The different cyclins are degraded by two
different E3 ligases
e.g. cyclin B, the G2/M cyclin is degraded by
APC = anaphase-promoting complex
Not to be confused with:
APC (adenomatous polyposis coli)
APC (antigen-presenting cell)
How does APC function?
M
APC
M
The cyclin must be degraded in
order for anaphase to take place
Remember?
cyclins must be removed for mitosis to
be completed
Protein
Level
cyclin A
cyclin B
Time
M
M
M
Negative feedback generates
a repeating oscillator
CDKs are positively regulated by cyclins
A Cyclin promotes synthesis of the next cyclin that in
turn, promotes destruction of the previous one
These regulatory activities are indirect
Mechanisms of CDKs regulation
1. Abundance of cyclins
2. CDK phosphorylation
3. Binding to CKIs (inhibitory proteins)
active
inactive
CDK
CDK
Cyclin
+
p21
Cyclin
p21
Activating phosphorylation is catalyzed by CdkActivating Kinases (CAKs). However, they are
abundant and not regulated
3
1
Cdk
Phosporylation of Thr by CAK
2
Cyclin
4
Substrate binding to the kinase
Inhibitory phosphorylation is also
involved in CDKs regulation
M-CDK
- e.g. Phosphorylation by Wee1 Tyr kinase
blocks the CDK’s active site
- e.g. Cdc25 is a phosphatase that removes this
inhibitory block
- The Wee1/Cdc25 switch event is regulated by
substrates and extrinsic signals
Mechanisms of CDKs regulation
1. Abundance of cyclins
2. CDK phosphorylation
3. Binding to CKIs (inhibitory proteins)
active
inactive
CDK
CDK
Cyclin
+
p21
Cyclin
p21
Cdk inhibitor proteins (CKIs)
- Discovered by asking : “what binds to CDKs”?
- The INK4 family proteins
(e.g. p16) bind to CDK4/6,
blocking cyclin D binding
- The Cip/Kip family proteins (e.g. p21, p27)
bind to blocking active site of multiple CDKs
- CKIs normally regulate entry into S phase
CKIs Regulate the G1-S Transition
(p16)
(p21, p27)
p16 is Frequently Mutated in Human Tumors
Table 1. D eleti ons i n tum or c ells a nd p rimar y tum ors.
9p21
Tum or t ype
L ines ( n)
Del etion s (n) D eleti ons ( %)
Astroc ytom a
B ladde r
B reast
C olon
G liom a
L euke mia
L ung
M elan oma
N euro blast oma
O steosarcom a
O vary
R enal
17
15
10
20
35
4
59
99
10
5
7
9
14
5
6
0
25
1
15
57
0
3
2
5
82
33
60
0
71
25
25
58
0
60
29
56
Total
29 0
13 3
46
See Kamb et al. (1994) Science 264: 436; Nobori et al. (1994) Nature 368:753 for detail
Chemical structures of small molecular cdk
inhibitors (none approved so far)
Senderowicz, A. M. et al. J Natl Cancer Inst 2000;92:376-387
Summary
- The cell cycle is controlled by Cdks, activated by
cyclins and CAKs, and inhibited by CKIs
- Cyclins are positively and negatively
regulated by cyclin-Cdks complexes
- Any process in the cell cycle is
dependent on the previous one
- The cell cycle progresses in the
right order
Cyclins regulate other cyclins, both
negatively and positively
But, it cannot be just an intrinsic cell cycle clock…
Mitogens stimulate the onset of the cell cycle
In this case, we are very different from yeasts
Mitogens control cyclin D expression
Mitogens control cyclin D
expression
- Mitogens act by activating
the D-Cdk4/6 complexes
- Mitogens act by inhibiting CKIs
- Mitogen signaling is
correlated with growth,
answering the question:
“have I grown enough?”
We actually have:
3 D-type cyclins
2 E-type cyclins
2 A-type cyclins
3 B-type cyclins
cyclin D and growth
w/o growth signals, subthreshold levels of enzymes
will lead to quiescence (G0)
START/Restriction point
- Activated D-Cdk4/6 initiates transcription of cyclin
E and activation of E-Cdk2
- Activated E-Cdk2 allows progression through START
- From here on, it’s a cell cycle clock game
In the next lectures we will focus on the molecular
machinery that acts at the START point in normal
and cancer cells