Cyclin E
CDK inhibitors are thought to prevent cell proliferation by negatively regulating cyclin-CDK
complexes. The opposite is also true: cyclin-CDK complexes in mammalian cells
can promote cell cycle progression by directly down-regulating CDK inhibitors. Expression of cyclin E-CDK2 in murine fibroblasts causes phosphorylation of the CDK inhibitor
p27Kip1 on T187, and cyclin E-CDK2 can directly phosphorylate p27 T187 in vitro. Cyclin E-CDK2-dependent phosphorylation of p27 results in elimination of p27 from the cell,
allowing cells to transit from G1 to S phase. Moreover, mutation of T187 in p27 to alanine creates a
p27 protein that causes a G1 block resistant to cyclin E whose level of expression is not modulated
by cyclin E. A kinetic analysis of the interaction between p27 and cyclin E-CDK2 explains how p27
can be regulated by the same enzyme it targets for inhibition. p27 interacts with cyclin
E-CDK2 in at least two distinct ways: one resulting in p27 phosphorylation and release, the other in
tight binding and cyclin E-CDK2 inhibition. The binding of ATP to the CDK governs which of these two states predominate. At low ATP (< 50 microM) p27 is primarily a CDK inhibitor, but at ATP concentrations
approaching physiological levels (> 1 mM) p27 is more likely to be a substrate. Thus, p27 is identified as a biologically relevant cyclin E-CDK2 substrate, the physiological
consequences of p27 phosphorylation has been demonstrated, and a kinetic model has been developed to explain how p27 can be both
an inhibitor and a substrate of cyclin E-CDK2 (Sheaff, 1997).
The p27(Kip1) protein associates with G1-specific cyclin-CDK complexes and inhibits their catalytic
activity. p27(Kip1) is regulated at various levels, including translation, degradation by the
ubiquitin/proteasome pathway and non-covalent sequestration. Point mutants of p27 are described that are
deficient in their interaction with either cyclins [p27(c-)], CDKs [p27(k-)] or both [p27(ck-)]. Each contact is critical for kinase inhibition and induction of G1 arrest. Through its
intact cyclin contact, p27(k-) associates with active cyclin E-CDK2 and, unlike wild type p27, p27(c-)
or p27(ck-), is efficiently phosphorylated by CDK2 on a conserved C-terminal CDK target site
(TPKK). Retrovirally expressed p27(k-) is rapidly degraded through the proteasome in Rat1 cells,
but is stabilized by secondary mutation of the TPKK site to VPKK. In this experimental setting,
exogenous wild-type p27 forms inactive ternary complexes with cellular cyclin E-CDK2, is not
degraded through the proteasome, and is not further stabilized by the VPKK mutation. p27(ck-),
which is not recruited to cyclin E-CDK2, also remains stable in vivo. Thus, selective degradation of
p27(k-) depends upon association with active cyclin E-CDK2 and subsequent phosphorylation.
Altogether, these data show that p27 must be phosphorylated by CDK2 on the TPKK site in order to
be degraded by the proteasome. It is proposed that cellular p27 must also exist transiently in a
cyclin-bound non-inhibitory conformation in vivo (Vlach, 1997).
Activation of the human cyclin E-cdk2 heterodimer in quiescent cells involves a Myc-dependent (Drosophila homolog: dmyc) step, but involves no significant change in the amount of cyclin-cdk complex. This activation involves the release of a 120 kDa cyclin E-cdk2 complex from a 250 kDa complex that is present in serum starved cells. The 250 kDa complex involves an association of cyclin-cdk with inhibitory molecule p27. Release of p27 involves a change of a change of affinity for p27 or p27 degradation. An additional step in activation of cyclin-dependent kinases by c-myc is dephosphorylation of cdk2 carried out by cdc25A, a transcriptional target of Myc/Max heterodimers (Steiner, 1995 and Galaktionov, 1996).
