Biology Reference
In-Depth Information
centrosomes was shown to promote dendrite growth and elaboration. The cen-
trosomal localization of Cdc20 was shown to be critical for this activity through an
elegant series of experiments involving rescue from RNAi depletion using a Cdc20
mutant lacking residues responsible for its accumulation at centrosomes that was
artificially targeted to centrosomes (Kim et al.
2009
) via the pericentrin and
AKAP450 centrosome targeting (PACT) domain (Gillingham and Munro
2000
).
8.8 SCF and the Centrosome
Skp1 (Freed et al.
1999
; Gstaiger et al.
1999
) and Cul1 (Freed et al.
1999
) have
been shown to localize to the centrosome in mammalian cells, and the F-box
protein bTrCP was found in the centrosomal proteome (Andersen et al.
2003
).
Moreover, several candidate F-box proteins were also found at centrosomes,
suggesting the existence of multiple centrosomal SCF complexes (Freed et al.
1999
). Interestingly, centrosomal targeting of Skp1 and Cul1 may be regulated by
modification with NEDD8, a ubiquitin-like protein (Freed et al.
1999
). Antibodies
against Skp1 or Cul1 block centriole separation in an in vitro assay using X. laevis
egg extracts, and proteasome inhibitors block centriole separation and centrosome
duplication in X. laevis embryos, suggesting that SCF function is directly relevant
to centrosome assembly (Freed et al.
1999
). Cyclin E is critical for the initiation of
centrosome duplication, and in addition to resulting in polyploidy and centrosome
overduplication in mice, Skp2 deletion leads to the elevation of cyclin E and the
Cdk inhibitor p27Kip1 (Nakayama et al.
2000
). Similarly, in fruit flies, mutations
in genes encoding either the F-box protein Slimb (the fly ortholog of bTrCP)
(Wojcik et al.
2000
) or SkpA (Murphy
2003
) lead to centrosome overproduction.
Because cyclin E has many functions in centrosome duplication, and has been
shown to be degraded in an SCF-dependent manner (Dealy et al.
1999
; Wang et al.
1999
), failure to degrade cyclin E might explain excess centrosomes in mice
lacking Skp2 and flies lacking Slimb or SkpA. In contrast, excess p27 would be
predicted to suppress centrosome duplication by inhibiting Cdk2, suggesting that a
Cdk inhibitor might be the relevant SCF substrate in the X. laevis in vitro centriole
assembly assay (Freed et al.
1999
). However, excess cyclin E could not rescue the
centriole duplication defect caused by the inhibitory Skp1 or Cul1 antibodies
(Freed et al.
1999
), and excess centrosomes were still observed in skpa cyc E
double mutant flies (Murphy
2003
), suggesting that additional centrosomal SCF
substrates whose degradation controls centrosome duplication remained to be
identified.
While it seems likely that excess cyclin E contributes to centrosome defects in
SCF-deficient cells, it was recently shown that Plk4 is regulated by SCF-dependent
degradation. Overexpression of Plk4 leads to centriole overproduction (Kleylein-
Sohn et al.
2007
; Habedanck et al.
2005
), demonstrating that the control of cen-
trosome duplication requires tight regulation of the levels of Plk4. The Cul1
subunit of the SCF localizes to the maternal centriole, where it antagonizes
