PERSPECTIVE
Interwoven Ubiquitination Oscillators and
Control of Cell Cycle Transitions
Xiaolu L. Ang and J. Wade Harper*
(Published 20 July 2004)
Dominant activity
An ever-increasing number of cell cycle events are now apA hierarchy of reversible protein phosphorylation, networked in
preciated to be representative of bi-stable switch systems, intightly regulated feedback loops, fundamentally drives progrescluding the ultimately stable states of phases G1 and S. Each is
sion through the cell cycle by mediating cyclin-dependent kimaintained by a positive feedback loop (or double negative
nase (Cdk) oscillations. The particular constellation of Cdk’s is
feedback loop) to promote and establish its character (7). Addisufficient to define a cell cycle phase. For example, phase G1
(gap 1 phase) may be viewed as a period of overall low Cdk actionally, amplification or cooperativity within the feedback cirtivity, whereas entry into S phase (the DNA replication phase)
cuit helps to efficiently create each state as a discrete entity
and mitosis reflects high Cdk activity. Accordingly, cells orfrom the other, and so the transition between the two states enchestrate multiple levels of regulation to precisely govern the
compasses a switch-like character because of the impossibility
activities of Cdk’s and the
duration of their signaling. In
addition to transcriptional
and posttranslational regulatory mechanisms, Cdk activity is controlled both directly
and indirectly by ubiquitin
(Ub)–mediated proteolysis
APCCdh1
SCFSkp2
(1, 2). A transition from one
phase to another can consequently be viewed as a
change in proteolytic activity.
Intrinsic to this idea of proteolytically driven cell cycle
regulation are the following
requirements: (i) Proteolysis
must have a role in the maintenance of phase characteristics, and (ii) changes in proteolytic activity, its targets, or
both can mediate cell cycle
phase transitions. The recent
findings of Bashir et al. (3)
G1
G1/S
and Wei et al. (4) regarding
the regulation of mammalian
Fig. 1. Oscillating E3 ligase activities in G1 and early S phase. G1 stability is sustained by Cdh1 and
G1 phase of the cell cycle re- the APCCdh1-dependent degradation of S-phase–promoting
factors, including components of the
veal a proteolysis-based Cks1 and SCFSkp2 complex (SCFSkp2/Cks1). The transition into S phase is mediated by a change in
mechanism that maintains G1 E3 ligase activity corresponding to the activation of the SCFSkp2 complex upon inhibition of APCCdh1.
stability in the face of the Consequently, proteolytic targets are altered in a phase-dependent manner according to the selectivicontinued presence and ex- ty of the active E3 ligase at that time. Maintenance of subsequent phases and their transitions is also
pression of genes encoding likely to involve a similar mechanism of E3 ligase oscillation.
factors that promote S phase.
Furthermore, when combined
of sustaining stable intermediate activity. The end result is thus
with previously known positive feedback loops driving entry ina binary output, or an all-or-none response. The irreversibility
to S phase, their findings support the view that G1 and S phases
may form a bi-stable system regulated by proteolytically driven
of the transition requires some degree of hysteresis in the sysmechanisms (5, 6). Establishment of this bi-stable system sets
tem, defined as the difficulty to re-flip the system back to its
the foundation for how changes in proteolysis may favor a
original state rather than maintaining the flipped state (8, 9).
switch-like progression from G1 to S phase.
Often this is modeled as a discontinuous or dissimilar transition
from state 1 to state 2 as compared to its reversal. In the cases
Program in Biological and Biomedical Sciences, Department of
of G1 and S phase, establishment of hysteresis may lie in the use
Pathology, Harvard Medical School, Boston, MA 02115, USA.
of Ub-mediated proteolysis to maintain each phase; as such, tar*Corresponding author. E-mail: [email protected]
geted proteolysis is both precise and irreversible.
