A proline transport system in Saccharomyces chevalieri

Proline transport and stress tolerance of ammonia-insensitive mutants of the PUT4-encoded proline-specific permease in yeast

The imino amino acid, proline, has roles in both cellular nutrition and response to stress. Proline uptake in Saccharomyces cerevisiae is largely mediated by a high affinity, specific permease, Put4p, and a low affinity general amino acid permease, Gap1p. Both are subject to nitrogen catabolite repression (NCR) and nitrogen catabolite inactivation (NCI). In order for proline to be fully exploited, its transport must be derepressed, as occurs upon depletion of preferred nitrogen sources, and molecular oxygen must be present to allow the first step of catabolism via proline oxidase. This study focuses on the isolation of variants of Put4p, which are insensitive to repression by a preferred nitrogen source (ammonia) and their subsequent effect on proline transport and stress tolerance. Specific amino acid residues in the carboxy-terminal region of Put4p were targeted by site-directed mutagenesis. Substitution at Serine(605), a potential phosphorylation target, led to the amelioration of ammonia-induced down-regulation of Put4p. When combined with a promoter mutation (-160), the S(605)A mutation resulted in increased proline uptake and accumulation. This increase in proline accumulation was associated with increased cell viability in conditions of high temperature and osmotic stress raising possible benefits in industrial fermentation applications.

Mutated-gamma-actin restores growth to a yeast amino acid transport defective mutant

A mutated yeast cell 22574d lacking all three proline transporters, PUT4, UGA4, and GAP1, and incapable of growth on proline recovers its lost ability to grow on proline as sole nitrogen source when transformed with a mutagenized mouse gamma-actin cDNA (M-gamma-A). Native mouse gamma-actin cDNA is ineffective. The 3'-region of gamma-actin cDNA was mutagenized to resemble E51 cDNA previously isolated from Ehrlich tumor cells. The E51 cDNA has an extended reading frame in the 3'-region compared to that in native gamma-actin. The extension of the open reading frame in E51 cDNA, was found to be due to an additional pair of bases (TG) at position 1104 of E51 cDNA. After site-directed mutagenesis of the 3'-region of native gamma-actin cDNA to resemble that of E51 cDNA, the construct, M-gamma-A cDNA, was expressed in the 22574d yeast. While the transformation with M-gamma-A increased the uptake of both proline and gamma-amino butyric acid, the transport of five other solutes was not changed by this transformation. Northern blotting of the nontransformed and the M-gamma-A-transformed 22574d cells with gene-specific probes for the three proline transporters showed the expression of an mRNA for UGA4 in both transformed and nontransformed cells but no evidence for the expression of GAP1 or PUT4. The mRNA for UGA4 was expressed at a lower level in strain 22574d than in the parent yeast sigma1278b. Furthermore, the message in the mutated cells is smaller in size by about 15%. These results are consistent with the synthesis of a mutated transporter which requires the coexpression of M-gamma-A, but not native gamma-actin, to restore physiological function, i.e., proline or gamma-amino acid transport.

Regulation of L-proline transport in Leishmania donovani by extracellular pH

We have previously shown that Leishmania donovani promastigotes adapted to long-term culture at acidic pH can serve as a model to study parasite development in a lysosomal-like environment. In this study we investigated the effect of growth pH on L. donovani L-proline transport systems. Reducing the pH of the growth medium causes an up to 7-fold decrease in the extent of L-proline transport. Transport resumes after switching the culture from pH 4.5 to pH 7 for 48 h by a protein synthesis-dependent process. The pH optimum for transport changes from 7.5 in promastigotes grown at pH 7 to 5.5 in cells grown at pH 4.5. In addition, kinetic analysis of L-proline transport showed that Vmax in pH 4.5-grown L. donovani promastigotes is one-tenth that of cells grown at pH 7 (4.5 and 44.7 nmol min-1 (10(8) cells)-1, respectively). The apparent Km for L-proline in pH 4.5 promastigotes is one-half of the Km in pH 7 cells (0.30 and 0.65 mM, respectively). In contrast to L-proline transport, D-glucose transport demonstrates a growth pH-independent activity: Km and Vmax as well as optimum pH of transport are similar in promastigotes grown at either pH 7 or pH 4.5. Taken together, the results indicate that in L. donovani, expression and activity of L-proline transport is regulated by culture pH. The pH-dependent expression of L-proline transporters may be of physiological significance during the promastigote-amastigote transition.

