THE N-END RULE PATHWAY CONTROLS THE IMPORT OF PEPTIDES THROUGH DEGRADATION OF A TRANSCRIPTIONAL REPRESSOR

Ubiquitin-dependent proteolytic systems underlie many processes, inclu ding the cell cycle, cell differentiation and responses to stress, One such system is the N-end rule pathway, which targets proteins bearing destabilizing N-terminal residues, Here we report that Ubr1p, the mai n recognition component of this pathway, regulates peptide import in t he yeast Saccharomyces cerevisiae through degradation of Cup9p, a 35 k Da homeodomain protein, Cup9p was identified using a screen for mutant s that bypass the previously observed requirement for Ubr1p in peptide import, We show that Cup9p is a short-lived protein (t(1/2) Alpha sim ilar to 5 min) whose degradation requires Ubr1p, Cup9p acts as a repre ssor of PTR2, a gene encoding the transmembrane peptide transporter. I n contrast to engineered N-end rule substrates, which are recognized b y Ubr1p through their destabilizing N-terminal residues, Cup9p is targ eted by Ubr1p through an internal degradation signal. The Ubr1p-Cup9p- Ptr2p circuit is the first example of a physiological process controll ed by the N-end rule pathway, An earlier study identified Cup9p as a p rotein required for an aspect of resistance to copper toxicity in S. c erevisiae. Thus, one physiological substrate of the N-end rule pathway functions as both a repressor of peptide import and a regulator of co pper homeostasis.