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Ub-mediated proteolysis begins through an enzymatic cascade involving E1 Ub-activating enzymes, E2 Ub-conjugating
enzymes, and E3 Ub-ligase enzymes. These enzymes ultimately
serve to mediate the covalent attachment of Ub molecules by
the E3 ligase to a lysine residue in the target substrate, or on the
growing multi-Ub chain extending from the “tagged” protein
[reviewed in (10)]. This Ub chain serves as a signal for the 26S
proteasome to unfold and digest the target. Most of the target
selectivity of this system is conferred by the E3 ligase because
of its direct interaction with the ubiquitination substrate. The
two most prominent E3 ligases in cell cycle control are the
anaphase-promoting complex [(APC); also known as the cyclosome (APC/C)] and the SCF (Skp1, Cullin, F-box) complexes.
The APC core consists of ~12 subunits and is regulated by two
activating subunits: Cdc20 (also known as Fizzy) and Cdh1
(also known as Hct1 or Fizzy-related) [reviewed in (11–13)].
These activating subunits each confer differential substrate selectivity to APC. Moreover, each associates with the APC core
under different circumstances: Cdc20 is more likely to associate
with the APC core under conditions of high Cdk activity, and
Cdh1 is more likely to activate the APC core under conditions
of low Cdk activity. Comparatively, the SCF E3 ligase comprises three subunits at its core; Skp1, Cullin, and Rbx1. This core
interacts with modular F-box proteins, which all share an F-box
sequence motif [reviewed in (14, 15)]. F-box proteins directly
bind to the target substrate and bridge the interaction between
the E3 ligase and target substrate, and so the identity of the F
box determines the target of SCF. There are various F-box proteins, including subfamilies with WD40 domains (Fbw’s) and
those with leucine-rich-repeats (Fbl’s). F-box proteins frequently recognize their substrates through phosphorylation-dependent mechanisms. In the case of Fbw’s, the WD40 motif recognizes a phosphodegron domain on the substrate that forms after
appropriate phosphorylation events.
The revelation that E3 ligases play a role in G1 maintenance
(G1 is characterized by low Cdk activity) was first seen in budding yeast (16), where APCCdh1 promotes the degradation of the
B-type cyclins, Cdc20, and Cdc5 [(17, 18), reviewed in (12)].
Each of these proteins is required to push cells through mitosis,
but once the transition is completed, their levels need to be reduced in order to efficiently pass through telophase and enter a
G1 state characterized by low Cdk activity.
Recent findings unveil another specific role for APCCdh1 activity in the maintenance of G1 phase in human somatic cells (3,
4). Several mechanisms conspire to keep Cdk activity low during G1 phase in mammalian cells, but a critical mechanism involves the inhibition of Cdk’s by inhibitors of the p27 class.
These inhibitors bind to cyclin and Cdk complexes
(cyclin/Cdk), such as cyclin E/Cdk2, and block phosphorylation
of proteins that promote S phase (19). Degradation of p27 by
the Ub-proteasome pathway is required for proper entry into S
phase, and this occurs by SCFSkp2 recognition of a p27 form that
is phosphorylated on Thr187 (20–22). Skp2 is aided in this process by Cks1, a protein that simultaneously interacts with both
Skp2 and with the Cdk2/cyclin E/p27 complex (23, 24). Controlling Skp2 activity is clearly important for proper cell cycle
control. SKP2 can function as an oncogene in model systems
and is overexpressed in a number of tumor types [reviewed in
(25)]. Additionally, SCFSkp2 has a direct role in the firing of
replication origins (26, 27). Now, new work has indicated a
mechanism by which G1 cells maintain low levels of SCFSkp2,
thereby putting a break on S-phase entry until the criteria for Sphase entry have been met (Fig. 1).