Nucleotide sequence of the Saccharomyces cerevisiae PUT4 proline-permease-encoding gene: similarities between CAN1, HIP1 and PUT4 permeases

The complete nucleotide (nt) sequence of the PUT4 gene, whose product is required for high-affinity proline active transport in the yeast Saccharomyces cerevisiae, is presented. The sequence contains a single long open reading frame of 1881 nt, encoding a polypeptide with a calculated Mr of 68,795. The predicted protein is strongly hydrophobic and exhibits six potential glycosylation sites. Its hydropathy profile suggests the presence of twelve membrane-spanning regions flanked by hydrophilic N- and C-terminal domains. The N terminus does not resemble signal sequences found in secreted proteins. These features are characteristic of integral membrane proteins catalyzing translocation of ligands across cellular membranes. Protein sequence comparisons indicate strong resemblance to the arginine and histidine permeases of S. cerevisiae, but no marked sequence similarity to the proline permease of Escherichia coli or to other known prokaryotic or eukaryotic transport proteins. The strong similarity between the three yeast amino acid permeases suggests a common ancestor for the three proteins.

Nitrogen catabolite regulation of proline permease in Saccharomyces cerevisiae. Cloning of the PUT4 gene and study of PUT4 RNA levels in wild-type and mutant strains

The proline permease gene PUT4 has been cloned. Nitrogen-source regulation ('ammonia sensitivity') of this and at least two other amino-acid permeases is believed to occur at two distinct levels, i.e. permease synthesis and permease activity. Therefore, PUT4 transcription/messenger stability was examined in the ammonia- and proline-grown wild type as well as in mutant strains supposedly affected at only one or at both of these levels. We report transcript-level repression of proline permease synthesis in ammonia-grown cells. Repression is lifted at this level in gdhCR, gln1ts and gdhA mutants which exhibit pleiotropically derepressed permease and catabolic enzyme activities. On the other hand, the npi1 and npi2 mutations, formerly called mut2 and mut4, relieve an inactivation process which seems only to affect permeases. These mutations do not affect the detected PUT4 RNA level. The only known positive factor in proline permease regulation, the nitrogen permease reactivator protein Npr1, is believed to counteract the inactivation process on derepressing media. This protein appears to have an additional, indirect effect on PUT4 transcription/messenger stability: it would actually mediate repression via its activating effect on ammonia uptake.

Participation of an extracellular deaminase in amino acid utilization by Neurospora crassa

A strain of Neurospora crassa defective in amino acid transport can utilize a variety of amino acids for growth when readily metabolizable nitrogen is limiting. Growth is accompanied by the production of an extracellular deaminase that converts the amino acid to its respective keto acid plus equimolar quantities of utilizable nitrogen in the ammonium ion form. Production of the deaminase is subject to ammonium repression. The relationship between the ability of an amino acid to trigger deaminase production and the presence of particular amino acid permease deficiencies is complex. Four classes of amino acids have been defined with respect to this relationship. The existence of multiple extracellular deaminases is discussed.

Stoichiometry of H+/amino acid cotransport in Neurospora crassa revealed by current-voltage analysis

