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.