Transport of L‐leucine in the fission yeast Schizosaccharomyces pombe

Abstract

AbstractTransport of L‐leucine into Schizosaccharomyces pombe cells from the stationary phase of growth (after preincubation for 60 min with 1% glucose) proceeds uphill, practically unidirectionally, and is mediated by at least two systems: a high‐affinity system with a KT of 0·045 mmol 1−1 and Jmax of 3·3 nmol min−1 (mg dry weight)−1 and a low‐affinity system with a KT of 1·25 mmol 1−1 and Jmax of 16·0 nmol min−1 (mg dry weight)−1. The high‐affinity system has a pH optimum at 3.2, the accumulation ratio is highest at a cell density of 2–4 mg dry weight per ml and decreases with increasing leucine concentration. Transport of leucine by the high‐affinity system is strongly inhibited by proton conductors, ammonium ions and by most amino acids, but only L‐phenylalanine, L‐isoleucine, L‐valine and L‐cysteine behave as fully competitive inhibitors. Systems of L‐leucine transport in S. pombe are not constitutive. Transport activity appears only after preincubation of cells with a suitable source of energy. If cycloheximide is added during preincubation with glucose, no transport systems for leucine are synthesized. After removal of glucose, the activity of transport systems decays with a half‐time of about 20 min. The presence of cyclic AMP increases the initial rate of leucine uptake only in cells preincubated with glucose and in the absence of cycloheximide.