Coupling of ions to the uptake of neutral and basic amino acids via a general amino acid transport system (System II), was studied in a mutant of Neurospora crassa (bat mtr) which lacks other transport systems for these solutes. All amino acids tested--including ones bearing no net charge--elicited rapid membrane depolarization, as expected for ion-coupled transport. (Since amino acid transport in Neurospora is not dependent on extracellular Na+ or K+, the associated ion is presumed to be H+.) Although the 14C-labeled amino acid fluxes through System II are largely independent of the identity of the amino acid, the depolarization caused by basic amino acids (L-lysine and L-ornithine) is 60-70% greater than that for neutral amino acids (e.g. L-leucine). This difference is consistent with a constant H+/amino acid stoichiometry of 2, the extra charge for lysine and ornithine being that on the amino acid itself, so that the charge ratio basic:neutral amino acids is 3:2. When actual membrane charge flow associated with amino acid uptake was compared with measured 14C-labeled amino acid influx, ratios of 2.07 charges/mol L-leucine and 3.40 charges/mol L-lysine were obtained, again in accord with a constant translocation stoichiometry of 2H+/amino acid. The advantages of this electrical method for estimating H+/solute stoichiometry in cotransport are discussed in relation to more familiar methods.

Amino acid uptake by yeasts. IV. Effect of thiol reagents on L-leucine transport in Saccharomyces cerevisiae

(1) N-Ethylmaleimide (a penetrating SH- reagent) inactivated L-[14C]leucine entrance (binding and translocation) into Saccharomyces cerevisiae, the extent of inhibition depending on the time of preincubation with N-ethylmaleimide, N-ethylmaleimide concentration, the amino acid external and internal concentration, and the energization state of the yeast cells. With D-glucose-energized yeast, N-ethylmaleimide inhibited L-[14C]leucine entrance in all the assayed experimental conditions, but with starved yeast and low (0.1 mM) amino acid concentration, it did not inhibit L-[14C]leucine binding, except when the cells were preincubated with L-leucine. With the rho- respiratory-deficient mutant (energized cells), N-ethylmaleimide inhibited L-[14C]leucine entrance as with the energized wild-type, though to a lesser extent. (2) Analysis of the N-ethylmaleimide effect as a function of L-[14C]leucine concentration showed a significant decrease of Jmax values of the high- (S1) and low- (S2) affinity amino acid transport systems, but KT values were not significantly modified. (3) When assayed in the presence of D-glucose, N-ethylmaleimide inhibition of D-glucose uptake and respiration contributed significantly to inactivation of L-[14C]leucine entrance. Pretreatment of yeast cells with 2,4-dinitrophenol enhanced the effect of L-[14C]leucine binding and translocation. (4) Bromoacetylsulfanilic acid and bromoacetylaminoisophthalic acid, two non-penetrating SH- reagents, did not inactivate L-[14C]leucine entrance, while p-chloromercuribenzoate, a slowly penetrating SH-reagent, inactivated it to a limited extent. When compared with the effect of N-ethylmaleimide, these negative results indicate that thiol groups of the L-[14C]leucine carrier were not exposed on the outer surface of the yeast cell permeability barrier.

L-Proline transport in Saccharomyces cerevisiae

Transport of L-proline into Saccharomyces cerevisiae K is mediated by two systems, one with a KT of 31 microM and Jmax of 40 nmol . s-1 . (g dry wt.)-1, the other with KT greater than 2.5 mM and Jmax of 150-165 nmol . s-1 . (g dry wt.)-1. The kinetic properties of the high-affinity system were studied in detail. It proved to be highly specific, the only potent competitive inhibitors being (i) L-proline and its analogs L-azetidine-2-carboxylic acid, sarcosine, D-proline and 3,4-dehydro-DL-proline, and (ii) L-alanine. The other amino acids tested behaved as noncompetitive inhibitors. The high-affinity system is active, has a sharp pH optimum at 5.8-5.9 and, in an Arrhenius plot, exhibits two inflection points at 15 degrees C and 20-21 degrees C. It is trans-inhibited by most amino acids (but probably only the natural substrates act in a trans-noncompetitive manner) and its activity depends to a considerable extent on growth conditions. In cells grown in a rich medium with yeast extract maximum activity is attained during the stationary phase, on a poor medium it is maximal during the early exponential phase. Some 50-60% of accumulated L-proline can leave cells in 90 min (and more if washing is done repeatedly), the efflux being insensitive to 0.5 mM 2,4-dinitrophenol and uranyl ions, the pH between 3 and 7.3, as well as to the presence of 10-100 mM unlabeled L-proline in the outside medium. Its rate and extent are increased by 1% D-glucose and by 10 micrograms nystatin per ml.

