical structures in relation to membrane permeabilization and fungicidal activity. Studies included circular dichroism measurements, evaluation of the effects on the cytoplasmic transmembrane potential and on the respiration of isolated mitochondria, and analysis of the peptide hydrophobicity/amphipathicity relationship (Eisenberg, D. (1984) Annu. Rev. Biochem. 53, 595-623). The 14-residue synthetic peptides used were dh-5, comprising the functional domain of histatin 5, and dhvar1 and dhvar4, both designed to maximize amphipathic characteristics. The results obtained show that the amphipathic analogs exhibited a high fungicidal activity, a high propensity to form an alpha-helix, dissipated the cytoplasmic transmembrane potential, and uncoupled the respiration of isolated mitochondria, similar to the pore-forming peptide PGLa (Peptide with N-terminal Glycine and C-terminal Leucine-amide). In contrast, histatin 5 and dh-5 showed fewer or none of these features. The difference in these functional characteristics between histatin 5 and dh-5 on the one hand and dhvar1, dhvar4, and PGLa on the other hand correlated well with their predicted affinity for membranes based on hydrophobicity/amphipathicity analysis. These data indicate that the salivary protein histatin 5 exerts its antifungal function through a mechanism other than pore formation.

A high affinity fungal nitrate carrier with two transport mechanisms

We have expressed the CRNA high affinity nitrate transporter from Emericella (Aspergillus) nidulans in Xenopus oocytes and used electrophysiology to study its properties. This method was used because there are no convenient radiolabeled substrates for the transporter. Oocytes injected with crnA mRNA showed nitrate-, nitrite-, and chlorite-dependent currents. Although the gene was originally identified by chlorate selection there was no evidence for transport of this anion. The gene selection is explained by the high affinity of the transporter for chlorite, and the fact that this ion contaminates solutions of chlorate. The pH-dependence of the anion-elicited currents was consistent with H(+)-coupled mechanism of transport. At any given voltage, currents showed hyperbolic kinetics with respect to extracellular H(+), and these data could be fitted with a Michaelis-Menten relationship. But this equation did not adequately describe transport of the anion substrates. At higher concentrations of the anion substrates and more negative membrane voltages, the currents were decreased, but this effect was independent of changes in external pH. These more complicated kinetics could be fit by an equation containing two Michaelis-Menten terms. The substrate inhibition of the currents could be explained by a transport reaction cycle that included two routes for the transfer of nitrate across the membrane, one on the empty carrier and the other proton coupled. The model predicts that the substrate inhibition of transporter current depends on the cytosolic nitrate concentration. This is the first time a high affinity nitrate transport activity has been characterized in a heterologous system and the measurements show how the properties of the CRNA transporter are modified by changes in the membrane potential, external pH, and nitrate concentration. The physiological significance of these observations is discussed.

The YNT1 gene encoding the nitrate transporter in the yeast Hansenula polymorpha is clustered with genes YNI1 and YNR1 encoding nitrite reductase and nitrate reductase, and its disruption causes inability to grow in nitrate

DNA sequencing in the phage lambda JA13 isolated from a lambda EMBL3 Hansenula polymorpha genomic DNA library containing the nitrate reductase-(YNR1) and nitrite reductase-(YNI1) encoding genes revealed an open reading frame (YNT1) of 1524 nucleotides encoding a putative protein of 508 amino acids with great similarity to the nitrate transporters from Aspergillus nidulans and Chlamydomonas reinhardtii. Disruption of the chromosomal YNT1 copy resulted in incapacity to grow in nitrate and a significant reduction in rate of nitrate uptake. The disrupted strain is still sensitive to chlorate, and, in the presence of 0.1 mM nitrate, the expression of YNR1 and YNI1 and the activity of nitrate reductase and nitrite reductase are significantly reduced compared with the wild-type. Northern-blot analysis showed that YNT1 is expressed when the yeast is grown in nitrate and nitrite but not in ammonium solution.

