Environmental study of subunit i, a F(o) component of the yeast ATP synthase

The topology of subunit i, a component of the yeast F(o)F(1)-ATP synthase, was determined by the use of cysteine-substituted mutants. The N(in)-C(out) orientation of this intrinsic subunit was confirmed by chemical modification of unique cysteine residues with 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid. Near-neighbor relationships between subunit i and subunits 6, f, g, and d were demonstrated by cross-link formation following sulfhydryl oxidation or reaction with homobifunctional and heterobifunctional reagents. Our data suggest interactions between the unique membrane-spanning segment of subunit i and the first transmembranous alpha-helix of subunit 6 and a stoichiometry of 1 subunit i per complex. Cross-linked products between mutant subunits i and proteins loosely bound to the F(o)F(1)-ATP synthase suggest that subunit i is located at the periphery of the enzyme and interacts with proteins of the inner mitochondrial membrane that are not involved in the structure of the yeast ATP synthase.

Evidence for a dynamic role for proline376 in the purine-cytosine permease of Saccharomyces cerevisiae

The purine-cytosine permease (PCP), a carrier located in the plasma membrane of Saccharomyces cerevisiae, mediates the active transport of purine (adenine, guanine and hypoxanthine) and cytosine into the cell. Previous studies [Ferreira, T, Brèthes, D., Pinson, B., Napias, C. & Chevallier, J. et al. (1997) J. Biol. Chem. 272, 9697-9702] suggest that the hydrophilic segment 371-377 (-I-A-N-N-I-P-N-) of the polypeptide chain may play a key role in the correct three-dimensional structure of the active carrier. This paper describes the effects of mutations in this particular segment: a four-residue deletion, Delta374-377, and two substitutions, P376G and P376R. The Delta374-377 PCP was expressed in tiny amounts and was totally inactive. When compared with the wild-type, the P376G PCP showed slightly decreased amounts and was able to transport the bases with significantly increased affinity and decreased turnover. The P376R PCP was normally expressed and targeted to the plasma membrane; however, despite a normal number of base-binding sites [1000-1200 pmol.(mg protein)-1], this mutated carrier was completely unable to transport any of its ligands. In addition, the Kd(app) for hypoxanthine binding was completely independent of the pH (within the range 3.5-6.0), showing that the conformational change induced by ligand binding was no longer present. Our results show that the 374-377 segment is essential for the expression and activity of this carrier. They also show that the P376 residue is part of an unusual secondary structure, probably a beta-turn motif, which must play a crucial dynamic role in the translocation process.

Screening of an intragenic second-site suppressor of purine-cytosine permease from Saccharomyces cerevisiae. Possible role of Ser272 in the base translocation process

The purine-cytosine permease from Saccharomyces cerevisiae mediates the active transport through the plasma membrane of adenine, hypoxanthine, guanine and cytosine using the proton electrochemical potential difference as an energy source. Analysis of the activity of strains mutated in a hydrophilic segment (371-377) of the polypeptidic chain has shown the involvement of this segment in the maintenance of the active three-dimensional structure of the carrier. In an attempt to identify permease domains that could interact functionally and/or physically with this segment, we looked for second-site mutations that could suppress the effects of amino acid changes in this region. This paper describes a positive screen that has allowed the isolation of one suppressor from a permease mutant displaying the N374I change (fcy2-20 allele), a substitution that induces a dramatic decrease in the affinity of the carrier for adenine, cytosine and hypoxanthine. The second-site mutation corresponds to the replacement of the Ser272 residue by Leu. Its suppressive effect is shown to be a partial restoration of the binding of cytosine and hypoxanthine to the permease. To test whether this second-site mutation is specific for the fcy2-20 allele, two double mutants were constructed (Fcy2pT213I, S272L and Fcy2pS272L, N377G). Results obtained with these two double mutants showed that the suppressive effect of S272 L replacement was not specific for the original N374I change. To understand the general effect of this amino acid replacement for the three distinct double mutants, a strain overexpressing Fcy2pS272I, was constructed. Kinetic analysis of this strain showed that, by itself, the S272 L change induced an improvement in the base-binding step that could account for its global suppressive effect. Moreover, S272 L induced a decrease in the turnover of the permease, thus showing the involvement of S272 in the translocation process. Taking into account the topological model of the permease proposed here, this Ser residue is probably located in a transmembrane amphipathic alpha-helix (TM5). The location and the observed decrease in the turnover of the carrier observed with the S272 L change lead us to propose that S272 could be part of a hydrophilic pore involved in the translocation of the base and/or the proton.

