R76 in transmembrane domain 3 of the aspartate:alanine transporter AspT is involved in substrate transport

Thermostability optimization of the aspartate/alanine exchange transporter from Tetragenococcus halophilus

Aspartate/alanine exchange transporter (AspT) is a secondary transporter isolated from the lactic acid bacterium Tetragenococcus halophilus D10 strain. This transporter cooperates with aspartate decarboxylase to produce proton-motive force through decarboxylative phosphorylation. A method that successfully analyzes the AspT mechanism could serve as a prototype for elucidating the substrate transport mechanism of other exchange transporters; therefore, the purpose of this study was to search for conditions that improve the thermal stability of AspT for 3D structure analysis. We used the fluorescence size-exclusion chromatography-based thermostability assay to evaluate conditions that contribute to AspT stability. We found that the AspT thermostability was enhanced at pH 5.0 to 6.0 and in the presence of Na+ and Li+. Pyridoxal phosphate, a coenzyme of aspartate decarboxylase, also had a thermostabilizing effect on AspT. Under the conditions obtained from these results, it was possible to increase the temperature at which 50% of dimer AspT remained by 14°C. We expect these conditions to provide useful information for future structural analysis of AspT.

Conformational transition induced in the aspartate:alanine antiporter by L-Ala binding

An aspartate:alanine antiporter (AspT) from the lactic acid bacterium Tetragenococcus halophilus catalyzes the electrogenic aspartate^1-:alanine⁰ exchange reaction. Our previous kinetic analyses of transport reactions mediated by AspT in reconstituted liposomes suggested that, although the substrate transport reactions are physiologically coupled, the putative binding sites of L-aspartate (-Asp) and L-alanine (-Ala) are independently located on AspT. By using the fluorescent probe Oregon Green maleimide (OGM), which reacts specifically with cysteine, we also found that the presence of L-Asp changes the conformation of AspT. In this study, we conducted an OGM labeling assay in the presence of L-Ala. The labeling efficiency of single cysteine mutants (G62C and P79C) in transmembrane helix 3 of the AspT showed novel patterns depending on the presence of L-Ala or analogs. A concentration-dependent shift of AspT from the conformation in the presence of one substrate to that specific to the substrate added subsequently (L-Ala or L-Asp) was observed. Moreover, size-exclusion-chromatography-based thermostability assays indicated that the thermal stability of AspT in the presence of L-Ala differed from that in the presence of L-Asp. From these results, we concluded that L-Ala binding yields a conformation different from the apo or L-Asp binding conformations.

Oligomeric state of the aspartate:alanine transporter (AspT) from Tetragenococcus halophilus

The aspartate:alanine exchanger family of membrane transporters includes industrially important transporters such as succinate exporter and glutamate exporter. No high-resolution structure is available from this family so far, and the transport mechanism of these transporters also remains unclear. In the present study, we focus on the oligomeric status of the aspartate:alanine antiporter (AspT) of Tetragenococcus halophilus, which is the prototype of this family. To investigate the oligomeric structure of AspT, we established a system that produces high yields of highly purified AspT and determined the oligomeric structure of AspT by analysis with size exclusion chromatography coupled with multi-angle light scattering and blue native PAGE and by comparison of the wild-type AspT with a single-cysteine mutant that forms spontaneous inter-molecular thiol crosslinking. All the results consistently support the notion that AspT is a homodimer in solutions and in membranes.