Bashir et al. and Wei et al. independently demonstrate that
APCCdh1 targets both Skp2 and Cks1 for ubiquitination and destruction during G1. Consequently, active APCCdh1 in G1 prevents premature SCFSkp2/Cks1–mediated degradation of p27,
thereby preventing premature S phase. Coupling coexpression
studies with complementary RNA interference (RNAi) experiments, both labs show that in vivo degradation of Skp2 and
Cks1 is directed by Cdh1. Consistent with the involvement of
APC, mutation of destruction boxes (sequences in substrates
that confer specificity toward APC-mediated degradation)
stabilizes both Skp2 and Cks1 throughout the cell cycle and in
response to overexpression of Cdh1. Consistent with the direct
involvement of APCCdh1, both Skp2 and Cks1 (but not destruction-box mutants) are ubiquitinated by APCCdh1 in vitro. Using
different approaches, both labs produced evidence of precocious S phase in the absence of proper Cdh1 regulation. Bashir
et al. demonstrate premature S phase in synchronized cells that
overexpressed a nondegradable form of Skp2, as well as in cells
where Cdh1 is knocked down by RNAi; and Wei et al. show
that DNA content increases earlier in cells treated with Cdh1
RNAi than in control cells. The molecular basis for this regulation, however, still remains to be determined.
These studies reveal a circuit that can stabilize the G1 state
through the down-regulation of S-phase–promoting factors by
an indirect proteolytic mechanism dependent on APCCdh1. In
some ways, this G1 maintenance is analogous to traffic pausing
at a red light. The cars (S-phase–promoting feedback loops) remain stopped at the intersection, ready to advance when the
light changes, and they are being regulated by break pads (protein degradation) that cause them to wait until the time of the
transition. Consequently, this process creates the G1 (gap 1)
phase.
What is responsible for conversion of the G1 state to the Sphase state? Moreover, once initiated, how is the S-phase state
maintained, so that cells do not reverse direction and end up
back in G1? The answers to these questions are not fully known,
because we understand relatively little about specif ic
mechanisms. Moreover, it is possible that several mechanisms
contribute to the bi-stable switch. Although the bulk of p27 is
destroyed by SCFSkp2-mediated degradation during S phase,
previous work has suggested that a small fraction of p27 is
relocalized to the cytoplasm during G1 (28, 29). This process
appears to reflect two distinct kinase pathways, one involving
phosphorylation of Thr187 by Akt and a second involving phosphorylation of Ser10 by hKIS (human kinase interacting with
stathmin) (30, 31). Presumably, phosphorylation of cytoplasmic
p27 by Akt leads to its retention in the cytoplasm, whereas
hKIS phosphorylates p27 in the nucleus, which leads to its export. This process appears to lead to activation of a small pool
of nuclear Cdk’s that could then initiate the signaling cascade
that leads to inactivation of Cdh1; in essence, a negative feedback loop. Induction of Cdk activity generates at least two
mechanisms for Cdh1 inactivation. First, Cdk-mediated E2F activation leads to increased expression of the gene encoding
Emi1, an inhibitor of APCCdh1 (Fig. 2). Second, E2F also stimulates expression of the gene encoding cyclin A, which in combination with Cdk2 is responsible for phosphorylation of Cdh1
and inactivation of its ability to bind to APC (32). Because
hKIS, Akt, or both are sensitive to the nutrient and growth fac-
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Low Plk1 and Cdk
activity impedes
phosphodegron formation
Cd
c20
Cdc14
G1
APC
Cyclin B/Cdk1
Cyclin A
h1
Cdh1
Cddh1
APCC
APC
APC
CP
R
β-T
SCF
Emi1
Cdk2
E2F
Rb
Sk
p2
Cyclin E
s1
Ck
Cdk2
SCF
p27
Low Plk1 and Cdk
activity impedes
phosphodegron formation
c2
Cd
0
Cd
Cyclin A
h1
G1/S
SCF
Emi1
Cdk2
Cdc14
APC
P
RC
β-T
APC
Cyclin B/Cdk1
E2F
Rb
Cyclin E
SCF
SCF
SCF
p2
Sk
Ck
s1
p27
Cdk2
Cytoplasmic
export
P
p27
Fig. 2. Feedback loops that are regulated by E3 ligase activity maintain bi-stability between G1 and S phases. In G1, APCCdh1
promotes stabilization of p27, which inhibits S-phase–promoting factors E2F and cyclin E/Cdk2 through the activity of the Rb repressor protein. The predominance of Cdc14 phosphatase activity over cyclin B/Cdk1 activity favors formation of the APCCdh1 complex instead of the APCCdc20 complex in G1. After an initial trigger signaling the completion of G1, S-phase entry is reinforced by Emi1 and cyclin A/Cdk2, which prevent APC Cdh1 degradation of the SCF Skp2/Cks1 complex. Upon stabilization of SCF Skp2/Cks1 activity,
S-phase–promoting factors are activated (cyclin E/Cdk2, E2F, cyclin A/Cdk2, and Emi1). Proteins whose activities are high at each
phase are shown in green; proteins whose activities are low are shown in gray.
