Transinhibition kinetics of the sulfate transport system of Penicillium notatum: analysis based on an iso uni uni velocity equation

Inactivation of Saccharomyces cerevisiae sulfate transporter Sul2p: use it and lose it

Saccharomyces cerevisiae SO(4)(=) transport is regulated over a wide dynamic range. Sulfur starvation causes ∼10,000-fold increase in the (35)SO(4)(=) influx mediated by transporters Sul1p and Sul2p; >80% of the influx is via Sul2p. Adding methionine to S-starved cells causes a 50-fold decline (t(1/2) ∼5 min) in SUL1 and SUL2 mRNA but a slower decline (t(1/2) ∼1 h) in transport. In contrast, SO(4)(=) addition does not affect mRNA but causes a rapid (t(1/2) = 2-4 min) decrease in transport. In met3Δ cells (unable to metabolize SO(4)(=)), addition of SO(4)(=) to S-starved cells causes inactivation of (35)SO(4)(=) influx over times in which cellular SO(4)(=) contents are nearly constant. The relationship between cellular SO(4)(=) and transport inactivation shows that cellular SO(4)(=) is not the signal for Sul2p inactivation. Instead, the transport inactivation rate has the same dependence on extracellular SO(4)(=) as (35)SO(4)(=) influx, indicating that Sul2p exhibits use-dependent inactivation; the transport process itself increases the probability of Sul2p inactivation and degradation. In addition, there is a transient efflux of SO(4)(=) shortly after adding >0.02 mM SO(4)(=) to S-starved met3Δ cells. This transient efflux provides further protection against excessive SO(4)(=) influx and may represent an alternate transport mode of Sul2p.

Molybdate transport through the plant sulfate transporter SHST1

Molybdenum is an essential micronutrient required by plants. The mechanism of molybdenum uptake in plants is poorly understood, however, evidence has suggested that sulfate transporters may be involved. The sulfate transporter from Stylosanthes hamata, SHST1, restored growth of the sulfate transport yeast mutant, YSD1, on media containing low amounts of molybdate. Kinetic analysis using 99MoO4(2-) demonstrated that SHST1 enhanced the uptake of molybdate into yeast cells at nM concentrations. Uptake was not inhibited by sulfate, but sulfate transport via SHST1 was reduced with molybdate. These results are the first measurement of molybdate transport by a characterised plant sulfate transport protein.

A high-affinity molybdate transporter in eukaryotes

Molybdenum is an essential element for almost all living beings, which, in the form of a molybdopterin-cofactor, participates in the active site of enzymes involved in key reactions of carbon, nitrogen, and sulfur metabolism. This metal is taken up by cells in form of the oxyanion molybdate. Bacteria acquire molybdate by an ATP-binding-cassette (ABC) transport system in a widely studied process, but how eukaryotic cells take up molybdenum is unknown because molybdate transporters have not been identified so far. Here, we report a eukaryotic high-affinity molybdate transporter, encoded by the green alga Chlamydomonas reinhardtii gene MoT1. An antisense RNA strategy over the MoT1 gene showed that interference of the expression of this gene leads to the inhibition of molybdate transport activity and, in turn, of the Mo-containing enzyme nitrate reductase, indicating a function of MoT1 in molybdate transport. MOT1 functionality was also shown by heterologous expression in Saccharomyces cerevisiae. Molybdate uptake mediated by MOT1 showed a K(m) of approximately 6 nM, which is the range of the lowest K(m) values reported and was activated in the presence of nitrate. Analysis of deduced sequence from the putative protein coded by MoT1 showed motifs specifically conserved in similar proteins present in the databases, and defines a family of membrane proteins in both eukaryotes and prokaryotes probably involved in molybdate transport and distantly related to plant sulfate transporters SULTR. These findings represent an important step in the understanding of molybdate transport, a crucial process in eukaryotic cells.

Evidence for two distinct intracellular pools of inorganic sulfate in Penicillium notatum

A strain of Penicillium notatum unable to metabolize inorganic sulfate can accumulate sulfate internally to an apparent equilibrium concentration 10(5) greater than that remaining in the medium. The apparent Keq is near constant at all initial external sulfate concentrations below that which would eventually exceed the internal capacity of the cells. Under equilibrium conditions of zero net flux, external 35SO42- exchanges with internal, unlabeled SO42- at a rate consistent with the kinetic constants with the sulfate transport system. Efflux experiments demonstrated that sulfate occupies two distinct intracellular pools. Pool 1 is characterized by the rapid release of 35SO42- when the suspension of preloaded cells is adjusted to 10 mM azide at pH 8.4 (t 1/2, 0.38 min). 35SO42- in pool 1 also rapidly exchanges with unlabeled medium sulfate. Pool 2 is characterized by the slow release of 35SO42- induced by azide at pH 8.4 or unlabeled sulfate (t 1/2, 32 to 49 min). Early in the 35SO42- accumulation process, up to 78% of the total transported substrate is found in pool 1. At equilibrium, pool 1 accounts for only about 2% of the total accumulated 35SO42-. The kinetics of 35SO42- accumulation is consistent with the following sequential process: medium----pool 1----pool 2. Monensin (33 microns) accelerates the transfer of 35SO42- from pool 1 to pool 2. Valinomycin (0.2 microM) and tetraphenylboron- (1 mM) retard the transfer of 35SO42- from pool 1 to pool 2. At the concentrations used, neither of the ionophores nor tetraphenylboron- affect total 35SO42- uptake. Pool 2 may reside in a vacuole or other intracellular organelle. A model for the transfer of sulfate from pool 1 to pool 2 is presented.

