RAS2/PKA pathway activity is involved in the nitrogen regulation of l-leucine uptake in Saccharomyces cerevisiae

The aim of the present work is to study the participation of RAS2/PKA signal pathway in the nitrogen regulation of l-leucine transport in yeast cells. The study was performed on Saccharomyces cerevisiae isogenic strains with the normal RAS2 gene, the RAS2 val19 mutant and the disrupted ras2:: LEU2. These strains bring about different activities of the RAS2/PKA signal pathway. l-( 14C)-Amino acid uptake measurements were determined in cells grown in a rich YPD medium with a mixed nitrogen source or in minimal media containing NH 4 + or l-proline as the sole nitrogen source. We report herein that in all strains used, even in those grown in a minimal proline medium, the activity of the general amino acid permease (GAP1) was not detected. l-Leucine uptake in these strains is mediated by two kinetically characterized transport systems. Their K T values are of the same order as those of S1 and S2 l-leucine permeases. Mutation in the RAS2 gene alters initial velocities and J max values in both high and low affinity l-leucine transport systems. Activation of the RAS2/PKA signalling pathway by the RAS2 val19 mutation, blocks the response to a poor nitrogen source whereas inactivation of RAS2 by gene disruption, results in an increase of the same response.