tor status of the cell, these components may be the critical sensors that determine when a cell will begin to convert from a G1
state to an S-phase state. The ability of Cdk2 to activate bulk
p27 turnover thereby enforces the positive feedback loop leading to E2F activation, while simultaneously setting the stage for
negative feedback to inactivate Cdh1 through phosphorylation
and direct inhibition by Emi1. It is additionally possible that in
G1 a small amount of Emi1 is present (32), which may regulate
Cdh1 before the E2F-mediated increase in Emi1 abundance at
the G1-to-S transition.
The abundance of Emi1, itself an F-box protein (Fbx5), is also subject to control by the Ub pathway. Emi1 functions to inhibit both APC Cdc20 and APC Cdh1 from early S phase to
prometaphase, at which time Emi1 is degraded by SCFβ-TRCP,
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thus allowing APC activation (33, 34). The trigger for Emi1 destruction is polo kinase (Plk1) phosphorylation to generate a
phosphodegron that is recognized by SCFβ-TRCP, although phosphorylation by cyclin B/Cdk1 may also play a supporting role
(35, 36). In mammalian cells, increasing Emi1 overcomes a
Cdh1-imposed G1 block, leading to S-phase entry, analogous to
that seen upon knockdown of Cdh1 (32). Likewise, in
Drosophila, overexpression of the Emi1 ortholog Rca1 in G1
causes precocious cyclin A/Cdk2 activation (37), which leads to
premature S phase. The ability of Emi1 to function in this manner probably reflects the absence of Plk1 and cyclin B/Cdk1
during G1, which themselves are degraded by APCCdh1 as cells
exit mitosis (18, 38, 39).
Although Ub-mediated degradation can be used to maintain
a particular cell cycle state such as G1, there are also cases in
which degradation serves as part of a feedback loop to promote
a phase transition. In this instance, initiation of the phase transition provides signals that lead to the destruction of proteins that
normally block the transition (for example, destruction of bulk
p27 by SCFSkp2-mediated degradation during G1-to-S phase).
Recent papers describe how this type of mechanism controls the
somatic mammalian G2-to-M transition, as well as the coupling
of the morphogenetic pathway to mitosis in budding yeast (40,
41). In mammalian cells, entry into mitosis is controlled by activation of Cdk1/cyclin B. In G2, Cdk1 is held in an inactive
form by Wee1- and Myt1-mediated phosphorylation of Cdk1 on
Thr14 and Tyr15 (42, 43). In response to appropriate signals, including activation of Plk1, the Cdc25 phosphatases are activated, leading to dephosphorylation and activation of Cdk1 (44). In
order to maintain the “on” state and propel cells into mitosis, a
negative feedback loop exists to extinguish the activity of Wee1
and Myt1 (45). Although it has been known that Wee1 activity
is blocked by phosphorylation and degradation, precisely how
this is coupled to Cdk1 activation and mitotic entry was unknown. New work now reveals that degradation of Wee1 occurs
through the SCFβ-TRCP–Ub ligase pathway (40). Interestingly,
rapid Wee1 ubiquitination depends on dual phosphorylation
mechanisms involving Plk1 and Cdk1. Each of these kinases
phosphorylates Wee1 to generate unconventional phosphodegrons that bind β–transducin repeat–containing protein
(β-TRCP), but both kinase activities are required for maximal
rates of Wee1 ubiquitination in vitro and turnover in vivo. This
provides clear evidence that Cdk1 activation is coupled to a regulatory feedback loop to degrade its inhibitor Wee1. A parallel
situation exists for the budding yeast homolog Swe1, but the details of the mechanism employed are distinct. During S phase,
