Regulation of amino acid transport system L in Chinese hamster ovary cells.

The regulation of the activity of transport System L for neutral amino acids has been investigated in Chinese hamster ovary cells. Incubation of the temperature-sensitive leucyl-tRNA synthetase mutant CHO-tsH1 at marginally permissive temperatures results in leucine-limited growth and increased transport of branched chain and aromatic amino acids. This temperature-dependent transport enhancement is restricted to transport System L and results in increased Vmax values of uptake of System L substrates with unchanged Km values of uptake. Several lines of evidence suggest that trans-stimulation by intracellular amino acids cannot account for the increased System L activity in CHO-tsH1. Other temperature-sensitive aminoacyl-tRNA synthetase mutants of the Chinese hamster ovary line show increased transport of the amino acid corresponding to the synthetase defect after incubation at elevated temperatures. Increased transport by System L occurs in CHO-S, the parental line of CHO-tsH1, after growth in limiting levels of leucine. The System L enhancement can be prevented by cycloheximide but not, at early times, by actinomycin D. We conclude that the activity of transport System L in Chinese hamster ovary cells is regulated by a mechanism which appears to act at the level of translation.