Proline transport in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is capable of utilizing proline as the sole source of nitrogen. Mutants of S. cerevisiae with defective proline transport were isolated by selecting for resistance to either of the toxic proline analogs L-azetidine-2-carboxylate or 3,4-dehydro-DL-proline. Strains carrying the put4 mutation are defective in the high-affinity proline transport system. These mutants could still grow when given high concentrations of proline, due to the operation of low-affinity systems whose existence as confirmed by kinetic studies. Both systems were repressed by ammonium ions, and either was induce by proline. Low-affinity transport was inhibited by histidine, so put4 mutants were unable to grow on a medium containing high concentrations of proline to which histidine has been added.

Kinetics of L-[14C]leucine transport in Saccharomyces cerevisiae: effect of energy coupling inhibitors

1. L-[14C]Leucine transport into Saccharomyces cerevisiae involves a high-affinity, low-velocity system (system 1) and a low-affinity, high-velocity system (system 2). These systems are characterized by the different values of the kinetic parameters KT and Jmax, and are both capable of concentrative transport. The general amino acid permease is assumed to be a part of the high-affinity system. 2. The kinetics of L-[14C]leucine entrance show and initial rapid phase (the 'very early uptake') before reaching the steady-state rate. The contribution of the very early uptake to total entrance values affects the values of KT and Jmax, especially when the steady-state rate is relatively slow, as with starved yeast, and then negative KT and Jmax values may result. The very early uptake is increased by pretreatment of starved yeast and D-glucose, this latter effect being counteracted by iodoacetate. 3. After energization of starved yeast by pretreatment with D-glucose or propionaldehyde, the apparent KT,2 value greatly decreases whilst the KT,1 value decreases to a much more limited extent, or does not vary. With the energized yeast, KT,2 decreases throughout incubation whilst KT,1 variation is insignificant. Energization increases Jmax,1 and Jmax,2 several-fold and with the energized yeast at the steady-state phase, Jmax,2 greater than or equal to 4Jmax,1. Variation of KT and Jmax values as a function of the metabolic state of yeast cells may be explained in terms of variation of rate constants k-1, k+1 and k+2 for each transport system. 4. Dicyclohexylcarbodiimide, quercetin and diethylstilbestrol inhibit tranport at 0.05 mM L-[14C]leucine, in good agreement with a function of the plasmalemma ATPase for the operation of system 1. Dio-9, propionic and isobutyric acids, pentachlorophenol, carbonylcyanide 3-chlorophenylhydrazone and carbonylcyanide 4-trifluoromethoxyphenylhydrazone, which affect the proton gradient and/or the membrane potential inhibit L-[14C]leucine uptake at all the assayed amino acid concentrations. 5. The polyene antibiotic, nystatin, which forms channels in membranes permeable to K+ and H+, inhibits systems 1 and 2 activity but enniatin (also a K+ ionophore) does not.

Characteristics of proline transport in normal and starved cells of Candida albicans

1. There was no apparent correlation between the rate of respiration and rate of accumulation of proline in Candida albicans cells. 2. In contrast to normal cells, the respiration in the starved cells became completely cyanide insensitive. The starvation of cells in the presence of cycloheximide prevented the cells from becoming cyanide insensitive. The addition of Fe(III), however, accelerated the process. 3. Oxidizable substrates e.g. NADH, acetate and glucose, when added to cyanide-insensitive starved cells, exhibited 40--280% stimulation in respiration rate. However, this enhancement in oxidation by various substrates was not coupled to a simultaneous increase in the proline uptake or in intracellular ATP levels. 4. There was 6-fold stimulation in proline uptake when cyanide-insensitive cells were preincubated with 50 mM glucose. The preincubation of starved cells resulted in a partial restoration of cyanide sensitivity and increased intracellular ATP levels. The preincubation of starved cells with other oxidizable substrates resulted in a partial restoration of cyanide sensitivity but had no stimulatory effect on intracellular ATP levels and proline accumulation. 5. Both the enhanced uptake and ATP levels in glucose preincubated cells were found to be completely abolished by iodoacetate. 6. It is proposed that the increased proline uptake in cells preincubated with glucose was mainly due to the production of glycolytic energy.