Swe1 becomes sequentially phosphorylated and is degraded
(46, 47). However, in response to defects in septin filament assembly at the bud neck, Swe1 is dephosphorylated and stabilized and inhibits Cdk1 (Cdc28) activity, thereby blocking mitotic entry (46). As a result of this block, cells fail to convert
polarized growth to isotropic growth, giving rise to highly elongated buds. Now recent work suggests that a dual kinase mechanism is also required for Swe1 degradation upon appropriate
septin filament assembly (42). Like Wee1, Swe1 degradation requires the polo kinase Cdc5, but unlike Wee1, the p21-activated
kinase Cla4 provides the initial signal that facilitates Cdc5-dependent phosphorylation. Precisely how these events are controlled is not clear, but localization of Cdc5 at the bud neck appears to be critical for Swe1 degradation, because Cdc5 mutants
defective in bud neck localization failed to properly degrade
Swe1. The Ub ligase for Swe1 is currently undefined, but given
the role of SCF in controlling phosphorylation-dependent protein destruction, it is conceivable that SCF is involved. Regardless, these studies emphasize the extent to which multiple phosphorylation-driven events conspire to control critical cell cycle
transitions.
The past several years have revealed many instances in
which phosphorylation drives the turnover of critical signaling
molecules through the SCF–Ub ligase pathway. Current efforts
seek to build on these fundamental pathways to understand how
individual turnover pathways are integrated into larger networks, forming both positive and negative feedback loops. Two
major trends are emerging. First, there are now many instances
in which multiple kinase signaling pathways converge on a particular substrate to generate a phosphodegron responsible for
mediating turnover of a particular protein by the SCF pathway.
Examples include β-catenin, Emi1, Wee1, c-Myc, and Cdc25A
(35, 40, 48–51). Presumably, the use of multiple inputs reflects
the fact that multiple independent processes are being simultaneously monitored, thereby allowing the timing of dependent
events to be tightly controlled. Second, there are clear indications that Ub ligase components themselves are regulated by
protein turnover. It has been known for some time that F-box
proteins are themselves subject to turnover by distinct Ub ligases through autocatalytic mechanisms (52), but it is becoming
clear that regulated proteolysis of E3 ligases provides a critical
underpinning for controlling cell cycle transitions and maintaining particular cell cycle phases. These protein kinase and proteolysis circuits are likely to provide fodder for systems biologists
for years to come.
References
1. S. I. Reed, Ratchets and clocks: The cell cycle, ubiquitylation and protein
turnover. Nat. Rev. Mol. Cell Biol. 4, 855–864 (2003).
2. M. Tyers, P. Jorgensen, Proteolysis and the cell cycle: With this RING I do
thee destroy. Curr. Opin. Genet. Dev. 10, 54–64 (2000).
3. T. Bashir, N. V. Dorrello, V. Amador, D. Guardavaccaro, M. Pagano,
Control of the SCF-skp2-cks1 ubiquitin ligase by the APC/C-cdh1 ubiquitin
ligase. Nature 428, 190–193 (2004).
4. W. Wei, N. G. Ayad, Y. Wan, G.-J. Zhang, M. W. Kirschner, W. G. Kaelin
Jr., Degradation of the SCF component Skp2 in cell-cycle phase G1 by the
anaphase-promoting complex. Nature 428, 194–198 (2004).
5. J. E. Ferrell Jr., W. Xiong, Bistability in cell signaling: How to make continuous processes discontinuous, and reversible processes irreversible. Chaos
11, 227–236 (2001).
6. F. R. Cross, Two redundant oscillatory mechanisms in the yeast cell cycle.
Dev. Cell 4, 741–752 (2003).