Characterization of the Saccharomyces cerevisiae cytosine transporter using energizable plasma membrane vesicles

The purine-cytosine permease is a carrier localized in the plasma membrane of the yeast Saccharomyces cerevisiae. The energetics of cytosine transport catalyzed by this permease has been studied in an artificial system obtained by fusion between proteoliposomes containing beef heart cytochrome c oxidase and plasma membrane-enriched fractions of a S. cerevisiae strain overexpressing the permease. Upon addition of an energy donor, a proton-motive force (inside alkaline and negative) is created in this system and promotes cytosine accumulation. By using different phospholipids, it is shown that cytosine uptake is dependent on the phospholipids surrounding the carrier. It was demonstrated that the purine-cytosine permease is able to catalyze a secondary active transport of cytosine. By using nigericin and valinomycin, the DeltapH component of the proton-motive force is shown to be the only force driving nucleobase accumulation. Moreover, transport measurements done at two pH values have shown that alkalinization of intravesicular pH leads to a significant increase in cytosine uptake rate. Finally, no specific role of K+ ions on cytosine transport could be demonstrated in this system.

Functional analysis of mutated purine-cytosine permease from Saccharomyces cerevisiae. A possible role of the hydrophilic segment 371-377 in the active carrier conformation

The purine-cytosine permease (PCP) is an active transporter located in the plasma membrane of the yeast Saccharomyces cerevisiae. This protein mediates purine (adenine, guanine, and hypoxanthine) and cytosine accumulation in the cell by using an electrochemical potential difference in proton as the energy source. Various mutant strains, with altered Kt(app) (apparent Michaelis constant of transport) of uptake for one or several bases, have already been selected. Their cloning and sequencing revealed that three of them presented substitutions in the same region of the putative sequence of the PCP: this region might correspond to the hydrophilic segment 371-377 (I-A-N-N-I-P-N). Two mutants displayed single mutations, resulting in only one amino acid residue change (N377I and N374I, respectively), and the other displayed three amino acid substitutions (I371V, I375V, and N377G). Therefore, to analyze the contribution of individual amino acid changes to the phenotype of the complex mutant, single (N377G) and double (I371V,I375V) mutants were constructed by site-directed mutagenesis. The influence of single mutations in this region was studied by measuring, for adenine, hypoxanthine, and cytosine, the uptake constants on cells and equilibrium binding parameters on plasma membrane-enriched fractions. Uptake and binding constant determinations showed that all the variations observed for the Kt(app) of uptake were correlated with variations of the binding Kd(app) for the corresponding solutes. Thus, our results emphasize the role of the two asparagine residues, located at positions 374 and 377, respectively, in the binding of the bases. In addition, the sole substitution of the 377 asparagine residue by glycine is responsible for the phenotype of the triple mutant. The effect of pH on the apparent hypoxanthine binding dissociation constant showed that the effects of N377G and N377I changes were, at least partially, due to a shift of the pKa of an ionizable amino acid residue of the unliganded permease. These two amino acid residue changes induced a shift of the pKa of this group in the unliganded, deprotonated permease about two units toward acidic pH. This result suggests that the 371-377 segment might play a key role in the proper three-dimensional structure of the active purine-cytosine permease.

In vivo phosphorylation of the purine/cytosine permease from the plasma membrane of the yeast Saccharomyces cerevisiae

The purine/cytosine permease, encoded by the FCY2 gene, is a carrier located in the plasma membrane of the yeast Saccharomyces cerevisiae. Polyclonal antibodies were raised against two peptides that corresponded to the sub-N-terminal and C-terminal sequences of the putative protein deduced from the FCY2 gene. Immunoprecipitation experiments performed with protein extracts labelled in vivo with 35S showed that purine/cytosine permease is specifically detected as a broad and diffuse band. The apparent molecular mass of this protein was 45-50 kDa. By means of in vivo pulse/chase 35S-labelling experiments, we observed a slight increase in the apparent molecular mass of purine/cytosine permease during the chase. This shift in electrophoretic mobility of the protein suggested a post-translational modification. This molecular mass increase was eliminated by alkaline phosphatase treatment of the immunoprecipitate, which strongly suggested phosphorylation of the carrier. This proposal was confirmed by in vivo [32P]P(i) labelling and immunoprecipitation of purine/cytosine permease with purified anti-(sub-N-terminal peptide) IgG or anti-(C-terminal peptide) IgG. Phosphoamino acid analysis indicated that phosphorylation occurred on seryl residues of purine/cytosine permease. By means of thermosensitive secretory-pathway-mutant strains, we demonstrated that purine/cytosine permease phosphorylation occurred either between the Golgi apparatus and the plasma membrane or in the plasma membrane itself.

