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Chapter 17 Control of Cell Division 181

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cycle–regulated genes and contributes to their repression. The The SCF complex consists of the three invariable components
DREAM complex is comprised of the RB-like protein p130 (RBL2), RBX1, CUL1, and SKP1. RBX1 is a RING finger protein that
E2F4 or E2F5, DP1 or DP2, and the 5-component MuvB complex functions as an E3 ubiquitin ligase, CUL1 is a scaffold protein, and
containing LIN9, LIN37, LIN52, LIN54, and RBBP4. Whereas the SKP1 is an adaptor protein. In addition to the three invariable
DREAM complex contributes to the expression of both S phase– and components, one variable coactivator, known as an F-box protein,
M phase–specific genes, the RB protein specifically represses S binds through its F-box motif to SKP1 and is responsible for substrate
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phase–specific genes. RB binds to the activating E2F transcription recognition. Approximately 70 putative F-box proteins have been
factors (E2F1–E2F3) and blocks E2F-mediated activation of S-phase identified in humans, and at least 4 of them are thought to be
genes. S- and M-phase genes possess distinct promoter elements that involved in cell cycle control: SKP2, FBXW7, β-transducin repeats-
mediate binding of RB-E2F and DREAM complexes. E2F promoter containing proteins (β-TRCP), and cyclin F. The SCF complex
elements can be found in S-phase genes and specifically mediate mediates degradation of cell cycle proteins during the progression
binding of E2F transcription factors. In contrast, M-phase genes from late G 1 phase through S phase until the onset of mitosis.
possess cell cycle genes homology region (CHR) promoter elements Important substrates include cyclin D and cyclin E; the transcrip-
that specifically mediate binding of the MuvB complex. tional regulators MYC, E2F1, and p130; the CDK inhibitors p27
When a quiescent cell is stimulated to enter the cell cycle, it begins and p21; and WEE1 kinase. WEE1 kinase deactivates cyclin B–
to express cyclin D that binds to CDK4 or CDK6 and is capable of CDK1 complexes. Degradation of these important cell cycle regula-
monophosphorylating RB. In late G 1 phase, Cyclin E–CDK2 com- tors allows for proper S-phase entry and completion, and onset of
plexes multiphosphorylate the monophosphorylated RB, leading to mitosis.
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the release of the activating E2F transcription factors. The activating The APC/C complex consists of several invariable components,
E2Fs dimerize with DP1 or DP2, bind to the promoters of genes and the central ones are structurally similar to the SCF complex
required for DNA synthesis through E2F promoter elements, and components. APC11 is a RING-finger protein related to RBX1, and
promote their expression during late G 1 phase and S phase. These APC2 is a scaffold protein related to CUL1. Similar to the SCF
G 1 /S genes encode many of the factors required for DNA replication. complex, APC/C contains a variable coactivator that confers substrate
During late S phase, E2F7 and E2F8, which are E2F targets them- specificity. Two of such variable coactivators function in the cell cycle:
selves, will replace E2F1–E2F3 and serve to repress the expression of CDC20 and CDH1. APC/C CDC20 mediates the degradation of sub-
the G 1 /S phase genes when DNA synthesis is completed. 13 strates during mitosis, whereas APC/C CDH1 functions primarily in G 1
Cyclin D–CDK4/6 and cyclin E–CDK2 are also capable of phase. Key substrates of APC/C CDC20 are cyclin A and cyclin B. With
phosphorylating p130, thereby enabling the release of p130 from the the onset of anaphase, activation of APC/C CDH1 leads to the degrada-
DREAM complex. The B-MYB (MYBL2) transcription factor is tion of CDC20, PLK1, and Aurora kinases.
encoded by a G 1 /S gene and binds to the MuvB core when p130, Similar to the other cell cycle control systems, SCF and APC/C
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E2F4, and DP are released from the DREAM complex. The newly are controlled within the cell cycle, in part by their own substrates.
