Differential transport properties of D-leucine and L-leucine in the archaeon,Halobacterium salinarum

Abstract

The transport of D-leucine was compared with that of L-leucine in Halobacterium salinarum. When a high-outside/low-inside Na+gradient was imposed, D-leucine as well as L-leucine accumulated in envelope vesicles, supporting the hypothesis that D-leucine is transported via a symport system along with Na+. Kinetic analyses, including inhibition experiments, indicated that both enantiomers are transported via a common carrier. However, a Hill plot indicated a single binding site for Na+during L-leucine transport, but dual binding sites for Na+during D-leucine transport. Furthermore, D-leucine transport was dependent on electrical membrane potential, suggesting that a transporter bound with D-leucine is positively charged. L-leucine transport was slightly, if at all, dependent on membrane potential, suggesting that a transporter bound with L-leucine is electrically neutral. These results indicate that the leucine carrier in Halobacterium salinarum translocates two moles of Na+per mole of D-leucine, and one mole of Na+per mole of L-leucine.Key words: D-leucine, sodium ion-dependent transport, stoichiometry, stereospecific recognition, halophilic archaea.