FIGURE 19-15 Six suboptimal phosphorylation sites in Sic1 create a switch-like cell cycle entry. (a) A single optimal G₁/S phase CDK phosphorylation site in Sic1 would result in a sluggish G₁–S phase transition. As G₁/S phase CDKs accumulate during G₁, Sic1 would be progressively degraded. As a result, S phase CDKs would slowly rise, and initiation of S phase would be a drawn-out event. (b) Six suboptimal phosphorylation sites ensure that Sic1 is fully phosphorylated, and hence recognized by SCF, only when G₁/S phase CDKs have reached high levels. This also ensures that Sic1 degradation occurs rapidly and when G₁/S phase CDKs have accomplished all their other G₁ tasks. See P. Nash et al., 2001, Nature 414:514–521, and D. O. Morgan, 2006.