Regulation of Centrosomes by Cyclin-Dependent Kinases - The Centrosome - page 188

stability of Mps1 protein (Fisk and Winey 2001 ). A deletion mutant of Mps1
(Mps1D12/13, deletion of exons 12 and 13) was found to remain at the centrosome
after CDK2 inhibition, suggesting that CDK2 phosphorylation of Mps1 within the
region coded by exons 12-13 is responsible for regulation of its protein stability
(Kasbek et al. 2007 ). Three phosphorylation sites were identified within this region
(S436, T453, T468), which are regulated by the activities of both CDK2-cyclin E
and CDK2-cyclin A kinases (Kasbek et al. 2007 ). A non-phosphorylable mutant of
Mps1 (Mps1T468A) resulted in a loss in accumulation of Mps1 at the centrosome,
suggesting that CDK2-cyclin A mediated phosphorylation of Mps1 at the cen-
trosome protects the protein from proteasome-mediated degradation (Kasbek et al.
2007 ) (Fig. 11.3 ). Preventing the degradation of Mps1 at the centrosome, in the
absence of CDK2 activity, was found to be sufficient to cause centrosome re-
replication in S phase arrested cells. Thus, phosphorylation of Mps1 by CDK2
controls the level of Mps1 protein at the centrosome and restricts the number of
centrosome replication cycles during each cell division cycle (Kasbek et al. 2007 ).
11.2.2.3 CP110
CP110 was identified as a CDK substrate during a screen of a human cDNA expression
library with a dominant negative form of CDK2-cyclin E (Chen et al. 2002 ). It was
found to be phosphorylated by CDK2-cyclin E, CDK2-cyclin A and CDK1-cyclin B in
vitro. CP110 was subsequently found to be a centrosomal protein, which specifically
co-localised with centrin to the centrioles (Chen et al. 2002 ). Similarly to NPM and
Mps1, CP110 depletion was found to suppress centrosome re-replication in S phase
arrested U2OS cells. Expression of a CP110 phosphorylation site mutant caused
premature centrosome separation, which resulted in unscheduled mitotic entry and
subsequent accumulation of polyploid cells (Chen et al. 2002 ). Phosphorylation of
CP110 by CDK2 therefore suppresses premature centrosome separation, thereby
regulating the timing of mitotic entry (Chen et al. 2002 )(Fig. 11.3 ).
CP110 has also been shown to play a number of roles at the centrosome which
are independent of phosphorylation by CDK-cyclins. CP110 contributes to the
regulation of centriole elongation, by localising to the distal end of both the mother
and daughter centrioles and functioning as a cap to limit centriole length (Schmidt
et al. 2009 ). CP110 also plays a role in cytokinesis, through interactions with the
proteins centrin 2 and calmodulin (Tsang et al. 2006 ).
11.2.3 CDK1-Cyclin B Control of Spindle Assembly and Mitosis
The G2-M transition is regulated by CDK1 in complex with cyclin B (Fig. 11.1 ).
CDK1-cyclin B becomes activated in prophase by the CDC25 phosphatases (Gavet
and Pines 2010 ), first at the centrosome and then in the nucleus (Jackman et al.
2003 ). Once activated, CDK1-cyclin B phosphorylates many mitotic substrates,