Biology Reference
In-Depth Information
association with first, Cyclin E, and later, Cyclin A. Cyclins E and A are subjected
to control by cell cycle-dependent degradation: Cyclin E by the SCF, in con-
junction with the F-box protein, Fbxw7, and Cyclin A by the APC/C, in con-
junction with first Cdc20, and later Cdh1. Cdk2 activity is also regulated by the
inhibitor, p27, though this is degraded by the SCF, in conjunction with Skp2, at the
onset of S-phase. Hence, cell cycle control of Cdk2 is mediated in large part by
protein degradation and provides a neat mechanism to couple the onset of DNA
replication and centriole duplication. However, while Cdk2 substrates that regulate
DNA replication have been well characterized, validated substrates that promote,
or possibly even inhibit, centrosome duplication remain frustratingly elusive.
Despite the uncertainty over how Cdk2 promotes centriole duplication, the
molecular events of centriole assembly are now beginning to emerge (Bettencourt-
Dias and Glover
2007
; Loncarek and Khodjakov
2009
; Strnad and Gonczy
2008
).
In brief, a core set of five proteins that are sufficient for centriole assembly was first
identified in C. elegans; these include four structural proteins, SPD-2, SAS-4,
SAS-5, and SAS-6, and a protein kinase, ZYG-1 (Pelletier et al.
2006
; Leidel et al.
2005
; Delattre et al.
2006
; Delattre et al.
2004
; Dammermann et al.
2004
). SPD-2
and ZYG-1 act upstream of SAS-5 and SAS-6 to promote the assembly of a central
tube. Singlet microtubules are then built on this tube under the action of SAS-4 to
generate the simple centrioles typical of this organism. In higher eukaryotes,
additional components are involved; however, these same five proteins have been
highly conserved and also play crucial roles in the biogenesis of more complex
centrioles. So, for example, SAS-6, together with the Bld10p/Cep135 protein,
forms the cartwheel structure that assembles at the proximal end of a growing
centriole and defines its 9-fold symmetry (Hiraki et al.
2007
; Kitagawa et al.
2011
;
Nakazawa et al.
2007
; van Breugel et al.
2011
), SAS-5/Ana-2/STIL proteins are
required for initiation of procentriole assembly (Stevens et al.
2010
; Tang et al.
2011
), while SAS-4/CPAP contributes to centriole elongation(Kohlmaier et al.
2009
; Schmidt et al.
2009
; Tang et al.
2009
). Importantly, it is becoming clear that
almost all these proteins are regulated at the level of protein degradation to ensure
numerical and spatial control of centriole biogenesis.
Crucially, it would appear that many of these centriole assembly proteins are
rate limiting with their expression level tightly regulated by protein degradation
(Fig.
9.2
). Perhaps the best studied of these so far is the polo-like kinase, Plk4, or
SAK, which are the human and Drosophila orthologues of the C. elegans ZYG-1
kinase, respectively. Typical of these rate limiting proteins, Plk4 localizes to
centrioles and experimental manipulation of Plk4 expression alters centriole
numbers. So, downregulation of Plk4 leads to loss of centrioles over successive
cell divisions, while overexpression leads to the formation of multiple procentri-
oles that form in a rosette around a single parent centriole (Bettencourt-Dias et al.
2005
; Kleylein-Sohn et al.
2007
; Habedanck et al.
2005
). Plk4 levels also regulate
centriole formation via the de novo assembly pathway, which occurs in the
absence of a pre-existing centriole (Eckerdt et al.
2011
; Peel et al.
2007
; Rodri-
gues-Martins et al.
2007
).
