Pre-steady-state currents in neutral amino acid transporters induced by photolysis of a new caged alanine derivative

Transient kinetics reveal the mechanism of competitive inhibition of the neutral amino acid transporter ASCT2

ASCT2 (alanine serine cysteine transporter 2), a member of the solute carrier 1 family, mediates Na+-dependent exchange of small neutral amino acids across cell membranes. ASCT2 was shown to be highly expressed in tumor cells, making it a promising target for anticancer therapies. In this study, we explored the binding mechanism of the high-affinity competitive inhibitor L-cis hydroxyproline biphenyl ester (Lc-BPE) with ASCT2, using electrophysiological and rapid kinetic methods. Our investigations reveal that Lc-BPE binding requires one or two Na+ ions initially bound to the apo-transporter with high affinity, with Na1 site occupancy being more critical for inhibitor binding. In contrast to the amino acid substrate bound form, the final, third Na+ ion cannot bind, due to distortion of its binding site (Na2), thus preventing the formation of a translocation-competent complex. Based on the rapid kinetic analysis, the application of Lc-BPE generated outward transient currents, indicating that despite its net neutral nature, the binding of Lc-BPE in ASCT2 is weakly electrogenic, most likely because of asymmetric charge distribution within the amino acid moiety of the inhibitor. The preincubation with Lc-BPE also led to a decrease of the turnover rate of substrate exchange and a delay in the activation of substrate-induced anion current, indicating relatively slow Lc-BPE dissociation kinetics. Overall, our results provide new insight into the mechanism of binding of a prototypical competitive inhibitor to the ASCT transporters.

Functional and Kinetic Comparison of Alanine Cysteine Serine Transporters ASCT1 and ASCT2

Neutral amino acid transporters ASCT1 and ASCT2 are two SLC1 (solute carrier 1) family subtypes, which are specific for neutral amino acids. The other members of the SLC1 family are acidic amino acid transporters (EAATs 1–5). While the functional similarities and differences between the EAATs have been well studied, less is known about how the subtypes ASCT1 and 2 differ in kinetics and function. Here, by performing comprehensive electrophysiological analysis, we identified similarities and differences between these subtypes, as well as novel functional properties, such as apparent substrate affinities of the inward-facing conformation (in the range of 70 μM for L-serine as the substrate). Key findings were: ASCT1 has a higher apparent affinity for Na⁺, as well as a larger [Na⁺] dependence of substrate affinity compared to ASCT2. However, the general sequential Na⁺/substrate binding mechanism with at least one Na⁺ binding first, followed by amino acid substrate, followed by at least one more Na⁺ ion, appears to be conserved between the two subtypes. In addition, the first Na⁺ binding step, presumably to the Na3 site, occurs with high apparent affinity (<1 mM) in both transporters. In addition, ASCT1 and 2 show different substrate selectivities, where ASCT1 does not respond to extracellular glutamine. Finally, in both transporters, we measured rapid, capacitive charge movements upon application and removal of amino acid, due to rearrangement of the translocation equilibrium. This charge movement decays rapidly, with a time constant of 4–5 ms and recovers with a time constant in the 15 ms range after substrate removal. This places a lower limit on the turnover rate of amino acid exchange by these two transporters of 60–80 s⁻¹.

Electrogenic Steps Associated with Substrate Binding to the Neuronal Glutamate Transporter EAAC1

Glutamate transporters actively take up glutamate into the cell, driven by the co-transport of sodium ions down their transmembrane concentration gradient. It was proposed that glutamate binds to its binding site and is subsequently transported across the membrane in the negatively charged form. With the glutamate binding site being located partially within the membrane domain, the possibility has to be considered that glutamate binding is dependent on the transmembrane potential and, thus, is electrogenic. Experiments presented in this report test this possibility. Rapid application of glutamate to the wild-type glutamate transporter subtype EAAC1 (excitatory amino acid carrier 1) through photo-release from caged glutamate generated a transient inward current, as expected for the electrogenic inward movement of co-transported Na(+) In contrast, glutamate application to a transporter with the mutation A334E induced transient outward current, consistent with movement of negatively charged glutamate into its binding site within the dielectric of the membrane. These results are in agreement with electrostatic calculations, predicting a valence for glutamate binding of -0.27. Control experiments further validate and rule out other possible explanations for the transient outward current. Electrogenic glutamate binding can be isolated in the mutant glutamate transporter because reactions, such as glutamate translocation and/or Na(+) binding to the glutamate-bound state, are inhibited by the A334E substitution. Electrogenic glutamate binding has to be considered together with other voltage-dependent partial reactions to cooperatively determine the voltage dependence of steady-state glutamate uptake and glutamate buffering at the synapse.