Expulsion of uracil and thymine from the yeast Saccharomyces cerevisiae: contrasting responses to changes in the proton electrochemical gradient

The outflow of uracil from the yeast Saccharomyces cerevisiae is known to be relatively fast in certain circumstances, to be retarded by proton conductors and to occur in strains lacking a uracil proton symport. In the present work, it was shown that uracil exit from washed yeast cells is an active process, creating a uracil gradient of the order of -80 mV relative to the surrounding medium. Glucose accelerated uracil exit, while retarding its entry. DNP or sodium azide each lowered the gradient to about -30 mV, simultaneously increasing the rate of uracil entry. They also lowered cellular ATP content. Manipulation of the external ionic conditions governing delta mu H+ at the plasma membrane had no detectable effect on uracil transport in yeast preparations thoroughly depleted of ATP. It was concluded that uracil exit is probably not driven by the proton gradient but may utilize ATP directly. It is known that thymine is not normally absorbed by yeast. However, thymine expulsion was here observed during deamination of the substrate 5-methylcytosine in the presence of glucose. In the absence of glucose, or following ATP depletion, thymine uptake from the medium only occurred when delta mu H+ was dissipated, either by DNP or azide, or by manipulation of the external ionic environment. The yeast expelled absorbed thymine when delta mu H+ was restored to the physiological range. The properties of the system corresponded to those of an H+/thymine antiport that is distinct from the mechanism expelling uracil.

Mechanisms regulating the transport of acetic acid in Saccharomyces cerevisiae

Cells of the yeast Saccharomyces cerevisiae IGC 4072 grown in medium with acetic acid produced a mediated transport system for acetic acid that behaved as an electroneutral proton symport for the anionic form of the acid. The system could transport propionate and formate but not lactate and pyruvate. Uptake of labelled lactic acid was negligible, no mediated transport system activity for this acid being found. The acetate transporter was also found in cells grown in lactic acid or ethanol media, suggesting that the carrier did not require the presence of an external inducer. When cells were grown in lactic acid medium, uptake of labelled acetic acid, at pH 5.0, was biphasic and consistent with the presence of two distinct transport modes for the acid. One of these components corresponded to the acetate/proton symport, and the higher affinity system corresponded to a more general monocarboxylate carrier that could also transport lactate, pyruvate and propionate. Both systems were subject to repression by glucose, fructose, sucrose, maltose or galactose. In glucose-repressed cells, the undissociated form of the acids appeared to be the only one that could cross the plasma membrane, a diffusion mechanism being involved in the acid uptake. Under these growth conditions and when the extracellular pH was lower than that of the cytosol, accumulation of the acid could also be observed, it being a function of the delta pH.

Cytosine accumulation as a measure of the proton electrochemical gradient acting on the overexpressed cytosine permease of Saccharomyces cerevisiae

The magnitude of the proton gradient (delta mu H+) driving solute accumulation in Saccharomyces cerevisiae has long been in doubt, principally because of the lack of an agreed method for assaying its electrical component, the membrane potential (delta psi). In the present work, the size of the cytosine gradient (delta mu cyt) that the yeast generated was used as a measure of the driving gradient (delta mu H+). The selected yeast lacked cytosine deaminase and overexpressed cytosine permease, a 1 H+/cytosine system. delta mu cyt, assayed in washed cell suspensions fermenting glucose and containing 0.5 or 50 mM KCl, was about 260 mV at pH 4 or 5, falling to about 194 mV at pH 7. As a first estimate, -delta mu H+ was thus at least as large at the respective pH value. A 20 mM solution of the lipophilic cation tetraphenylphosphonium lowered delta mu cyt to a value roughly equal to the magnitude of the pH gradient (delta pH). A mathematical model was used to correct the first estimates of delta mu H+ for the effect of cytosine leakage outside the symport. In such a system, delta mu cyt cannot exceed the equivalent ratio Vmax/KmL, where Vmax and Km are kinetic parameters of the symport and L is the rate coefficient for leakage. The feasibility of assaying delta mu H+ depends on it not being much larger than that ratio. The model was tested successfully against observations made with yeast preparations depleted of ATP. After correction, -delta mu H+ during fermentation was estimated to be up to 25 mV larger than delta mu cyt and at least 70 mV larger than previous estimates in the literature involving lipophilic cations. From a knowledge of delta pH, delta psi was in turn deduced and compared with the maximum methylamine gradient (delta mu M) the yeast formed. The results supported the claim in the literature that, at acid pH, delta mu M is a measure of delta psi.

