Transport activity dependent regulation of the yeast general amino acid permease

Abstract

The general amino acid permease Gap1p of Saccharomyces cerevisiae scavenges amino acids from the extracellular medium for use as nitrogen sources in starvation conditions. Because unlimited uptake of both naturally occurring amino acids and amino acid analogs is toxic, Gap1p is active at the plasma membrane only when amino acid levels are low. Gap1p is down regulated when amino acids are abundant by two distinct post-translational mechanisms. Gap1p is regulated post-translationally to respond quickly and efficiently to changing amino acid concentration. An increase in amino acids causes accumulation of Gap1p in the vacuole and inactivation of Gap1p located at the plasma membrane. Conversely, a decrease in amino acid levels allows for redistribution of Gap1p from internal membranes to the cell surface. Here I examine the mechanism of amino acid regulation of Gap1p. Previous studies of Gap1p sorting have focused on the trans-acting factors required for the distribution of Gap1p between the plasma membrane and internal compartments. To complement this body of work, these studies focus on the cis-elements required for Gap1p sorting. We find that post-translational regulation of Gap1p requires the catalytic activity of Gap1p, indicating that sorting and activity of Gap1p are controlled in cis. Gap1p therefore can serve as an amino acid sensor to control its activity in response to nutrient levels. This finding suggested that post-translational regulation of Gap1p could apply to other transporter proteins in yeast. I examined the activity and localization of a related transporter protein, the histidine-specific permease Hip1p in response to various amino acids, and found that although Hip1p is down regulated only in response to histidine, this regulation is less tightly controlled than the regulation of Gap1p. This observation supports previous assertions that the function of Gap1p in the cell is distinct among yeast amino acid transporters.