formed MMB (B-MYB–MuvB) complex does not contain any E2F Most important, SCF and APC/C can regulate each other. The cell
transcription factor and thus binds exclusively to genes that possess cycle protein FBXO5 (EMI1) functions as an inhibitor of APC/
CHR promoter elements. These genes are highly expressed in late G 2 C CDC20 and is sent to proteasomal degradation by SCF β-TRCP when
phase and M phase, and encode for proteins that carry out essential mitosis starts, and SCF cyclin F mediates degradation of CDH1. Simi-
functions in mitosis. The MMB complex recruits FOXM1, a third larly, ubiquitination of CDC20 and cyclin F by APC/C CDH1 leads to
transcription factor, to promote the expression of these G 2 /M genes. their destruction when cells exit mitosis. Further regulation occurs
Notably, FOXM1 is a G 2 /M gene itself, and together with the impor- on the transcriptional level because substrate-specific components of
tant role of FOXM1 in promoting the expression of G 2 /M genes, SCF and APC/C are encoded by cell cycle–regulated genes, including
FOXM1 has emerged as one of the most robust biomarkers for strati- SKP2, FBXO5, Cyclin F, and CDC20.
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fying high-risk and low-risk cancers. The B-MYB and FOXM1
factors are ubiquitinated and destroyed by the proteasome during late
mitosis, resulting in decrease in expression of the G 2 /M genes when DNA REPLICATION
a cell exits mitosis. Therefore the DREAM complex represses all cell
cycle genes during quiescence, with the G 1 /S cell cycle genes being The initiation of DNA replication represents a commitment to cell
activated by E2F1–E2F3 and the G 2 /M late cell cycle genes being proliferation, and is a central event in the growth and division of all
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activated by B-MYB–MuvB–FOXM1. organisms. The assembly of replication machineries is coordinated
Together, proliferation signals will enable MYC-dependent gene by multiple proteins, which ensure that DNA synthesis begins at the
expression that contributes to the growth in size of the cell, whereas correct chromosomal locus. Strict regulation of initiation is crucial to
the E2F and B-MYB/FOXM1-dependent gene expression induces viability because inappropriate replication start is linked to genetic
genes required for DNA replication during S phase and cell division instability, including alterations in gene copy number and DNA
during mitosis. damage. DNA replication starts with the assembly of prereplication
complexes (pre-RCs) at multiple DNA replication origins during the
G 1 phase. At the transition from G 1 to S phase, the replicative helicase
UBIQUITINATION is activated, leading to a change from pre-RCs to preinitiation com-
plexes, which unwind DNA and initiate DNA synthesis. The recog-
Cells employ a variety of mechanisms to control the cell division nition of pre-RC sites is known as replication origin licensing, and
cycle, ensuring one-way progression through S and M phases of the the activation of DNA synthesis is known as origin firing. Separation
cell cycle. Cell cycle genes are cell cycle–dependently expressed at of these two steps is critical for preventing rereplication within the
certain times, and cyclin-CDK complexes drive the cell cycle through same cell cycle.
the control of the cell cycle transcription machinery and regulation Replication origin licensing in G 1 phase involves sequential
of critical events in S and M phases. The mRNA of many cyclins and assembly of different proteins. The origin recognition complex
CDKs is also cell cycle–dependently regulated, and in addition, (ORC) consists of the subunits ORC1–ORC6 and binds initially to
cyclin-CDK activity is regulated by CDK inhibitors. Yet, cells employ replication origins. Then, CDC6 and CDT1 are recruited to the
an additional layer of cell cycle control through the timed destruction ORC, leading to the recruitment of the minichromosome mainte-
of essential cell cycle proteins, including cyclins, CDKs, and cell nance (MCM) complex, which consists of six subunits: MCM2–
cycle transcription factors (Fig. 17.3). Two ubiquitin ligases, the MCM7. The MCM complex is a helicase that forms a double
Skp, Cullin, F-box–containing complex (SCF) and the anaphase- hexamer and encircles double-stranded DNA. Together, the inactive
promoting complex/cyclosome (APC/C), are responsible for the assembly of ORC, CDC6, CDT1, and MCM are known as the
specific ubiquitination of many of these cell cycle proteins. 15 pre-RC. Notably, the Meier-Gorlin syndrome, a form of primordial