The calmodulin-dependent protein kinase-II (CaMK-II) inhibitor KN-93 reversibly arrests mouse and human cells in the G1 phase of the cell cycle. The
stimulation of Ca(2+)-independent (autonomous) CaMK-II enzymatic activity, a
barometer of in situ activated CaMK-II, is prevented by the same KN-93
concentrations that cause G1 phase arrest. KN-93 causes the retinoblastoma protein pRB
to become dephosphorylated and the activity of both cdk2 and cdk4 (Drosophila homolog: Cyclin-dependent kinase 4/6), two potential pRb
kinases, to decrease. Neither the activity of p42MAP kinase, an early response G1
signaling molecule, nor the phosphorylation status or DNA-binding capability of the
transcription factors serum response factor and cAMP responsive element-binding
protein is altered during this G1 arrest. The protein levels of cyclin-dependent kinase 2
(cdk2) and cdk4 are unaffected during this G1 arrest and the total cellular levels of the
cdk inhibitors p21cip1 and p27kip1 are not increased. Instead, the cdk4 activity
decreases resulting from KN-93 are the result of a 75% decrease in cyclin D1 levels. In
contrast, cyclin A and E levels are relatively constant. Cdk2 activity decreases are
primarily the result of enhanced p27kip1 association with cdk2/cyclin E. All of these
phenomena are unaffected by KN-93's inactive analog, KN-92, and are reversible
upon KN-93 washout. The kinetics of recovery from cell cycle arrest are similar to
those reported for other G1 phase blockers. These results suggest a mechanism by which
G1 Ca2+ signals can be linked via calmodulin-dependent phosphorylations to the cell
cycle-controlling machinery through cyclins and cdk inhibitors (Morris, 1998).
This study examines in vivo the role and functional interrelationships of components regulating exit from the
G1 resting phase into the DNA synthetic (S) phase of the cell cycle. The approach made use of several key
experimental attributes of the developing mouse lens, namely its strong dependence on pRb in maintenance of
the postmitotic state, the down-regulation of cyclins D and E and up-regulation of the p57(KIP2) inhibitor in
the postmitotic lens fiber cell compartment, and the ability to target transgene expression to this compartment.
These attributes provide an ideal in vivo context from which to examine the consequences of forced cyclin
expression and/or of loss of p57(KIP2) inhibitor function in a cellular compartment. This location permits an accurate
quantitation of cellular proliferation and apoptosis rates in situ. Despite substantial
overlap in cyclin transgene expression levels, D-type and E cyclins exhibit clear functional differences in
promoting entry into S phase. In general, forced expression of the D-type cyclins is more efficient than
cyclin E in driving lens fiber cells into S phase. In the case of cyclins D1 and D2, ectopic proliferation
requires their enhanced nuclear localization through CDK4 coexpression. High nuclear levels of cyclin E and
CDK2, while not sufficient to promote efficient exit from G1, act synergistically with ectopic cyclin
D/CDK4. The functional differences between D-type and E cyclins is most evident in the
p57(KIP2)-deficient lens wherein cyclin D overexpression induces a rate of proliferation equivalent to that of
the pRb null lens, while overexpression of cyclin E does not increase the rate of proliferation over that induced
by the loss of p57(KIP2) function. These in vivo analyses provide strong biological support for the prevailing
view that the antecedent actions of cyclin D/CDK4 act cooperatively with cyclin E/CDK2 and antagonistically
with p57(KIP2) to regulate the G1/S transition in a cell type highly dependent on pRb (Gomez Lahoz, 1999).
During a normal cell cycle, entry into S phase is dependent on completion of mitosis and subsequent
activation of cyclin-dependent kinases (Cdks) in G1. These events are monitored by checkpoint pathways. After treatment with microtubule inhibitors (MTIs), cells
deficient in the Cdk inhibitor p21(Waf1/Cip1) enter S phase with a 4N DNA content or greater, a process known as
endoreduplication, which results in polyploidy. To determine how p21 prevents MTI-induced
endoreduplication, the G1/S and G2/M checkpoint pathways were examined in two isogenic cell systems:
1). HCT116 p21(+/+) and p21(-/-) cells and 2). H1299 cells containing an inducible p21 expression vector (HIp21).
Both HCT116 p21(-/-) cells and noninduced HIp21 cells endoreduplicate after MTI treatment. Analysis of
G1-phase Cdk activities demonstrates that the induction of p21 inhibits endoreduplication through direct
cyclin E/Cdk2 regulation. The kinetics of p21 inhibition of cyclin E/Cdk2 activity and binding to
proliferating-cell nuclear antigen in HCT116 p21(+/+) cells parallels the onset of endoreduplication in
HCT116 p21(-/-) cells. In contrast, loss of p21 does not lead to deregulated cyclin D1-dependent kinase
activities, nor does p21 directly regulate cyclin B1/Cdc2 activity. MTI-induced
endoreduplication in p53-deficient HIp21 cells is due to levels of p21 protein below a threshold required for
negative regulation of cyclin E/Cdk2, since ectopic expression of p21 restores cyclin E/Cdk2 regulation and
prevents endoreduplication. Based on these findings, it is proposed that p21 plays an integral role in the
checkpoint pathways that restrain normal cells from entering S phase after aberrant mitotic exit due to defects
in microtubule dynamics (Stewart, 1999).