Dual system for potassium transport in Saccharomyces cerevisiae

In a newly formulated growth medium lacking Na+ and NH4+, Saccharomyces cerevisiae grew maximally at 5 microM K+. Cells grown under these conditions transported K+ with an apparent Km of 24 microM, whereas cells grown in customary high-K+ medium had a significantly higher Km (2 mM K+). The two types of transport also differed in carbonyl cyanide-m-chlorophenyl hydrazone sensitivity, response to ATP depletion, and temperature dependence. The results can be accounted for either by two transport systems or by one system operating in two different ways.

Proton-sulfate co-transport: mechanism of H+ and sulfate addition to the chloride transporter of human red blood cells

Proton and sulfate inhibition of the obligatory chloride-chloride exchange of human erythrocytes was measured at 0 degrees C to determine their mechanism of reaction with the anion transporter. The proton and sulfate that are co-transported by this mechanism at higher temperatures behaved as nontransported inhibitors at 0 degrees C. We analyzed the data in terms of four molecular mechanisms: (1) HSO4- addition to the transporter; (2) ordered addition with the proton first; (3) ordered addition with the sulfate first; (4) random addition to the transporter. The Dixon plots of 1/MCl vs. [SO4] at different proton concentrations were not parallel. Thus protons and sulfate ions were not mutually exclusive inhibitors. The slope of these Dixon plots was independent of pH above 7.0, which indicates that sulfate could bind to the unprotonated carrier and excludes the first two mechanisms. Protons were inhibitors of chloride flux in the absence of sulfate, which indicates that protons could bind to the unloaded carrier and excludes mechanism 3. The KI for sulfate was 4.35 +/0 0.36 mM. The pK for the protonatable group was 5.03 +/- 0.02. The binding of either a proton or sulfate to the carrier decreased the KI of the other by ninefold. The only simple mechanism consistent with the data is a random-ordered mechanism with more transporters loaded with a sulfate than loaded with a proton at the pH and sulfate concentrations of plasma.

Adaptive changes in phosphate uptake by the fungus Neurospora crassa in response to phosphate supply

The phosphate uptake rate of Neurospora crassa germlings growing exponentially in media containing phosphate at concentrations between 10 mM and 50 micronM was virtually constant. The uptake characteristics of these germlings were studied in detail assuming the simultaneous operation of two uptake systems, one of low affinity and one of high affinity. The Km of the low-affinity system was constant after growth at phosphate concentrations greater than 1 mM but became progressively lower as the concentration was reduced below 1 mM. In contrast, the Km of the high-affinity system was independent of the phosphate concentration of the growth medium. The Vmax of each system was highest after growth at low phosphate concentrations. As the phosphate concentration was increased to a maximum of 100 mM, the Vmax of the low-affinity system fell gradually, whereas that of the high-affinity system at first fell rapidly but then reached a constant minimum value at concentrations of 2.5 mM and higher. The differences in the kinetic parameters fully account for the constancy of uptake rate shown by the germlings.

Characterization of cytosine permeation in Saccharomyces cerevisiae

Cytosine permeation in Saccharomyces cerevisiae has been studied. Cytosine uptake is mediated by a permease which is also responsible for purines transport. The Km for the transport of various substrates of this permease have been determined. By means of appropriate selective techniques, mutants with altered Km and mutants lacking the permease have been selected. Cytosine transport is active and is inhibited by 2,4-dinitrophenol, an uncoupler of oxidative phosphorylation, and by N-ethylmaleimide, a reagent of--SH group. Internal labeled cytosine is chased by addition of unlabeled cytosine in the medium. These results support the hypothesis of a carrier-mediated transport, with reduced internal affinity, allowing the release and accumulation of cytosine in the inner compartment. The efflux of cytosine from cytosine permease-less cells has also been studied and shows first order kinetics. A diffusion coefficient of 5.7 per 10- minus 8 cm per S- minus 1 has been evaluated for this efflux.

Regulation of methionine transport activity in Escherichia coli

Methionine transport activity in cells of Escherichia coli K-12 is regulated by the level of the internal methionine pool. Transport activity is depressed either in cells grown in the presence of methionine or in cells exposed to methionine immediately prior to harvest. Alpha-Keto-gamma-methiol-butyrate, D-methionine, or methionine sulfoxide have little effect on the initial rate of uptake of L-methionine when they are added simultaneously with the substrate. However, methionine transport is markedly reduced in cells exposed to these sources of L-methionine before the addition of substrate. This reduction is prevented if the cells are treated with amino oxyacetic acid. The initial rate of uptake into L-methionine-loaded cells was lower than that into unloaded cells. This inhibition affected both methionine transport systems and the inhibition by the internal pool appeared to be non-competitive with the external methionine concentration. Two classes of mutants with increased methionine pools have decreased rates of uptake. Conversely, starvation for methionine in a methionine auxotroph with high rates of methionine degradation resulted in a substantial increase in the rate of methionine transport. Thus, these transport systems are subject to regulation by the internal pool size and possibly by repression.