Energy requirements for the uptake of L-leucine by Saccharomyces cerevisiae

(1) Substrates capable of activating mitochondrial electron transfer and oxidative phosphorylation, namely, pyruvate, acetate, propionaldehyde and butanol, stimulated the concentrative uptake (transport and accumulation) of L-[14-C]leucine by Saccharomyces cerevisiae (wild type strain 207, starved cells). Under adequate experimental conditions, the L-[14-C]leucine uptake versus the oxygen uptake ratio was almost the same with either pyruvate, acetate or D-glucose as energy sources. Substrate oxidation also increased L-[14-C]leucine incorporation into the cell protein. (2) With S. cerevisiae D261 and D247-2 and propionaldehyde as an energy source, or with strain 207 and glucose as energy source, 2,4-dinitrophenol (50 muM) inhibited L-[14-C]leucine uptake, the inhibition being accompanied by stimulation of respiration. With S. cerevisiae 207 and propionaldehyde as energy source, 2,4-dinitrophenol inhibited both respiration and L-[14-C]leucine uptake, but with respiration being less affected than uptake. Displacement of accumulated L-[14-C]leucine was also inhibited by 2,4-dinitrophenol. (3) In the presence of glucose, and for relatively brief incubation periods, anaerobically grown cells of S. cerevisiae 207 and of a p-minus "petite" mutant of this strain incorporated L-[14-C]leucine with less efficiency than the original wild type strain 207, grown aerobically. With D-glucose as energy source, 2,4-dinitrophenol and iodoacetate inhibited alike L-[14-C]leucine uptake by the respiration competent cells. (4) It is postulated that in respiration-competent yeasts, the mitochondrion contributes to 6-[14-C]leucine uptake by supplying high-energy compounds required for amino acid transport and accumulation. Conversely, the promitochondrion in the anaerobically grown yeast, or the modified mitochondrion in the respiratory deficient mutant, competes for high energy compounds generated by glycolysis in the cytosol.

The investigator and the control of clinical investigations

Cells of Saccharomyces cerevisiae showed differential growth inhibition when cultured on various carbon sources in the presence of the proline analogue thiazolidine-4-carboxylic acid (TZ). On 0.5% yeast extract, 2% glucose and TZ (10 mg/ml) medium, growth lags from 8 to 10 h were observed, after which cells recovered and growth proceeded normally. Growth was totally inhibited on a medium of 0.5% yeast extract, 3% ethanol, and 5 mg of TZ per ml. This inhibition was not due to the inability of cells to undergo aerobic respiration, since similar media containing glycerol instead of ethanol allowed growth. Proline added to the culture medium reversed the lag on glucose and TZ medium but did not promote recovery on ethanol and TZ medium. TZ was found to have two probable modes of action in yeast. It was a noncompetitive inhibitor of yeast alcohol dehydrogenase, and it was also found to be incorporated into cellular protein. Uptake studies using ¹⁴C-labeled TZ showed that the recovery on glucose was correlated with the progressive exclusion of the analogue from cells.

Uptake and utilization of glutamic acid by Cryptococcus albidus

Cryptococcus albidus utilizes glutamate as a sole carbon source. The kinetics of uptake of this amino acid were studied. l-Glutamic acid was taken up by two saturable systems: a high affinity system with a Michaelis constant (K(m)) of 1.15 x 10(-5) M and a V(max) of 0.049 mumol per mg per h and a low affinity system with a K(m) of 2.5 x 10(-3) M and a V(max) of 3.61 mumol per mg per h. Both systems possessed characteristics of active transport which were dependent on temperature and pH and which required metabolic energy. Uptake was inhibited at 37 C but the temperature-sensitive step was reversible. Chemical fractionation of cells with 5% trichloroacetic acid showed that glutamic acid initially entered a soluble pool which decreased after 1 h as the amino acid was incorporated into the protein and nucleic acid fractions of the yeast. Some of the glutamate was completely oxidized and could be recovered as (14)CO(2). Therefore, the amino acid was also used as an energy source.

