18 P. Doerner
control the crucial decision to enter a round of DNA replication. The now-
active SBF stimulates the transcription oftwodistinct types of cyclins (late
G1-type and S-phase type) sequentially required for full-blown commitment
and entry into S-phase. This proceeds by a build-up of thepotentialfor S-
phase CDK activity by progressive accumulation of cyclin subunits required
for S-phase CDK activity and their association with appropriate CDK pro-
teins. Importantly, these complexes initially lack protein kinase activity, as
the nascent complexes are held inactive by association with CDK inhibitory
proteins (CKI). The late G1 cyclin–CDK complexes that gradually accumulate
in an SBF-dependent manner mediate the activation of these latent S-phase
cyclin–CDK complexes. When sufficient amounts of late G1-type cyclins have
accumulated, the cyclin–CDK complexes they are part of surpass an acti-
vation threshold and phosphorylate CKI. CKI phosphorylation leads to its
recognition by an SCF-dependent ubiquitin ligase (see next section), resulting
in its destruction by the proteasome. The now liberated S-phase cyclin–CDK
complexes also recognize CKI as a substrate, resulting in a runaway activation
of S-phase cyclin–CDK complexes that constitutes the initiation of S-phase.
Thus, the principal contribution of cyclin–CDK complexes to cell cycle
control is to mediate key cell cycle transitionsdecisively. CDK activity is con-
trolled in a two-stage activation process. This is analogous to the cocking,
and subsequent firing of a gun by the activation of a trigger mechanism.
The trigger is provided by the accumulation of sufficient late G1-type (but
not Cln3p) cyclin–CDK activity to phosphorylate the CKI proteins holding
S-phase cyclin–CDK complexes in check. This multistage activation process
allows the final activation of cyclin–CDK complexes at a phase transition to
be subject to many controls. A similar process of stepwise activation occurs
prior to mitosis, although here, it is generally the phosphorylation state of the
CDK subunit that regulates its activity; additional protein kinases, which are
controlled by cyclin–CDK complexes, function as well.
1.1.3
Proteolysis in the Eukaryotic Cell Cycle
Regulated stability of regulatory proteins is a crucial aspect of eukaryotic
cell cycle control to impart irreversibility on cell cycle transitions. Proteol-
ysis occurs by ubiquitin-dependent mechanisms that are distinguished by
their mode of regulation. In G1 and S-phase, the SCF-mediated mechanism
dominates; while in M-phase and for exit from M-phase, the APC-dependent
mechanism prevails. The SCF complex has ubiquitin ligase activity and rec-
ognizes its substrates when these are phosphorylated; in contrast, subunits of
the APC complex are activated by phosphorylation, which leads to substrate
recognition and subsequent degradation.
SCF-dependent protein degradation is coupled to CDK-dependent cell
cycle control at G1/S through shared substrates. At this transition, SCF-