Amino acids regulate the transcription, internal sorting, and intrinsic activity of the general amino acid permease (GAP1) in S. cerevisiae

Abstract

The high capacity general amino acid permease in Saccharomyces cerevisiae (GAP1) is regulated such that it actively imports amino acids into the cell from the extracellular medium only when internal amino acid levels are low. Regulation of activity allows the cell to rapidly and reversibly modulate amino acid import according to the nitrogen requirements of the cell. I have explored three distinct modes of action by which amino acids repress GAP1 activity: transcriptionally, by regulation of ubiquitin-mediated intracellular sorting, and by transport-dependent inactivation at the plasma membrane. Transcriptional regulation of GAP1 by two nutrient responsive GATA transcription factors, Nillp and Gln3p, allows the cell to modulate expression of the permease in response to both amino acid quantity and nitrogen source quality. Any Gaplp that is expressed in the presence of elevated internal amino acids is sorted to the vacuole and degraded or stored in internal compartments from which the permease can be rapidly mobilized to the plasma membrane when amino acid levels become limiting.

(cont.) Redistribution of Gap Ilp from the plasma membrane to internal compartments upon an increase in internal amino acid levels involves three ubiquitin-mediated sorting steps that each require unique cis- and trans-acting factors. A constitutively expressed, non-ubiquitinateable form of Gap Ip can also be downregulated by the addition of amino acids through reversible, transport-dependent inactivation of the permease at the plasma membrane. Since amino acids are the primary source of nitrogen in the cell, upregulation of Gap Ip activity allows the cell to rapidly import nitrogen-containing compounds when internal amino acid pools are limiting. Conversely, downregulation of Gaplp activity when sufficient intracellular amino acids have accumulated allows the cell to avoid toxicity that results from unrestricted amino acid uptake. Therefore, amino acid regulated transcription, sorting, and activity of Gap 1 p are important to maintain the proper balance of intracellular amino acid levels in diverse and rapidly changing nutritional environments.