Transport of S-adenosylmethionine in Saccharomyces cerevisiae

The properties of a specific system for the transport of S-adenosylmethionine in yeast are described. The process was pH-, temperature-, and energy-dependent, and showed saturation kinetics. The K(m) for the system was 3.3 x 10(-6)m. Of the S-adenosylmethionine moieties tested, only S-adenosylhomocysteine competitively inhibited the uptake of the adenosylsulfonium compound. Adenine, adenosine, methionine, homocysteine, and the sulfonium compound S-methylmethionine were without effect. The analogue S-adenosylethionine showed competitive inhibition. Under conditions of inhibition of protein synthesis by cycloheximide or methionine starvation, permease activity was stable. The mutant sam-p3 apparently was able to transport S-adenosylmethionine only by diffusion. Uptake by diploids containing this mutation was directly proportional to the gene dose.

Mutation affecting activity of several distinct amino acid transport systems in Saccharomyces cerevisiae

Mutations at the apf locus selectively depress the activity of a number of distinct amino acid permeases in Saccharomyces cerevisiae. The activity of the general amino acid permease and specific amino acid permeases is decreased, but the uptake of pyrimidines and adenine is unaffected. Mutations at the apf locus are allelic to the aap mutation isolated by Surdin et al. Amino acid uptake is normal in a heterozygous diploid (apf/+) and in a tetraploid strain with only one functional allele at the apf locus.

Interctions between potassium ions and glycine transport in the yeast Saccharomyces carlsbergensis

A study has been made of the effects of both varying the pH and extracellular [K(+)] on the initial rate of uptake of glycine (v) by a strain of Saccharomyces carlsbergensis that concentrated the amino acid, with respect to the extracellular phase, by up to 1400 times. When no other substrate than glycine was provided and [glycine] was relatively small (</=0.2mm) (1) v increased fivefold when the pH was lowered from 7 to 4.5; (2) v fell by up to about 80% as [K(+)] rose, K(+) behaving as a non-competitive inhibitor of the system, with K(i) 0.33mequiv./l at pH7; (3) the absorption of glycine caused up to about 2 or 3 equiv. of K(+) to leave the yeast cells. These three phenomena were each less evident when glucose was present. An analogy is drawn between the respective interactions of H(+) and K(+) with the yeast system and the well recognized effects of Na(+) and K(+) on amino acid transport in certain mammalian systems.

Multiplicity of the amino acid permeases in Saccharomyces cerevisiae. IV. Evidence for a general amino acid permease

Kinetic and genetic evidences are presented to show that, in addition to specific amino acid permeases, Saccharomyces cerevisiae has a general amino acid permease which catalyzes the transport of basic and neutral amino acids, but most probably not that of proline. The general amino acid permease appears to be constitutive, and its activity is inhibited when ammonium ions are added to the culture medium. A mutant which has lost the general amino acid permease activity was isolated. Its mutation, named gap (general amino acid permease), is not allelic to the aap (amino acid permease) mutation of Surdin et al., which has a quite different phenotype and cannot be considered as having selectively lost the general amino acid permease activity.

Quantitative predictions of a noncarrier model for glucose transport across the human red cell membrane

There is an increasing amount of experimental data on transport across biological membranes which cannot be readily accommodated by classical mobile carrier models. We propose models for membrane transport based upon current concepts in molecular enzymology, in which the membrane component involved in transport is an oligomeric protein which undergoes substrate-induced conformational changes. A number of paradoxical observations on glucose transport in the human erythrocyte are explained if the protein involved is a tetramer possessing two classes of binding sites with different affinities for glucose. We develop in detail a particular model of this type, the internal transfer model, in which transport occurs by transfer of substrate from one subunit to another of the protein. The fit of the predictions of the internal transfer model with most of the experimental data is very good. Those data which cannot be fitted by the model cannot be accounted for by any presently available model. We extend our model qualitatively to include the sodium-activated cotransport systems for sugars and amino acids.