Proton production and consumption pathways in yeast metabolism. A chemostat culture analysis

In this investigation, a method for the accurate quantitative determination of net proton production or consumption in biological cultures has been devised. Cells are cultured under constant pH conditions. The specific rate of proton production or consumption by the culture (qH+, mmol h-1 per g biomass) is proportional to the mmol of base or acid required to maintain constant pH per unit time, and this equivalence is independent of the buffering capacity of the culture medium. The above method has been applied to chemostat cultures of Candida utilis growing on glucose or glycerol as carbon source, and different nitrogen sources. The results indicate that the nitrogen assimilation pathway alone determines the value of qH+, and a fixed stoichiometric relationship between nitrogen uptake rate qN (meq h-1 per g biomass) and qH+ has been found for each nitrogen source employed. Thus, qH+/qN values of +1, 0 and -1 were found for ammonium ions, urea and nitrate respectively. Under oxidative metabolism, the contribution of carbon catabolism to the value of qH+ was undetectable. Sine qN may be related to growth and production of type 1 compounds in fermentation processes, the parameter qH+ was incorporated into a model of growth and energy metabolism in chemostat culture (Castrillo and Ugalde, Yeast 10, 185 - 197, 1994), resulting in adequate simulations of experimentally observed culture performance. Thus, it is suggested that qH+ may be employed as a simple and effective control parameter for biotechnological processes involving biomass-related products.

NO3- transport across the plasma membrane of Arabidopsis thaliana root hairs: kinetic control by pH and membrane voltage

High-affinity nitrate transport was examined in intact root hair cells of Arabidopsis thaliana using electrophysiological recordings to characterise the response of the plasma membrane to NO3- challenge and to quantify transport activity. The NO3(-)-associated membrane current was determined using a three-electrode voltage clamp to bring membrane voltage under experimental control and to compensate for current dissipation along the longitudinal cell axis. Nitrate transport was evident in the roots of seedlings grown in the absence of a nitrogen source, but only 4-6 days postgermination. In 6-day-old seedlings, additions of 5-100 microM NO3- to the bathing medium resulted in membrane depolarizations of 8-43 mV, and membrane voltage (Vm) recovered on washing NO3- from the bath. Voltage clamp measurements carried out immediately before and following NO3- additions showed that the NO3(-)-evoked depolarizations were the consequence of an inward-directed current that appeared across the entire range of accessible voltages (-300 to +50 mV). Both membrane depolarizations and NO3(-)-evoked currents recorded at the free-running voltage displayed quasi-Michaelian kinetics, with apparent values for Km of 23 +/- 6 and 44 +/- 11 microM, respectively and, for the current, a maximum of 5.1 +/- 0.9 muA cm-2. The NO3- current showed a pronounced voltage sensitivity within the normal physiological range between -250 and -100 mV, as could be demonstrated under voltage clamp, and increasing the bathing pH from 6.1 to 7.4-8.0 reduced the current and the associated membrane depolarizations 3- to 8-fold. Analyses showed a well-defined interaction between the kinetic variables of membrane voltage, pHo and [NO3-]o. At a constant pHo of 6.1, depolarization from -250 to -150 mV resulted in an approximate 3-fold reduction in the maximum current but a 10% rise in the apparent affinity for NO3-. By contrast, the same depolarization effected an approximate 20% fall in the Km for transport as a function in [H+]o. These, and additional characteristics of the transport current implicate a carrier cycle in which NO3- binding is kinetically isolated from the rate-limiting step of membrane charge transit, and they indicate a charge-coupling stoichiometry of 2(H+) per NO3- anion transported across the membrane. The results concur with previous studies showing a high-affinity NO3- transport system in Arabidopsis that is inducible following a period of nitrogen-limiting growth, but they underline the importance of voltage as a kinetic factor controlling NO3- transport at the plant plasma membrane.

Utilization of Lactic Acid by Fusarium oxysporum var. lini : Regulation of Transport and Metabolism

Lactic acid was transported in Fusarium oxysporum var. lini ATCC 10960 by a saturable transport system that had a half-saturation constant of 56.6 ± 7.5 μM and a maximum velocity of 0.61 ± 0.10 mmol h -1 g -1 (dry weight) at 26°C and pH 5.0. This transport system was inducible and was not expressed in the presence of a repressing substrate. Evidence is presented that the anionic form lactate - was taken up by the cells. Propionic, acetic, pyruvic, and bromoacetic acids but not succinic acid competitively inhibited the transport of lactic acid. Bromoacetic acid, which was not metabolized, was taken up to a steady-state level when intracellular and extracellular concentrations were identical, indicating that the transport system was not accumulative. The enzymatic activity that was physiologically more relevant in the metabolism of lactic acid was lactate: ferricytochrome c oxidase. This enzyme did not exhibit stereospecifity and was induced by lactic acid.