7. J. E. Ferrell Jr., Self-perpetuating states in signal transduction: Positive
feedback, double-negative feedback and bistability. Curr. Opin. Cell Biol.
14, 140–148 (2002).
8. J. R. Pomerening, E. D. Sontag, J. E. Ferrell Jr., Building a cell cycle oscillator: Hysteresis and bistability in the activation of Cdc2. Nat. Cell Biol. 5,
346–351 (2003).
9. D. Angeli, J. E. Ferrell Jr., E. D. Sontag, Detection of multistability, bifurcations, and hysteresis in a large class of biological positive-feedback
systems. Proc. Natl. Acad. Sci. U.S.A. 101, 1822–1827 (2004).
10. A. Hershko, A. Ciechanover, The ubiquitin system. Annu. Rev. Biochem.
67, 425–479 (1998).
11. H. C. Vodermaier, J. M. Peters, APC activators caught by their tails? Cell
Cycle 3, 265–266 (2004).
12. J. W. Harper, J. L. Burton, M. J. Solomon, The anaphase-promoting complex: It’s not just for mitosis any more. Genes Dev. 16, 2179–2206 (2002).
13. W. Zachariae, K. Nasmyth, Whose end is destruction: Cell division and the
anaphase-promoting complex. Genes Dev. 13, 2039–2058 (1999).
14. R. J. Deshaies, SCF and Cullin/Ring H2-based ubiquitin ligases. Annu.
Rev. Cell Dev. Biol. 15, 435–467 (1999).
15. D. M. Koepp, J. W. Harper, S. J. Elledge, How the cyclin became a cyclin:
Regulated proteolysis in the cell cycle. Cell 97, 431–434 (1999).
16. A. Amon, S. Irniger, K. Nasmyth, Closing the cell cycle circle in yeast: G2
cyclin proteolysis initiated at mitosis persists until the activation of G1 cy-
www.stke.org/cgi/content/full/sigtrans;2004/242/pe31
Page 4
PERSPECTIVE
clins in the next cycle. Cell 77, 1037–1050 (1994).
17. S. Prinz, E. S. Hwang, R. Visintin, A. Amon, The regulation of Cdc20 proteolysis reveals a role for APC components cdc23 and Cdc27 during S
phase and early mitosis. Curr. Biol. 8, 750–760 (1998).
18. M. Shirayama, W. Zachariae, R. Ciosk, K. Nasmyth, The polo-like kinase
Cdc5p and the WD-repeat protein Cdc20p/fizzy are regulators and substrates of the anaphase promoting complex in Saccharomyces cerevisiae.
EMBO J. 17, 1336–1349 (1998).
19. C. J. Sherr, J. M Roberts, CDK inhibitors: Positive and negative regulators
of G1-phase progression. Genes Dev. 13, 1501–1512 (1999).
20. M. Pagano, S. W. Tam, A. M. Theodoras, P. Beer-Romero, G. Del Sal, V.
Chau, P. R. Yew, G. F. Draetta, M. Rolfe, Role of the ubiquitin-proteasome
pathway in regulating abundance of the cyclin-dependent kinase inhibitor
p27. Science 269, 682–685 (1995).
21. A. C. Carrano, E. Eytan, A. Hershko, M. Pagano, SKP2 is required for
ubiquitin-mediated degradation of the CDK inhibitor p27. Nat. Cell Biol. 1,
193–199 (1999).
22. L. M. Tsvetkov, K. H. Yeh, S. J. Lee, H. Sun, H. Zhang, p27(Kip1) ubiquitination and degradation is regulated by the SCF(Skp2) complex through
phosphorylated Thr187 in p27. Curr. Biol. 9, 661–664 (1999).
23. C. Spruck, H. Strohmaier, M. Watson, A. P. Smith, A. Ryan, T. W. Krek, S.
I. Reed, A CDK-independent function of mammalian Cks1: Targeting of
SCF(Skp2) to the CDK inhibitor p27Kip1. Mol. Cell 7, 639–650 (2001).
