REGULATION OF THE CELL CYCLE BY SCF UBIQUITIN
LIGASES
Joanna Bloom and Michele Pagano*
Department of Pathology and New York University Cancer Institute,
NYU School of Medicine, New York, New York 10016, USA
*[email protected]
Lab website: http://www.med.nyu.edu/Path/Pagano
INTRODUCTION. Ubiquitin-mediated proteolysis controls the timed
destruction of numerous cellular regulatory proteins. The specificity of the
ubiquitin system is achieved by the use of a large number of ubiquitin
ligases, which are either single subunits or multiprotein complexes that act
as recognition factors for substrates to be ubiquitinated. SCF complexes
represent a large family of ubiquitin ligases, and are so called, because they
are composed of Skp1, Cul1, an F-box protein and Roc1. It is the F-box
protein (FBP) component of the SCF complex that is directly responsible
for substrate recognition. FBPs are defined by a 40 amino acid motif called
the F-box and characterized by additional protein-protein interaction
domains such as WD-40 repeats or leucine-rich repeats (LRRs). FBPs bind
Skp1 through their F-box to link to Cul1, Roc1 and the E2 enzyme that
ultimately ubiquitinylates the substrtae (see Fig. 1). In turn, the FBPs
recruit phosphorylated substrates through their diverse protein-protein
interaction domains. This system allows for specific recognition of a
variety of substrates by SCF
complexes through different
FBPs. It is well documented that
SCF complexes are key in
controlling the abundance of cell
cycle
regulatory
proteins
including cyclins and cyclindependent kinase
(CDK)
Figure 1. The SCF ubiquitin ligase.
inhibitors.
RESULTS. The SCF complex containing the FBP Skp2 coordinates the
ubiquitinylation of p21 and p27, two CDK inhibitors, which function during
G1 phase to block the activity of complexes containing Cdk2 (1-3).
Degradation of p21 and p27 allows for Cdk2 activation and entry into S
phase. Ubiquitinylation of p21 and p27 by SCFSkp2 also requires an
accessory factor called Cks1, which promotes the binding of
phosphorylated p27 to Skp2 (3,4). Recognition of p27 by the LRRs of Skp2
requires that p27 be phosphorylated on Thr187 and part of a trimeric
complex with Cdk2 and either Cyclin E or Cyclin A 5. The free amino
group of the N-terminal methionine of p21 is a site for ubiquitinylation in
vivo demonstrating that the presence of lysines is dispensable for p21
ubiquitinylation 6. Skp2 and its cofactor Cks1 are also unstable proteins in
G1, and their degradation prevents unscheduled degradation of p21 and
p27, and premature entry into S-phase. Significantly, degradation of Skp2
and Cks1 during G1 is controlled by another cell cycle-regulated ubiquitin
ligase, the anaphase-promoting complex/cyclosome (APC/C) (Bashir and
Pagano, unpublished).
Not only does the APC/C control SCF but the opposite is also true.
The SCF complex containing the FBP ß-Trcp1 is responsible for the
ubiquitin-dependent degradation of Emi1, an inhibitor of the APC/C, during
prometaphase 7. Inactivation in mice of the BTRC gene, encoding ß-Trcp1,
results in reduced fertility correlating with an accumulation of metaphase I
spermatocytes. In addition, ß-Trcp1-/- mouse embryo fibroblasts (MEFs)
display a lengthened mitosis, centrosome overduplication, multipolar
metaphase spindles, and misaligned chromosomes. Furthermore, Emi1 is
stabilized in mitotic ß-Trcp1-/- MEFs and extracts from these cells are
unable to sustain Emi1 ubiquitinylation unless supplemented with
recombinant ß-Trcp1. These results show that ß-Trcp1 regulates the timely
order of meiotic and mitotic events, and demonstrate that in addition to their
role at the G1/S transition, SCF complexes function during meiosis and
mitosis.
Studies on p27 degradation have been expanded to the clinic. It was
originally found that aggressive human carcinomas contain high p27specific degradation activity 8. Additionally, the absence of p27 is a
powerful prognostic marker for poor survival in patients with breast,
esophageal, lung and colorectal carcinomas 9. Although transformation has
often been shown to involve the inactivation of some CDK inhibitors, no
homologous deletions or mutations of the CDKN1b gene, encoding p27,
have been found so far in human tumors. In contrast, low p27 expression
observed in human carcinomas can result from its enhanced ubiquitinmediated degradation of p27 rather than altered p27 gene expression.
Significantly, Skp2 levels inversely correlate with p27 expression in human
cancers, such as breast cancers 10 and lymphomas, and Skp2 cooperates
with activated N-Ras in an in vivo model of lymphomagenesis 11.
DISCUSSION. Temporally coordinated destruction of cell cycle proteins
by the ubiquitin-proteasome pathway represents an important regulatory
mechanism that drives progression through the cell division cycle in a
unidirectional and irreversible manner. There is increasing evidence that in
addition to genetic alterations,
aberrant proteolysis of cell cycle
regulators contributes significantly
to tumorigenesis, and is indeed
found in many types of human
cancer 9. For instance, Skp2 is an
oncoprotein, due to its involvement
in p27 degradation (see Fig. 2).
Future studies will focus on
whether additional FBPs have
tumor suppressive or oncogenic
properties due to the nature of their
substrates.
Figure 2. The p27 pathway
ACKNOWLEDGEMENTS. Work in the Pagano lab is supported by
grants from the NIH (R01-CA76584 and R01-GM57587) and by a grant
from Celgene Corp.
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