Fluorocytosine causes uncoupled dissipation of the proton gradient and behaves as an imperfect substrate of the yeast cytosine permease

At pH 5-6 ATP-depleted washed cell preparations of strain NC233-10b[pII4-9], in which the cytosine permease was overexpressed, absorbed cytosine, hypoxanthine or fluorocytosine stoichiometrically with, respectively, about 1, 1.4 and 5 proton equivalents. The cellular pH fell proportionately. The membrane depolarization caused by each compound was assayed in the presence of glucose with a voltage-sensitive dye and increased in the same order. Fluorocytosine significantly lowered the growth yield that a 'petite' strain of the yeast formed at limiting glucose concentrations. At pH 5.6 with extracellular [K+] below 1 mM, each of the three substrates was accumulated about 200-fold from a dilute solution at the expense of the proton gradient. This concentration ratio corresponds to a solute gradient (delta mu(s)) of 13 kJ mol-1. Raising [K+]o systematically lowered the substrate accumulation ratio and delta muH. The mean ratio delta mu(s)/delta muH was 0.82 for all three substrates. It was concluded that whereas the behaviour of cytosine approximated to that expected for a symport of unit proton stoichiometry, the absorption of protons with fluorocytosine and, to a lesser extent, hypoxanthine, was only partly conserved as useful work. A possible mechanism of this novel phenomenon is outlined.

Transport of lactic acid in Kluyveromyces marxianus: evidence for a monocarboxylate uniport

Lactic acid-grown cells of a strain of Kluyveromyces marxianus transported D- and L-lactic acid by a saturable mechanism that was partially inducible and subject to glucose repression, with the following kinetic parameters at pH 5.4: Vmax = 1.00 (+/- 0.13) mmol h-1 per g dry weight and Ks = 0.42 (+/- 0.08) mM. Lactic acid transport was competitively inhibited by pyruvic, glycolic, acetic and bromoacetic acids. The latter, a non-metabolizable analogue, was transiently accumulated, the extent depending on the extracellular pH. The pH dependence of the Ks values for undissociated lactic acid and for the lactate anion indicated that the latter was the transported species. Lactate uptake was not accompanied by the simultaneous uptake of protons, potassium ions or sodium ions excluding symport mechanisms. Initial lactic acid uptake led to transient membrane hyperpolarization as measured with a fluorescent dye excluding also an electroneutral anion antiport mechanism. It was concluded that lactate anions use a monocarboxylate uniport and that the counter anion, possibly bicarbonate, uses a separate channel, the coupling being electrical and loose.

Physiology of yeasts in relation to biomass yields

The stoichiometric limit to the biomass yield (maximal assimilation of the carbon source) is determined by the amount of CO2 lost in anabolism and the amount of carbon source required for generation of NADPH. This stoichiometric limit may be reached when yeasts utilize formate as an additional energy source. Factors affecting the biomass yield on single substrates are discussed under the following headings: Energy requirement for biomass formation (YATP). YATP depends strongly on the nature of the carbon source. Cell composition. The macroscopic composition of the biomass, and in particular the protein content, has a considerable effect on the ATP requirement for biomass formation. Hence, determination of for instance the protein content of biomass is relevant in studies on bioenergetics. Transport of the carbon source. Active (i.e. energy-requiring) transport, which occurs for a number of sugars and polyols, may contribute significantly to the calculated theoretical ATP requirement for biomass formation. P/O-ratio. The efficiency of mitochondrial energy generation has a strong effect on the cell yield. The P/O-ratio is determined to a major extent by the number of proton-translocating sites in the mitochondrial respiratory chain. Maintenance and environmental factors. Factors such as osmotic stress, heavy metals, oxygen and carbon dioxide pressures, temperature and pH affect the yield of yeasts. Various mechanisms may be involved, often affecting the maintenance energy requirement. Metabolites such as ethanol and weak acids. Ethanol increases the permeability of the plasma membrane, whereas weak acids can act as proton conductors. Energy content of the growth substrate. It has often been attempted in the literature to predict the biomass yield by correlating the energy content of the carbon source (represented by the degree of reduction) to the biomass yield or the percentage assimilation of the carbon source. An analysis of biomass yields of Candida utilis on a large number of carbon sources indicates that the biomass yield is mainly determined by the biochemical pathways leading to biomass formation, rather than by the energy content of the substrate.

