= Solution
\b[Selective destruction makes key transitions depend on rebuilding <proteins>, rather than merely reversing their <phosphorylation> state.] <ubiquitin-proteasome system> supplies this directionality while <cyclin-dependent kinases> and phosphatases provide more rapidly reversible regulation. An <ubiquitin-activating enzyme> uses <ATP> to activate <ubiquitin>; transfer through an <ubiquitin-conjugating enzyme> and recognition by an <ubiquitin ligase> directs substrate modification. Suitable polyubiquitin chains target <proteins> to the <proteasome> for unfolding and destruction. Substrate recognition, rather than a general increase in <protein> breakdown, gives temporal specificity.
At the G1/S transition, an <SCF ubiquitin ligase> can recognize a phosphorylated inhibitor of a cyclin-dependent kinase. In budding yeast, multisite <phosphorylation> of <Sic1> promotes its destruction, releasing S-phase kinase activity. This links a threshold of previous kinase activity to removal of an inhibitor. Different SCF substrate receptors recognize different <proteins>, and phosphorylation-dependent recognition is important for many, but not every, SCF target. Destruction reinforces entry into the new state while the kinase network supplies additional feedback.
The <anaphase-promoting complex> controls a particularly decisive transition. Its <Cdc20>-associated form is restrained by the <spindle checkpoint> until chromosome attachment conditions permit progression. Ubiquitination and destruction of <securin> release <separase>, which cleaves the <cohesin> component that holds sister chromatids together. Chromatid separation cannot be undone by putting the phosphate groups back onto an intact <securin> molecule, because that molecule has been degraded and the cohesion linkage has been cleaved. The combination of selective proteolysis and substrate cleavage explains why a brief regulatory transition commits the cell to anaphase.
Destruction of <cyclin B> lowers <CDK1> activity and enables mitotic exit. Nondegradable cyclin blocked inactivation of maturation-promoting activity and exit from <mitosis> in frog experimental systems, directly testing necessity of cyclin destruction rather than merely observing a correlation.<The role of cyclin synthesis and degradation in the control of maturation promoting factor activity by Andrew W. Murray and others>[Murray, Solomon and Kirschner, 1989, abstract and nondegradable-cyclin interventions]{ref} The <Cdh1>-associated anaphase-promoting complex helps keep mitotic cyclins low through late <mitosis> and G1, creating an interval suitable for replication licensing. In animal cells, destruction of <geminin> also permits <Cdt1> to support licensing. Subsequent kinase activation and licensing-factor regulation help prevent rereplication before the next cycle.
The mechanism is irreversible on the timescale of the transition, not an assertion that the cell can never make cyclin or <securin> again. ATP-dependent <ubiquitin> activation and proteasomal processing occur in an open, energy-consuming system; fresh <gene expression> later restores substrates under a different regulatory state. Conversely, ubiquitination alone is not synonymous with degradation: chain type and context can give other signals. Tests should compare normal and degron-mutant substrates at similar expression levels, monitor actual <protein> loss and kinase activity, and distinguish checkpoint arrest from failure of destruction itself. \b[Proteolysis commits the cell by removing selected regulators, while checkpoint and feedback circuits determine when that commitment occurs.]
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