24. D. Ganoth, G. Bornstein, T. K. Ko, B. Larsen, M. Tyers, M. Pagano, A. Hershko, The cell-cycle regulatory protein Cks1 is required for SCF(Skp2)mediated ubiquitinylation of p27. Nat. Cell Biol. 3, 321–324 (2001).
25. M. Pagano, R. Benmaamar, When protein destruction runs amok, malignancy is on the loose. Cancer Cell 4, 251–256 (2003).
26. L. Furstenthal, C. Swanson, B. K. Kaiser, A. G. Eldridge, P. K. Jackson,
Triggering ubiquitination of a CDK inhibitor at origins of DNA replication.
Nat. Cell Biol. 3, 715–722 (2001).
27. L. Furstenthal, B. K. Kaiser, C. Swanson, P. K. Jackson, Cyclin E uses
Cdc6 as a chromatin-associated receptor required for DNA replication. J.
Cell Biol. 152, 1267–1278 (2001).
28. G. Rodier, A. Montagnoli, L. Di Marcotullio, P. Coulombe, G. F. Draetta, M.
Pagano, S. Meloche, p27 cytoplasmic localization is regulated by phosphorylation on Ser10 and is not a prerequisite for its proteolysis. EMBO J.
20, 6672–6682 (2001).
29. N. Ishida, T. Hara, T. Kamura, M. Yoshida, K. Nakayama, K. I. Nakayama,
Phosphorylation of p27Kip1 on serine 10 is required for its binding to
CRM1 and nuclear export. J. Biol. Chem. 277, 14355–14358 (2002).
30. N. Fujita, S. Sato, K. Katayama, T. Tsuruo, Akt-dependent phosphorylation
of p27Kip1 promotes binding to 14-3-3 and cytoplasmic localization. J. Biol. Chem. 277, 28706–28713 (2002).
31. M. Boehm, T. Yoshimoto, M. F. Crook, S. Nallamshetty, A. True, G. J.
Nabel, E. G. Nabel, A growth factor-dependent nuclear kinase
phosphorylates p27(Kip1) and regulates cell cycle progression. EMBO J.
21, 3390–3401 (2002).
32. J. Y. Hsu, J. D. Reimann, C. S. Sorensen, J. Lukas, P. K. Jackson, E2Fdependent accumulation of hEmi1 regulates S phase entry by inhibiting
APC(Cdh1). Nat. Cell Biol. 4, 358–366 (2002).
33. D. Guardavaccaro, Y. Kudo, J. Boulaire, M. Barchi, L. Busino, M. Donzelli,
F. Margottin-Goguet, P. K. Jackson, L. Yamasaki, M. Pagano, Control of
meiotic and mitotic progression by the F box protein beta-Trcp1 in vivo.
Dev. Cell 4, 799–812 (2003).
34. F. Margottin-Goguet, J. Y. Hsu, A. Loktev, H. M. Hsieh, J. D. Reimann, P.
K. Jackson, Prophase destruction of Emi1 by the SCF(betaTrCP/Slimb)
ubiquitin ligase activates the anaphase promoting complex to allow progression beyond prometaphase. Dev. Cell 4, 813–826 (2003).
35. Y. Moshe, J. Boulaire, M. Pagano, A. Hershko, Role of Polo-like kinase in
the degradation of early mitotic inhibitor 1, a regulator of the anaphase promoting complex/cyclosome. Proc. Natl. Acad. Sci. U.S.A. 101, 7937–7942
(2004).
36. P. K. Jackson, personal communication.
37. X. Dong, K. H. Zavitz, B. J. Thomas, M. Lin, S. Campbell, S. L. Zipursky,
Control of G1 in the developing Drosophila eye: rca1 regulates Cyclin A.
Genes Dev. 11, 94–105 (1997).
38. G. Fang, H. Yu, M. W. Kirschner, Direct binding of CDC20 protein family
members activates the anaphase-promoting complex in mitosis and G1.
Mol. Cell 2, 163–171 (1998).
