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

972 Chapter 17: The Cell Cycle
The Cell-Cycle Control System Functions as a Network of
Biochemical Switches
Table 17–2 summarizes some of the major components of the cell-cycle control
system. These proteins are functionally linked to form a robust network, which
operates essentially autonomously to activate a series of biochemical switches,
each of which triggers a specific cell-cycle event.
When conditions for cell proliferation are right, various external and internal
signals stimulate the activation of G 1 -Cdk, which in turn stimulates the expression
of genes encoding G 1 /S- and S-cyclins (Figure 17–16). The resulting activation of
G 1 /S-Cdk then drives progression through the Start transition. By mechanisms
we discuss later, G 1 /S-Cdks unleash a wave of S-Cdk activity, which initiates
(A) control of proteolysis by APC /C
activating
subunit (Cdc20)
inactive APC /C
M-cyclin
polyubiquitin
chain
Cdk
active APC /C
ubiquitin ( )
+
E1
E2
ubiquitylation
enzymes
DEGRADATION
OF M-CYCLIN IN
PROTEASOME
(B) control of proteolysis by SCF
active
SCF
complex
F-box
protein
polyubiquitin
chain
kinase
P
P
DEGRADATION
OF CKI IN
PROTEASOME
Cdk inhibitor protein
(CKI)
ubiquitin ( )
+
E1
E2
ubiquitylation
enzymes
Figure 17–15 The control of proteolysis by APC/C and SCF during the cell cycle. (A) The
APC/C is activated in mitosis by association with Cdc20, which recognizes specific amino acid
sequences on M-cyclin and other target proteins. With the help of two additional proteins called
E1 and E2, the APC/C assembles polyubiquitin chains on the target protein. The polyubiquitylated
target is then recognized and degraded in a proteasome. (B) The activity of the ubiquitin ligase SCF
depends on substrate-binding subunits called F-box proteins, of which there are many different
types. The phosphorylation of a target protein, such as the CKI shown, allows the target to be
recognized by a specific F-box subunit.
MBoC6 m17.20/17.15