A theoretical evaluation of growth yields of yeasts

Growth yields of Saccharomyces cerevisiae and Candida utilis in carbon-limited chemostat cultures were evaluated. The yields on ethanol and acetate were much lower in S. cerevisiae, in line with earlier reports that site I phosphorylation is absent in this yeast. However, during aerobic growth on glucose both organisms had the same cell yield. This can be attributed to two factors: --S. cerevisiae had a lower protein content than C. utilis; --uptake of glucose by C. utilis requires energy whereas in S. cerevisiae it occurs via facilitated diffusion. Theoretical calculations showed that, as a result of these two factors, the ATP requirement for biomass formation in C. utilis is 35% higher than in S. cerevisiae (theoretical YATP values of 20.8 and 28.1, respectively). The experimental YATP for anaerobic growth of S. cerevisiae on glucose was 16 g biomass.mol ATP-1. In vivo P/O-ratios can be calculated for aerobic growth on ethanol and acetate, provided that the gap between the theoretical and experimental ATP requirements as observed for growth on glucose is taken into account. This was done in two ways: --via the assumption that the gap is independent of the growth substrate (i.e. a fixed amount of ATP bridges the difference between the theoretical and experimental values). --alternatively, on the assumption that the difference is a fraction of the total ATP expenditure, that is dependent on the substrate. Calculations of P/O-ratios for growth of both yeasts on glucose, ethanol, and acetate made clear that only by assuming a fixed difference between theoretical and experimental ATP requirements, the P/O-ratios are more or less independent of the growth substrate. These P/O-ratios are approximately 30% lower than the calculated mechanistic values.

Cytosolic calcium homeostasis in fungi: roles of plasma membrane transport and intracellular sequestration of calcium

Cytosolic free calcium ([Ca2+]c) has been measured in the mycelial fungus Neurospora crassa with Ca2(+)-selective microelectrodes. The mean value of [Ca2+]c is 92 +/- 15 nM and it is insensitive to external pH values between 5.8 and 8.4. Simultaneous measurement of membrane potential enables the electrochemical potential difference for Ca2+ across the plasma membrane to be estimated as about -60 kJ.mol-1-a value that cannot be sustained either by a simple Ca2(+)-ATPase, or, in alkaline conditions, by straightforward H+/Ca2+ exchange with a stoichiometric ratio of less than 5 H+/Ca2+. We propose that the most likely alternative mechanism of Ca2+ efflux is ATP-driven H+/Ca2+. In accord with this proposal, depletion of the ATP level from 2.5 to 0.5 mM by CN- elicits an increase in [Ca2+]c, but only in alkaline conditions in which the putative H+/Ca2(+)-ATPase would be selectively stalled. The insensitivity of Ca2+ homeostasis to CN- in more acid conditions implies that the Km (ATP) of the transport system is 100 microM or less. The increase in [Ca2+]c in the presence of CN- at pH 8.4 (50 nM.min-1) is compared with 45Ca2+ influx (0.62 mM.min-1) under the same conditions. The proportion of entering Ca2+ remaining free in the cytosol is only 8 x 10(-5), and since the concentration of available chelation sites on Ca2(+)-binding proteins is unlikely to exceed 100 microM, a major role for the fungal vacuole in short-term Ca2+ homeostasis is indicated. This notion is supported by the observation that cytosolic Ca2+ homeostasis is disrupted by a protonophore, which rapidly abolishes the driving force (a transmembrane pH difference) for Ca2+ uptake into fungal vacuoles.

Evidence for cotransport of nitrate and protons in maize roots : I. Effects of nitrate on the membrane potential

The electrical response of nitrate-grown maize (Zea mays L.) roots to 0.1 millimolar nitrate was comprised of two sequential parts: a rapid and transient depolarization of the membrane potential, followed by a slower, net hyperpolarization to a value more negative than the original resting potential. The magnitude of the response was smaller in roots of seedlings grown in the absence of nitrate, but, within 3 hours of initial exposure to 0.1 millimolar nitrate, increased to that of nitrate-grown roots. Chloride elicited a separate electrical response with a pattern similar to that of the nitrate response. However, the results presented in this study strongly indicate that the electrical response to nitrate reflects the activity of a nitrate-inducible membrane transport system for nitrate which is distinct from that for chloride. Inhibitors of the plasmalemma H(+)-ATPase (vanadate, diethylstilbestrol) completely inhibited both parts of the electrical response to nitrate, as did alkaline external pH. The magnitude of the initial nitrate-dependent, membrane potential depolarization was independent of nitrate concentration, but the subsequent nitrate-dependent hyperpolarization showed saturable dependence with an apparent K(m) of 0.05 millimolar. These results support a model for nitrate uptake in maize roots which includes a depolarizing NO(3) (-)/H(+) symport. The model proposes that the nitrate-dependent membrane potential hyperpolarization is due to the plasma membrane proton pump, which is secondarily stimulated by the operation of the NO(3) (-)/H(+) symport.

