Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1014 Chapter 17: The Cell Cycle
Additional layers of control promote an overwhelming increase in S-Cdk
activity at the beginning of S phase. We mentioned earlier that the APC/C activator
Cdh1 suppresses cyclin levels after mitosis. In animal cells, however, G 1 -
and G 1 /S-cyclins are resistant to Cdh1–APC/C and can therefore act unopposed
by the APC/C to promote Rb protein phosphorylation and E2F-dependent gene
expression. S-cyclin, by contrast, is not resistant, and its level is initially restrained
by Cdh1–APC/C activity. However, G 1 /S-Cdk also phosphorylates and inactivates
Cdh1–APC/C, thereby allowing the accumulation of S-cyclin, further promoting
S-Cdk activation. G 1 /S-Cdk also inactivates CKI proteins that suppress S-Cdk
activity. The overall effect of all these interactions is the rapid and complete activation
of the S-Cdk complexes required for S-phase initiation.
DNA Damage Blocks Cell Division: The DNA Damage Response
Progression through the cell cycle, and thus the rate of cell proliferation, is controlled
not only by extracellular mitogens but also by other extracellular and intracellular
signals. One of the most important influences is DNA damage, which can
occur as a result of spontaneous chemical reactions in DNA, errors in DNA replication,
or exposure to radiation or certain chemicals (discussed in Chapter 5). It is
essential that damaged chromosomes are repaired before attempting to duplicate
or segregate them. The cell-cycle control system can readily detect DNA damage
and arrest the cycle at either of two transitions—one at Start, which prevents entry
into the cell cycle and into S phase, and one at the G 2 /M transition, which prevents
entry into mitosis (see Figure 17–16).
DNA damage initiates a signaling pathway by activating one of a pair of related
protein kinases called ATM and ATR, which associate with the site of damage and
phosphorylate various target proteins, including two other protein kinases called
Chk1 and Chk2. These various kinases phosphorylate other target proteins that
lead to cell-cycle arrest. A major target is the gene regulatory protein p53, which
stimulates transcription of the gene encoding p21, a CKI protein; p21 binds to
G 1 /S-Cdk and S-Cdk complexes and inhibits their activities, thereby helping to
block entry into the cell cycle (Figure 17–62 and Movie 17.8).
DNA damage activates p53 by an indirect mechanism. In undamaged cells, p53
is highly unstable and is present at very low concentrations. This is largely because
it interacts with another protein, Mdm2, which acts as a ubiquitin ligase that targets
p53 for destruction by proteasomes. Phosphorylation of p53 after DNA damage
reduces its binding to Mdm2. This decreases p53 degradation, which results
in a marked increase in p53 concentration in the cell. In addition, the decreased
binding to Mdm2 enhances the ability of p53 to stimulate gene transcription (see
Figure 17–62).
The protein kinases Chk1 and Chk2 also block cell-cycle progression by
phosphorylating members of the Cdc25 family of protein phosphatases, thereby
inhibiting their function. As described earlier, these phosphatases are particularly
important in the activation of M-Cdk at the beginning of mitosis (see Figure
17–20). Chk1 and Chk2 phosphorylate Cdc25 at inhibitory sites that are distinct
from the phosphorylation sites that stimulate Cdc25 activity. The inhibition of
Cdc25 activity by DNA damage helps block entry into mitosis (see Figure 17–16).
The DNA damage response can also be activated by problems that arise when
a replication fork fails during DNA replication. When nucleotides are depleted, for
example, replication forks stall during the elongation phase of DNA synthesis. To
prevent the cell from attempting to segregate partially replicated chromosomes,
the same mechanisms that respond to DNA damage detect the stalled replication
forks and block entry into mitosis until the problems are resolved.
A low level of DNA damage occurs in the normal life of any cell, and this damage
accumulates in the cell’s progeny if the DNA damage response is not functioning.
Over the long term, the accumulation of genetic damage in cells lacking
the DNA damage response leads to an increased frequency of cancer-promoting
mutations. Indeed, mutations in the p53 gene occur in at least half of all human
cancers (discussed in Chapter 20). This loss of p53 function allows the cancer cell