The cellular abundance of the cyclin-dependent kinase (Cdk) inhibitor p27 is regulated by the ubiquitin-proteasome system. Activation of p27 degradation is seen in
proliferating cells and in many types of aggressive human carcinomas. p27 can be phosphorylated on threonine 187 by Cdks, and cyclin E/Cdk2 overexpression can
stimulate the degradation of wild-type p27, but not of a threonine 187-to-alanine p27 mutant [p27(T187A)]. However, it remains unknown whether threonine 187 phosphorylation
stimulates p27 degradation through the ubiquitin-proteasome system or some alternative pathway. In this study, it is demonstrated that p27 ubiquitination (as
assayed in vivo and in an in vitro reconstituted system) is cell-cycle regulated and that Cdk activity is required for the in vitro ubiquitination of p27. Furthermore,
ubiquitination of wild-type p27, but not of p27(T187A), can occur in G1-enriched extracts only on addition of cyclin E/Cdk2 or cyclin A/Cdk2. Using a
phosphothreonine 187 site-specific antibody for p27, it has been shown that threonine 187 phosphorylation of p27 is also cell-cycle dependent, being present in proliferating
cells but undetectable in G1 cells. In addition to threonine 187 phosphorylation, efficient p27 ubiquitination requires formation of a trimeric
complex with the cyclin and Cdk subunits. In fact, cyclin B/Cdk1, which can phosphorylate p27 efficiently, but cannot form a stable complex with it, is unable to
stimulate p27 ubiquitination by G1 extracts. Furthermore, another p27 mutant [p27(CK-)] that can be phosphorylated by cyclin E/Cdk2 but cannot bind this kinase
complex, is refractory to ubiquitination. Thus throughout the cell cycle, both phosphorylation and trimeric complex formation act as signals for the ubiquitination of a
Cdk inhibitor (Montagnoli, 1999).
The Cdk2 inhibitor, p27Kip1, is degraded in a phosphorylation- and ubiquitylation-dependent manner at the G1 -S transition of the cell cycle. Degradation of p27Kip1 requires import into the nucleus for phosphorylation by Cdk2. Phosphorylated p27Kip1 is thought to be subsequently re-exported and degraded in the cytosol. Using two-hybrid screens, p27Kip1 has been shown to interact with a nuclear pore-associated protein, mNPAP60: the interaction maps to the
310
helix of p27 and a point mutant in p27Kip1 has been identified that is deficient for interaction (R90G). Whether in vivo or in vitro, the loss-of-interaction mutant is poorly transported into the nucleus, while ubiquitination of p27R90G occurs normally. Co-expression of cyclin E and Cdk2 in vivo rescues the import defect. However, mutant p27Kip1 accumulates in a phosphorylated form in the nucleus and is not efficiently degraded, arguing that at least one step in the degradation of phosphorylated p27Kip1 requires an interaction with the nuclear pore. These results identify a novel component involved in p27Kip1 degradation and suggest that degradation of p27Kip1 is tightly linked to its intracellular transport (Muller, 2000).
The ability of cyclin-dependent kinases (CDKs) to promote cell proliferation is opposed by cyclin-dependent
kinase inhibitors (CKIs), proteins that bind tightly to cyclin-CDK complexes and block the phosphorylation of exogenous
substrates. Mice with targeted CKI gene deletions have only subtle proliferative abnormalities, however, and cells
prepared from these mice seem remarkably normal when grown in vitro. One explanation may be the operation of
compensatory pathways that control CDK activity and cell proliferation when normal pathways are inactivated. Mice lacking the CKIs p21(Cip1) and p27(Kip1) were used to investigate this issue, specifically with respect to CDK regulation
by mitogens. p27 is the major inhibitor of Cdk2 activity in mitogen-starved wild-type murine
embryonic fibroblasts (MEFs). Nevertheless, inactivation of the cyclin E-Cdk2 complex in response to mitogen starvation
occurs normally in MEFs that have a homozygous deletion of the p27 gene. Moreover, CDK regulation by mitogens is
also not affected by the absence of both p27 and p21. A titratable Cdk2 inhibitor compensates for the absence of both
CKIs, and this inhibitor is identified as p130, a protein related to the retinoblastoma gene product Rb. Thus, cyclin E-Cdk2
kinase activity cannot be inhibited by mitogen starvation of MEFs that lack both p27 and p130. In addition, cell types that
naturally express low amounts of p130, such as T lymphocytes, are completely dependent on p27 for regulation of the
cyclin E-Cdk2 complex by mitogens. It is concluded that inhibition of Cdk2 activity in mitogen-starved fibroblasts is usually
performed by the CKI p27, and to a minor extent by p21. Remarkably p130, a protein in the Rb family that is not related
to either p21 or p27, will directly substitute for the CKIs and restore normal CDK regulation by mitogens in cells lacking
both p27 and p21. p130 has the 'RxL' (Arg-X-Lys) motif, which is present in other cyclin-binding proteins and is required for CDK inhibition by members of the p21/p27 family. It is not clear, however, whether or not this motif is required for p130 to inhibit cyclin E-Ck2 in vitro, because the motif is absent from the amino-terminal inhibitory fragment of p130. Hence, the mode of inhibition of p130 may not mimic the one employed by the p21/p27 proteins. This compensatory use of p130 may be important in settings in which CKIs are not expressed at standard
levels, as is the case in many human tumors (Coats, 1999).