Purine-cytosine permease of Saccharomyces cerevisiae. Effect of external pH on nucleobase uptake and binding

The cloned FCY2 gene (strain pAB4) of the purine-cytosine permease (PCP) of Saccharomyces cerevisiae and the cloned allele fcy2-21 (strain pAB25) introduced into an S. cerevisiae strain carrying a chromosomal deletion at the FCY2 locus [Weber, E., Rodriguez, C., Chevallier, M. R. & Jund, R. (1990) Mol. Microbiol. 4, 585-596] were studied. The influence of external pH (varying over 3.5-6) has been analysed on the uptake of adenine, hypoxanthine and cytosine (Ktapp, apparent Michaelis constant and Vm) and on the binding constants of these three solutes (Kdapp, apparent half-saturation constant and Bmax, total binding sites) determined on plasma membranes. For pAB4, the variations of Ktapp and Vm were the same for the three bases, i.e. an increase in Ktapp when the pH increased and a maximum Vm around pH 5. For pAB25, Ktapp values varied in the same way and were significantly higher for the three bases than those found in pAB4. There was almost no variation of Vm for adenine, and there was a continuous decrease when the pH increased in the Vm of hypoxanthine and cytosine. Equilibrium binding measurements were performed for the three bases with plasma membrane isolated from pAB4 and pAB25. One single class of binding sites was detected. For pAB4, the affinity increased when the pH decreased for the three bases. The affinity of PCP for adenine was always greater than for cytosine or hypoxanthine. For pAB25, the same phenomenon was observed. However, the curves showing the variation of Kdapp as a function of pH were shifted towards more acidic pH values. A model was used to fit the experimental binding data obtained with hypoxanthine for the calculation of the dissociation constants of its binding to PCP and to determine the ionization constants of an amino acid involved in ligand binding. For pAB4, at acid pH, the dissociation constant was 1.7 +/- 0.4 microM. An amino acid displaying a pK of 3.8 was determined; this value was shifted to pK 4.8 when hypoxanthine was bound. For pAB25, the main effects of the mutation were a large decrease in the affinity of PCP for hypoxanthine (Kd of 14.4 +/- 4.3 microM) and a shift in the pK of the amino acid towards a more acidic pH (about 2.9). The pK of this group remained similar to the value obtained with pAB4 when hypoxanthine was bound. From these data, it is proposed that the binding of hypoxanthine and H+ is a random process.

In vivo and in vitro studies of the purine-cytosine permease of Saccharomyces cerevisiae. Functional analysis of a mutant with an altered apparent transport constant of uptake

The FCY2 gene of the purine-cytosine permease (PCP) of Saccharomyces cerevisiae and the allele fcy2-21 have been cloned on the yeast multicopy plasmid pJDB207. The corresponding plasmids were introduced into a S. cerevisiae strain carrying a chromosomal deletion at the FCY2 locus. The resulting strains were designated pAB4 and pAB25 respectively. The pAB25 strain, which carries the fcy2-21 allele, contains four amino acid changes in the open reading frame of the PCP (Weber et al., 1989). The influence of these mutations was studied on cells by determination of the uptake constants of purine bases and cytosine [apparent Michaelis constant of transport (Ktapp) and Vmax] and on plasma-membrane preparations, by measurements of binding parameters at equilibrium [(Kd and maximum amount of binding sites/Bmax)]. For strain pAB4, the Ktapp and Vmax of uptake were almost similar for all solutes considered [1.8-2.6 microM and 8.5-10.2 nmol.min-1.(10(7) cells)-1]. The main effect of the mutations in strain pAB25 was based on a large increase in Ktapp for all ligands except adenine. Plasma membranes of each strain displayed one class of specific binding sites. Variations in Kd of 0.4-1 microM were observed for pAB4. These slight variations had no effect on the Ktapp of uptake measured for the corresponding solutes. In contrast, using pAB25 membranes, Kd increased dramatically; 2.6 microM, 40 microM and 96 microM for adenine, cytosine and hypoxanthine, respectively. These increments were correlated to variations in Ktapp of the uptake for cytosine and hypoxanthine. Therefore, we conclude that modification in the Ktapp of uptake in the strain carrying fcy2-21 allele is merely due to a modification of the binding ability of the permease for its ligands.

Photoaffinity labelling of the purine-cytosine permease of Saccharomyces cerevisiae

8-Azidoadenine was used as a photoaffinity reagent to characterize the purine-cytosine permease of Saccharomyces cerevisiae. It is a potent competitive inhibitor of cytosine uptake and irradiation of the cells incubated with the label induced the irreversible inactivation of cytosine uptake. Addition of excess cytosine prevented this labelling which was restricted to the outer face of the plasma membrane since it was not accumulated by the cells. In the strain with the amplified purine-cytosine permease gene the maximum cytosine uptake rate was increased 4-5-fold relative to wild type without a modification of the Michaelis constant of uptake (Kt); no uptake could be measured in the deleted strain. The relative amounts of specific labelling determined for the cells and for membrane preparations were 0, 1 and 4 for the null, the wild-type and the amplified strains, respectively. One major band specifically labelled by [3H]azidoadenine, corresponding to a polypeptide with an apparent molecular mass of 45 kDa, was observed in the wild type, amplified in the strain carrying the multicopy plasmid and not detected in the deleted strain. Therefore this polypeptide corresponds to the purine-cytosine permease.

