Multisite phosphorylation of a CDK inhibitor sets a threshold for the onset of DNA replication

SCF ubiquitin ligases target phosphorylated substrates for ubiquitin-depend ent proteolysis by means of adapter subunits called F-box proteins. The F-b ox protein Cdc4 captures phosphorylated forms of the cyclin-dependent kinas e inhibitor Sic1 for ubiquitination in late G1 phase, an event necessary fo r the onset of DNA replication. The WD40 repeat domain of Cdc4 binds with h igh affinity to a consensus phosphopeptide motif (the Cdc4 phospho-degron, CPD), yet Sic1 itself has many sub-optimal CPD motifs that act in concert t o mediate Cdc4 binding. The weak CPD sites in Sic1 establish a phosphorylat ion threshold that delays degradation in vivo, and thereby establishes a mi nimal G1 phase period needed to ensure proper DNA replication. Multisite ph osphorylation may be a more general mechanism to set thresholds in regulate d protein-protein interactions.