The kinase inhibitor p27Kip1 regulates the G1 cell cycle phase. Data is presented indicating that the oncogenic kinase Src regulates p27 stability through phosphorylation of p27 at tyrosine 74 and tyrosine 88. Src inhibitors increase cellular p27 stability, and Src overexpression accelerates p27 proteolysis. Src-phosphorylated p27 is shown to inhibit cyclin E-Cdk2 poorly in vitro, and Src transfection reduces p27-cyclin E-Cdk2 complexes. These data indicate that phosphorylation by Src impairs the Cdk2 inhibitory action of p27 and reduces its steady-state binding to cyclin E-Cdk2 to facilitate cyclin E-Cdk2-dependent p27 proteolysis. Furthermore, it was found that Src-activated breast cancer lines show reduced p27 and observe a correlation between Src activation and reduced nuclear p27 in 482 primary human breast cancers. Importantly, it is reported that in tamoxifen-resistant breast cancer cell lines, Src inhibition can increase p27 levels and restore tamoxifen sensitivity. These data provide a new rationale for Src inhibitors in cancer therapy (Chu, 2007).
Polycomb group (PcG) proteins are transcriptional repressors of genes involved in development and differentiation, and also maintain repression of key genes involved in the cell cycle, indirectly regulating cell proliferation. The human SCML2 gene, a mammalian homologue of the Drosophila PcG protein SCM, encodes two protein isoforms: SCML2A that is bound to chromatin and SCML2B that is predominantly nucleoplasmic. SCML2B was purified and found to form a stable complex with CDK/CYCLIN/p21 and p27, enhancing the inhibitory effect of p21/p27. SCML2B participates in the G1/S checkpoint by stabilizing p21 and favoring its interaction with CDK2/CYCE, resulting in decreased kinase activity and inhibited progression through G1. In turn, CDK/CYCLIN complexes phosphorylate SCML2, and the interaction of SCML2B with CDK2 is regulated through the cell cycle. These findings highlight a direct crosstalk between the Polycomb system of cellular memory and the cell-cycle machinery in mammals (Leconam 2013).
Retroviral expression of the cyclin-dependent kinase (CDK) inhibitor p16(INK4a) in rodent fibroblasts
induces dephosphorylation of pRb, p107 and p130 and leads to G1 arrest. Prior expression of cyclin E
allows S-phase entry and long-term proliferation in the presence of p16. Cyclin E prevents neither the
dephosphorylation of pRb family proteins, nor their association with E2F proteins in response to p16.
Thus, cyclin E can bypass the p16/pRb growth-inhibitory pathway downstream of pRb activation.
Retroviruses expressing E2F-1, -2 or -3 also prevent p16-induced growth arrest but are ineffective
against the cyclin E-CDK2 inhibitor p27(Kip1), suggesting that E2F cannot substitute for cyclin E
activity. Thus, cyclin E possesses an E2F-independent function required to enter S-phase. However,
cyclin E may not simply bypass E2F function in the presence of p16, since it restores expression of
E2F-regulated genes such as cyclin A or CDC2. Finally, c-Myc bypasses the p16/pRb pathway with
effects indistinguishable from those of cyclin E. It is suggested that this effect of Myc is mediated by its
action upstream of cyclin E-CDK2, and occurs via the neutralization of p27(Kip1) family proteins,
rather than induction of Cdc25A. These data imply that oncogenic activation of c-Myc, and possibly also
of cyclin E, mimics loss of the p16/pRb pathway during oncogenesis (Alevizopoulos, 1997).