The intrinsic as opposed to the apparent stoichiometry of the glycine-proton symport of the yeast Saccharomyces carlsbergensis

1. Various ways of computing the proton stoichiometry of glycine absorption were examined in relation to the problem of distinguishing the proton flow (i) through the symport from the basal proton flow (ii) outside it. By depolarizing the plasma membrane, i will tend to inhibit ii. 2. A series of 23 yeast (Saccharomyces carlsbergensis) preparations grown with proline or glutamate were used, some of which were starved in the presence of glucose. Consequently, after ATP depletion, the rate of glycine uptake from a 0.2 mM solution varied through the series from 3 to 14 nmol.min-1.mg-1. Basal proton uptake in the absence of glycine was fairly constant at 3-4 nmol.min-1.mg-1. 3. After addition of glycine, the number of extra equivalents of protons entering the yeast with each amino acid equivalent in 30 s was 0.5 at the lowest rate of glycine absorption and 1.8 equivalents at the fastest rate. However, total proton absorption in 30 s increased in direct proportion to the amount of glycine absorbed. The proportionality factor, indicative of the carrier stoichiometry, was 2.25 +/- 0.13 (23) S.E.M. The effective basal proton uptake was negligibly small. 4. Progress of proton and glycine absorption by each yeast preparation in the period up to 180 s fitted the mathematical model described in the preceding paper by Eddy, Hopkins & Johnson [(1988) Biochem. J. 251, 111-114]. The analysis led to two estimates of the constant ratio of the inflow of protons to the inflow of glycine that would apply when the basal proton flow vanished. These further estimates of the carrier stoichiometry were also near 2, being 2.07 +/- 0.24 (6) and 2.22 +/- 0.07 (17).

Some general characteristics of the transport of nutrients and toxic compounds in yeasts and fungi

This essay deals in turn with the six main mechanisms that are believed to be involved in the passage of small molecules across the cell membrane of yeasts and fungi. Such solutes include amino acids, carbohydrates, purines and pyrimidines and other nutrients, among which are the ions K+,,or. The subject has been reviewed extensively (see, for example, Eddy, 1982; Cooper, 1982; Serrano, 1984; Harold, 1986). Transport systems relate to chemotherapy in two ways. First, they may themselves be potential targets for drug action. Second, they may deliver a drug to its intracellular target, the drug being an illicit passenger in a transport system with a different primary role. Most of our knowledge about yeast and fungal transport is currently based on studies with relatively few species selected because they are known to be amenable to genetic and biochemical analysis. The organisms include representative strains ofNeurospora crassaor ofSaccharomyces cerevisiaeand selected species ofRhodotorulaandSchizosaccharomyces. Fungal organisms pathogenic to man, such asCandida albicans, those causing either systemic fungal infections (see Kerridge, 1986; Shepherd, Poulter & Sullivan, 1985), or the numerous fungal diseases of plants have been little used as models of nutrient transport, despite their clinical or economic importance. Whether all these ecologically diverse organisms, which in evolutionary terms are only distantly related, employ only the transport mechanisms so far discovered inSaccharomyces cerevisiaeis an open question.

Transport of lactate and other short-chain monocarboxylates in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae IGC4072 grown in lactic acid medium transported lactate by an accumulative electroneutral proton-lactate symport with a proton-lactate stoichiometry of 1:1. The accumulation ratio measured with propionate increased with decreasing pH from ca. 24-fold at pH 6.0 to ca. 1,400-fold at pH 3.0. The symport accepted the following monocarboxylates (Km values at 25 degrees C and pH 5.5): D-lactate (0.13 mM), L-lactate (0.13 mM), pyruvate (0.34 mM), propionate (0.09 mM), and acetate (0.05 mM), whereas apparently a different proton symport accepted formate (0.13 mM). The lactate system was inducible and was subject to glucose repression. Undissociated lactic acid entered the cells by simple diffusion. The permeability of the plasma membrane for undissociated lactic acid increased exponentially with pH, and the diffusion constant increased 40-fold when the pH was increased from 3.0 to 6.0.