How cells use proteolysis to control their growth. Three basic strategies for using Ub-mediated proteolysis are shown. (A) Proteolysis is used to destroy a protein at a very specific point in the cell cycle. In this case, timed destruction of the S-phase inhibitor Sic1p allows cells to pass through the G1-to-S transition. (B) Proteolysis is used to keep the levels of a regulatory molecule low, and coupled to its synthesis. In this case, the continuous destruction of the oncoprotein Myc, coupled with relatively low rates of transcription of the Myc gene, keeps intracellular levels of Myc at a level compatible with controlled cell growth. (C) Regulated destruction is used to induce a rapid change in protein levels in response to a signal. In this case, DNA damage by UV light brings about a cessation of p53 destruction, resulting in a rapid rise in p53 levels and a halt to cell proliferation. See text for more details.