Simultaneous preparation of membrane fractions from small amounts of skeletal muscle: a study on mitochondrial and microsomal fractions from MedJ mice

This work is the first biochemical study of skeletal muscle membranes isolated from mice displaying an inherited neuromuscular disease: MedJ strains. It is focused on the research of a possible alteration of membrane biological activities related to this disease. We describe a procedure which allows the simultaneous preparation of mitochondrial and microsomal fractions from a small amount of skeletal muscle. When EGTA and BSA are present in the buffers, functional mitochondria can be prepared. Under these conditions we found that no major modification occurs for this disease at the mitochondrial inner membrane level. A dramatic impairment of a calcium active transport activity found in the microsomal fraction obtained from MedJ is noticed, suggesting that some modification may occur at this level.

Dihydropicrotoxinin binding sites in mammalian brain: interaction with convulsant and depressant benzodiazepines

The specific binding of [3H] alpha-dihydropicrotoxinin to rat brain membranes was inhibited competitively and potently (IC50 congruent to 100 nM) by a convulsant benzodiazepine drug, RO5-3663. This compound did not inhibit high affinity flunitrazepam binding to the same tissue under similar conditions, and its reported pharmacological activity as an antagonist of GABAergic synaptic transmission, which resembles that of picrotoxinin, appears to involve the picrotoxinin binding sites. Other benzodiazepines such as diazepam, in micromolar concentrations, inhibited picrotoxinin binding in a stereospecific and chemically specific manner. However, the order of potency of a series of depressant benzodiazepines did not correlate well with pharmacological activities nor with reported activities for displacement of high affinity benzodiazepine 'receptor' binding sites (although heterogeneity of both picrotoxinin and benzodiazepine binding site populations may make difficult such comparisons). A comparison of benzodiazepine-displaceable benzodiazepine binding and benzodiazepine-displaceable picrotoxinin binding for different brain regions and subcellular fractions revealed a very similar though not identical distribution of these two classes of drug receptor, again suggesting that the two are not identical. Both classes of drug binding site also showed a very similar distribution to sodium-independent GABA receptor binding sites, which is consistent with other evidence that at least part of these 3 receptor types may be found at least sometimes coupled together in the postsynaptic membrane GABA receptor-ionophore complex.

Phospholipase A2 activity and substrate specificity of snake venom presynaptic toxins

Beta-Neurotoxins from certain snake venoms are highly specific toxins acting at the presynaptic side of the neuromuscular junction. In this study biochemical aspects of this high specificity have been investigated. When toxins (notexin and Naja nigricollis basic phospholipase) act on a mixture of subcellular fractions obtained from brain cortex (synaptosomes, myelin, and mitochondria), the synaptosomal fraction is preferentially attacked and shows the highest release of membrane protein. As seen from isolated fractions, however, even the mitochondria are rapidly and strongly attached. Examining the phospholipase A2 activity of the toxin instead of the release of proteins reveals that synaptosomes represent the best substrate. In contrast to nonneurotoxic phospholipases A2, that from neurotoxin preferentially uses synaptosomal phosphatidylcholine as a substrate when pure phospholipids isolated from subcellular fractions are used. A relationship between the cholesterol/phospholipid ratio and the sensitivity to toxin action in the various subcellular fractions was found. These data suggest that the neurotoxic effect is mainly due to the substrate specificity of the beta-neurotoxins. It is suggested that synaptosomal phosphatidylcholine, embedded in a membrane containing a low amount of cholesterol, is a highly specific substrate for beta-neurotoxins.

Phosphate transport in yeast mitochondria: purification and characterization of a mitoribosomal synthesis dependent proteolipid showing a high affinity for phosphate

It is possible to obtain from yeast mitochondria a proteolipid able to bind phosphate, by two different procedures. One of them, generally used for lipid extraction, leads to the preparation of a more active crude proteolipid. This crude proteolipid has been purified by various chromatographic procedures and the active fraction, in phosphate binding, is always associated with cardiolipin. Its molecular weight seems to be close to 10000. The phosphate binding shows ligand saturation behavior and is inhibited by arsenate and N-ethylmaleimide; succinate is noninhibitory. This protein seems to be dependent on the mitoribosomal synthesis since it is not present in mitochrondria of mutant "petite colonie" and its amount largely decreases in mitochondria from yeast grown in the presence of chloramphenicol. It is possible to extract a proteolipid from the oligomycin sensitive ATPase, showing the same activity and properties. The hypothesis that this proteolipid acts as a part of the Pi carrier and constitutes the oligomycin-sensitive ATPase complex is discussed.