The N-Terminal Domain of the Yeast Permease Bap2p Plays a Role in Its Degradation

Abstract

The amino acid permease Bap2p in Saccharomyces cerevisiae mediates a major part of the uptake of leucine, isoleucine, and valine from media containing a preferred nitrogen source. Although the transcriptional controls of BAP2 have been well studied, the posttranslational down-regulation mechanisms for Bap2p have not been established. Here we show that Bap2p is subject to a starvation-induced degradation upon rapamycin treatment or cultivation with proline as the sole nitrogen source. The starvation-induced degradation of Bap2p was dependent on the cellular functions of ubiquitination and endocytosis. Down-regulation of the permease required the most probable ubiquitination sites, the lysine residues situated in the N-terminal 49 residues, as well as the C-terminal domain. Furthermore, when the N-terminal domain of Bap2p was fused to the general amino acid permease Gap1p, the resultant chimeric permease became susceptible to the starvation-induced degradation, indicating that the Bap2p N-terminus contains a determinant responsive to the starvation signals.