In cells of higher eukaryotes, cyclin D-dependent kinases Cdk4 and Cdk6 (and possibly cyclin
E-dependent Cdk2) positively regulate the G1- to S-phase transition by phosphorylating the
retinoblastoma protein (pRb), thereby releasing E2F transcription factors that control S-phase genes.
Ectopic expression of cyclin E, but not cyclin D1, can override G1 arrest imposed by either the
p16INK4a Cdk inhibitor specific for Cdk4 and Cdk6 or a novel phosphorylation-deficient mutant pRb. The cyclin E-induced S phase and completion of the cell division cycle can occur
in the absence of E2F-mediated transactivation. Together with the ability of cyclin E to overcome a G1
block induced by expression of dominant-negative mutant DP-1, a heterodimeric partner of E2Fs, these
results provide evidence for a cyclin E-controlled S phase-promoting event in somatic cells downstream
of or parallel to phosphorylation of pRb and independent of E2F activation.
A lack of E2F-mediated transactivation can be compensated for by hyperactivation of this cyclin
E-controlled event (Lukas, 1997).
In mammalian cells, the retinoblastoma protein (Rb) is thought to negatively regulate progression
through the G1 phase of the cell cycle by its association with the transcription factor E2F.
Rb-E2F complexes suppress transcription of genes required for DNA synthesis; the prevailing view is that phosphorylation of Rb by complexes of cyclin-dependent kinases
(Cdks) and their regulatory cyclin subunits, and the subsequent release of active E2F, is required for
S-phase entry. This view is based, in part, on the fact that ectopic expression of cyclin-Cdks
leads to Rb phosphorylation and that this modification correlates with S-phase entry. In
Drosophila, however, cyclin E expression can bypass a requirement for E2F, suggesting that cyclins
may activate replication independent of the Rb/E2F pathway. Is Rb
phosphorylation a prerequisite for S-phase entry in Rb-deficient SAOS-2 osteosarcoma cells? This was examined, employing a
commonly used cotransfection assay. A G1 arrest in SAOS-2 cells
mediated by an Rb mutant lacking all 14 consensus Cdk phosphorylation sites is bypassed by
coexpressing G1-specific E-type or D-type cyclin-Cdk complexes; injection of purified
cyclin-Cdks during G1 accelerates S-phase entry. These results indicate that Rb phosphorylation is not
essential for S-phase entry when G1 cyclin-Cdks are overexpressed, and that other substrates of these
kinases can be rate-limiting for the G1 to S-phase transition. These data also reveal that the SAOS-2
cotransfection assay is complicated by Rb-independent effects of the coexpressed Cdks (Leng, 1997).
The Xenopus Cdc6 protein is essential for the initiation of DNA replication. The Xenopus homolog of Yeast and Drosophila ORC2, Xorc2 cannot bind to chromatin at M phase in Xenopus extracts, suggesting that its ability to bind to the origin DNA is regulated by the cell cycle. This difference may be due to the fact that chromosomes of Xenopus and other vertebrates undergo extensive condensation at mitosis. At the end of mitosis, when the chromatin decondenses and the Cdc2-cyclin B complex (for the fly homolog see Cyclin B) undergoes inactivation, both Xorc2 and Xcdc6 can reassociate with chromatin. Binding of Xcdc6 to chromatin requires Xorc2. Following the association of Xorc2 with Xenopus chromatin, the Xcdc6 and Xmcm3 (for a fly homolog see Disc proliferation abnormal) proteins bind shortly thereafter. Xmcm3 cannot bind to chromatin lacking Xcdc6. At some point, S-phase promoting factor (SPF) triggers the firing of the replication origins. In Xenopus, the Cdk2-cyclinE complex is necessary and perhaps sufficient to fulfill the role of SPF. The critical targets of Cdk2-cyclinE may be Xcdc6 and/or components of the ORC. Phosphorylation of Xcdc6 by Cdk2-cyclinE could lead to both the firing of the origin and the subsequent elimination of Xcdc6 from the origin. In postreplicative nuclei, Xcdc6 is associated with the nuclear envelope (Coleman, 1996 and references).
A novel cyclin gene was discovered by searching an expressed sequence tag database with a cyclin box
profile. The human cyclin E2 gene encodes a 404-amino-acid protein that is most closely related to cyclin E.