Light activation of Staphylococcus aureus toxin YoeBSa1 reveals guanosine-specific endoribonuclease activity

The Staphylococcus aureus chromosome harbors two homologues of the YefM-YoeB toxin-antitoxin (TA) system. The toxins YoeBSa1 and YoeBSa2 possess ribosome-dependent ribonuclease (RNase) activity in Escherichia coli. This activity is similar to that of the E. coli toxin YoeBEc, an enzyme that, in addition to ribosome-dependent RNase activity, possesses ribosome-independent RNase activity in vitro. To investigate whether YoeBSa1 is also a ribosome-independent RNase, we expressed YoeBSa1 using a novel strategy and characterized its in vitro RNase activity, sequence specificity, and kinetics. Y88 of YoeBSa1 was critical for in vitro activity and cell culture toxicity. This residue was mutated to o-nitrobenzyl tyrosine (ONBY) via unnatural amino acid mutagenesis. YoeBSa1-Y88ONBY could be expressed in the absence of the antitoxin YefMSa1 in E. coli. Photocaged YoeBSa1-Y88ONBY displayed UV light-dependent RNase activity toward free mRNA in vitro. The in vitro ribosome-independent RNase activity of YoeBSa1-Y88ONBY, YoeBSa1-Y88F, and YoeBSa1-Y88TAG was significantly reduced or abolished. In contrast to YoeBEc, which cleaves RNA at both adenosine and guanosine with a preference for adenosine, YoeBSa1 cleaved mRNA specifically at guanosine. Using this information, a fluorometric assay was developed and used to determine the kinetic parameters for ribosome-independent RNA cleavage by YoeBSa1.

Voltage-dependent processes in the electroneutral amino acid exchanger ASCT2

Neutral amino acid exchange by the alanine serine cysteine transporter (ASCT)2 was reported to be electroneutral and coupled to the cotransport of one Na⁺ ion. The cotransported sodium ion carries positive charge. Therefore, it is possible that amino acid exchange is voltage dependent. However, little information is available on the electrical properties of the ASCT2 amino acid transport process. Here, we have used a combination of experimental and computational approaches to determine the details of the amino acid exchange mechanism of ASCT2. The [Na⁺] dependence of ASCT2-associated currents indicates that the Na⁺/amino acid stoichiometry is at least 2:1, with at least one sodium ion binding to the amino acid–free apo form of the transporter. When the substrate and two Na⁺ ions are bound, the valence of the transport domain is +0.81. Consistently, voltage steps applied to ASCT2 in the fully loaded configuration elicit transient currents that decay on a millisecond time scale. Alanine concentration jumps at the extracellular side of the membrane are followed by inwardly directed transient currents, indicative of translocation of net positive charge during exchange. Molecular dynamics simulations are consistent with these results and point to a sequential binding process in which one or two modulatory Na⁺ ions bind with high affinity to the empty transporter, followed by binding of the amino acid substrate and the subsequent binding of a final Na⁺ ion. Overall, our results are consistent with voltage-dependent amino acid exchange occurring on a millisecond time scale, the kinetics of which we predict with simulations. Despite some differences, transport mechanism and interaction with Na⁺ appear to be highly conserved between ASCT2 and the other members of the solute carrier 1 family, which transport acidic amino acids.

Electrophysiological characterization of membrane transport proteins

Active transport in biological membranes has been traditionally studied using a variety of biochemical and biophysical techniques, including electrophysiology. This review focuses on aspects of electrophysiological methods that make them particularly suited for the investigation of transporter function. Two major approaches to electrical recording of transporter activity are discussed: (a) artificial planar lipid membranes, such as the black lipid membrane and solid supported membrane, which are useful for studies on bacterial transporters and transporters of intracellular compartments, and (b) patch clamp and voltage clamp techniques, which investigate transporters in native cellular membranes. The analytical power of these methods is highlighted by several examples of mechanistic studies of specific membrane proteins, including cytochrome c oxidase, NhaA Na(+)/H(+) exchanger, ClC-7 H(+)/Cl(-) exchanger, glutamate transporters, and neutral amino acid transporters. These examples reveal the wealth of mechanistic information that can be obtained when electrophysiological methods are used in combination with rapid perturbation approaches.

A view on phosphate ester photochemistry by time-resolved solid state NMR. Intramolecular redox reaction of caged ATP

The light-driven intramolecular redox reaction of adenosine-5'-triphosphate-[P3-(1-(2-nitrophenyl)-ethyl)]ester (caged ATP) has been studied in frozen aqueous solution using time-resolved solid state NMR spectroscopy under continuous illumination conditions. Cleavage of the phosphate ester bond leads to 0.3, 1.36, and 6.06 ppm downfield shifts of the alpha-, beta-, and gamma-phosphorus resonances of caged ATP, respectively. The observed rate of ATP formation is 2.4 +/- 0.2 h(-1) at 245 K. The proton released in the reaction binds to the triphosphate moiety of the nascent ATP, causing the upfield shifts of the 31P resonances. Analyses of the reaction kinetics indicate that bond cleavage and proton release are two sequential processes in the solid state, suggesting that the 1-hydroxy,1-(2-nitrosophenyl)-ethyl carbocation intermediate is involved in the reaction. The beta-phosphate oxygen atom of ATP is protonated first, indicating its proximity to the reaction center, possibly within hydrogen bonding distance. The residual linewidth kinetics are interpreted in terms of chemical exchange processes, hydrogen bonding of the beta-phosphate oxygen atom and evolution of the hydrolytic equilibrium at the triphosphate moiety of the nascent ATP. Photoreaction of caged ATP in situ gives an opportunity to study structural kinetics and catalysis of ATP-dependent enzymes by NMR spectroscopy in rotating solids.