39. M. Schwab, M. Neutzner, D. Mocker, W. Seufert, Yeast Hct1 recognizes
the mitotic cyclin Clb2 and other substrates of the ubiquitin ligase APC.
EMBO J. 20, 5165–5175 (2001).
40. N. Watanabe, H. Arai, Y. Nishihara, M. Taniguchi, N. Watanabe, T. Hunter,
H. Osada, M-phase kinases induce phospho-dependent ubiquitination of
somatic Wee1 by SCFbeta-TrCP. Proc. Natl. Acad. Sci. U.S.A. 101,
4419–4424 (2004).
41. K. Sakchaisri, S. Asano, L.-R. Yu, M. J. Shulewitz, C. J. Park, J.-E. Park,
Y.-W. Cho, T. D. Veenstra, J. Thorner, K. S. Lee, Coupling morphogenesis
to mitotic entry. Proc. Natl. Acad. Sci. U.S.A. 101, 4124–4129 (2004).
42. N. Watanabe, M. Broome, T. Hunter, Regulation of the human WEE1Hu
CDK tyrosine 15-kinase during the cell cycle. EMBO J. 14, 1878–1891
(1995).
43. F. Liu, J. J. Stanton, Z. Wu, H. Piwnica-Worms, The human Myt1 kinase preferentially phosphorylates Cdc2 on threonine 14 and localizes to the endoplasmic
reticulum and Golgi complex. Mol. Cell. Biol. 17, 571–583 (1997).
44. T. Izumi, D. H. Walker, J. L. Maller, Periodic changes in phosphorylation of
the Xenopus cdc25 phosphatase regulate its activity. Mol. Biol. Cell 3,
927–939 (1992).
45. D. O. Morgan, Cyclin-dependent kinases: Engines, clocks, and
microprocessors. Annu. Rev. Cell Dev. Biol. 13, 261–291 (1997).
46. R. A. Sia, E. S. Bardes, D. J. Lew, Control of Swe1p degradation by the
morphogenesis checkpoint. EMBO J. 17, 6678–6688 (1998).
47. J. N. McMillan, C. L. Theesfeld, J. C. Harrison, E. S. Bardes, D. J. Lew,
Determinants of Swe1p degradation in Saccharomyces cerevisiae. Mol. Biol. Cell 13, 3560–3575 (2002).
48. C. Liu, Y. Li, M. Semenov, C. Han, G. H. Baeg, Y. Tan, Z. Zhang, X. Lin, X.
He, Control of beta-catenin phosphorylation/degradation by a dual-kinase
mechanism. Cell 108, 837–847 (2002).
49. M. Welcker, A. Orian, J. Jin, J. A. Grim, J. W. Harper, R. N. Eisenman, B.
E. Clurman, The Fbw7 tumor suppressor regulates glycogen synthase kinase 3 phosphorylation-dependent c-Myc protein degradation. Proc. Natl.
Acad. Sci. U.S.A., in press.
50. J. Jin, T. Shirogane, L. Xu, G. Nalepa, J. Qin, S. J. Elledge, J. W. Harper,
SCF-beta-TRCP links Chk1 signaling to degradation of the Cdc25A protein
phosphatase. Genes Dev. 17, 3062–3074 (2003).
51. L. Busino, M. Donzelli, M. Chiesa, D. Guardavaccaro, D. Ganoth, N. V.
Dorrello, A. Hershko, M. Pagano, G. F. Draetta, Degradation of Cdc25A by
beta-TrCP during S phase and in response to DNA damage. Nature 426,
87–91 (2003).
52. P. Zhou, P. M. Howley, Ubiquitination and degradation of the substrate
recognition subunits of SCF ubiquitin-protein ligases. Mol. Cell 2, 571–580
(1998).
Citation: X. L. Ang, J. W. Harper, Interwoven ubiquitination oscillators
and control of cell cycle transitions. Sci. STKE 2004, pe31 (2004).
www.stke.org/cgi/content/full/sigtrans;2004/242/pe31
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