Multisite phosphorylation of a CDK inhibitor sets a threshold for the onset of DNA replication.
SCF ubiquitin ligases target phosphorylated substrates for ubiquitin-dependent proteolysis by means of adapter subunits called F-box proteins. The F-box protein Cdc4 captures phosphorylated forms of the cyclin-dependent kinase inhibitor Sic1 for ubiquitination in late G1 phase, an event necessary for the onset of DNA replication. The WD40 repeat domain of Cdc4 binds with high affinity to a consensus phosphopeptide motif (the Cdc4 phospho-degron, CPD), yet Sic1 itself has many sub-optimal CPD motifs that act in concert to mediate Cdc4 binding. The weak CPD sites in Sic1 establish a phosphorylation threshold that delays degradation in vivo, and thereby establishes a minimal G1 phase period needed to ensure proper DNA replication. Multisite phosphorylation may be a more general mechanism to set thresholds in regulated protein-protein interactions.
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Programme in Molecular Biology and Cancer, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto M5G 1X5, Canada.
Tang
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Orlicky
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Chen
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Gertler
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Mendenhall
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Sicheri
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Pawson
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Tyers
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D001665:Binding Sites; D002453:Cell Cycle; D018797:Cell Cycle Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D016384:Consensus Sequence; D050756:Cyclin-Dependent Kinase Inhibitor Proteins; D004261:DNA Replication / Q000502:physiology*; D004271:DNA, Fungal / Q000096:biosynthesis; D004791:Enzyme Inhibitors; D044783:F-Box Proteins; D005656:Fungal Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000502:physiology*; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D029701:Saccharomyces cerevisiae Proteins; D013379:Substrate Specificity; D025801:Ubiquitin / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases
D018797:Cell Cycle Proteins; D050756:Cyclin-Dependent Kinase Inhibitor Proteins; D004271:DNA, Fungal; D004791:Enzyme Inhibitors; D044783:F-Box Proteins; D005656:Fungal Proteins; C086926:SIC1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; D044767:Ubiquitin-Protein Ligases