Control of leucine transport in yeast by periplasmic binding proteins

Abstract

The concentrative inward transport of leucine in Saccharomyces carlsbergensis involves two transport systems (S 1 and S 2); S 1 is a system of high affinity and low translocation velocity, and S 2 is a system of low affinity and high translocation velocity. The inward transport process of the amino acid is discriminated into two kinetically defined steps: first, binding to periplasmic proteins and second, translocation across the plasmalemma. When cells were incubated with glucose to increase the metabolic energy charge, we observed that J Tmax (maximum flux that each system can exhibit for the translocation step) increased for both systems. This increase in J Tmax is due to variations in the parameters defining the initial step ( K s (apparent dissociation constant) and N (concentration of binding sites)):for S 1, N 1 increases and for S 2, K S 2 diminishes. Dissipation of the electrochemical proton gradient produced an increase of K S1 and a decrease of N 2, resulting in a decrease of J Tmax in both systems. Instead, osmotic shock decreases N 1 and N 2, which suggests that periplasmic components were removed, resulting also in a decrease of J Tmax in both systems. These results are consistent with the proposition that the total unidirectional flux of the amino acid proceeds by means of a system of multiple components, with the simultaneous operation of two independent transport processes. We propose that the initial interaction of leucine with components of the cellular envelope might be the essential step for the subsequent translocation of the amino acid across the permeability barrier.