The N-end rule pathway controls the import of peptides through degradation of a transcriptional repressor.
Ubiquitin-dependent proteolytic systems underlie many processes, including 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 main recognition component of this pathway, regulates peptide import in the yeast Saccharomyces cerevisiae through degradation of Cup9p, a 35 kDa homeodomain protein. Cup9p was identified using a screen for mutants that bypass the previously observed requirement for Ubr1p in peptide import. We show that Cup9p is a short-lived protein (t1/2 approximately 5 min) whose degradation requires Ubr1p. Cup9p acts as a repressor of PTR2, a gene encoding the transmembrane peptide transporter. In contrast to engineered N-end rule substrates, which are recognized by Ubr1p through their destabilizing N-terminal residues, Cup9p is targeted by Ubr1p through an internal degradation signal. The Ubr1p-Cup9p-Ptr2p circuit is the first example of a physiological process controlled by the N-end rule pathway. An earlier study identified Cup9p as a protein required for an aspect of resistance to copper toxicity in S.cerevisiae. Thus, one physiological substrate of the N-end rule pathway functions as both a repressor of peptide import and a regulator of copper homeostasis.
Byrd
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Division of Biology, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.
Turner
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GC
Varshavsky
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A
D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D018398:Homeodomain Proteins / Q000378:metabolism; D006868:Hydrolysis; D008025:Ligases / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D026901:Membrane Transport Proteins; D010455:Peptides / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors / Q000378:metabolism; D044763:Ubiquitin-Conjugating Enzymes
C090457:CUP9 protein, S cerevisiae; D002352:Carrier Proteins; D005656:Fungal Proteins; D018398:Homeodomain Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; C093495:PTR2 protein, S cerevisiae; D010455:Peptides; D011994:Recombinant Proteins; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D044763:Ubiquitin-Conjugating Enzymes; C063109:ubiquitin-conjugating enzyme UBC4; D008025:Ligases