Cyclin E2 associates with Cdk2 in a functional kinase complex that is inhibited by both p27(Kip1) and
p21(Cip1). The catalytic activity associated with cyclin E2 complexes is cell cycle regulated and peaks at the
G1/S transition. Overexpression of cyclin E2 in mammalian cells accelerates G1, demonstrating that cyclin E2
may be rate limiting for G1 progression. Unlike cyclin E1, which is expressed in most proliferating normal
and tumor cells, cyclin E2 levels are low to undetectable in nontransformed cells and increase significantly
in tumor-derived cells. The discovery of a novel second cyclin E family member suggests that multiple unique
cyclin E-CDK complexes regulate cell cycle progression (Gudas, 1999).
In animal cells, the interphase centrosome reproduces (duplicates) only once per cell cycle, thereby ensuring a strictly bipolar mitotic spindle axis. Because there is no cell cycle checkpoint that monitors the number of spindle poles, uncontrolled duplications of the centrosome can contribute to genomic instability through the formation of multipolar mitotic spindles. To facilitate
investigation of the mechanisms that control centrosome reproduction, a frog egg extract arrested in S
phase of the cell cycle that supports repeated assembly of daughter centrosomes was developed.
Multiple rounds of centrosome reproduction are blocked by selective inactivation of cyclin-dependent
kinase 2-cyclin E (Cdk2-E) and are restored by addition of purified Cdk2-E. Confocal
immunomicroscopy reveals that cyclin E is localized at the centrosome. These results demonstrate
that Cdk2-E activity is required for centrosome duplication during S phase and suggest a mechanism
that could coordinate centrosome reproduction with cycles of DNA synthesis and mitosis. This suggests that a licensing event during S phase restores the reproductive capacity to the daughter centrosomes, theeby permitting them to duplicate again during the next cell cycle. Perhaps Cdk-2-E is the licensing factor that restores the reproductive capacity to the daughter centrosomes (Hinchcliffe, 1999).
Centrosome duplication consists of three distinct steps: (1) loss of orthogonal configuration and separation of the paired centrioles; (2) synthesis of a procentriole next to each preexisting centriole, and (3) elongation of the procentrioles. The activation of
CDK2/cyclin E is necessary for initiation of centrosome duplication (both separation of centrioles and procentriole formation). Moreover, constitutive activation of CDK2/cyclin E in cells results in uncoupling of initiation of centrosome and DNA duplication. In these cells, centrosomes duplicate immediately after entry into G1, much before the onset of DNA synthesis, indicating that initiation of
centrosome duplication primarily depends on activation of CDK2/cyclin E, while initiation of DNA replication requires additional events before being triggered
by CDK2/cyclin E. In normal cells, the activation of CDK2/cyclin E at late G1 triggers initiation of both centrosome and DNA duplication, and thus coordinates
these two events. Moreover, inhibition of CDK2 by CDK inhibitors abolishes the ability of CDK2/cyclin E to induce initiation of centrosome duplication,
demonstrating that the kinase activity of CDK2 is required. Thus, it is reasonable
to predict that certain centrosomal protein(s) are phosphorylated by CDK2/cyclin E, and that this phosphorylation event may trigger the initiation of centrosome
duplication (Okuda, 2000 and references therein).
Nucleophosmin (NPM/B23) has been identified as a substrate of CDK2/cyclin E in centrosome duplication. NPM/B23 associates
specifically with unduplicated centrosomes, and NPM/B23 dissociates from centrosomes by CDK2/cyclin E-mediated phosphorylation. An anti-NPM/B23
antibody, which blocks this phosphorylation, suppresses the initiation of centrosome duplication in vivo. Moreover, expression of a nonphosphorylatable mutant
NPM/B23 in cells effectively blocks centrosome duplication. Thus, NPM/B23 is a target of CDK2/cyclin E in the initiation of centrosome duplication (Okuda, 2000).
NPM/B23 is known to be involved in assembly and/or intranuclear transport of preribosomal particles, and in cytoplasmic/nuclear trafficking. NPM/B23 possesses molecular chaperoning activities, including
preventing protein aggregation, protecting enzymes during thermal denaturation, and facilitating renaturation of chemically denatured proteins. Since the centrosome consists of a large number of different proteins, it is interesting to consider the possibility that the chaperone activity of
NPM/B23 may be necessary for preventing irreversible aggregation within the centrosome proper. Moreover, its dissociation from the centrosome may induce a
dynamic structural change, leading to initiation of centrosome duplication (Okuda, 2000).
Continued: Cyclin E: Evolutionary Homologs part 3/3 | back to part 1/3
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