Biology Reference
In-Depth Information
NPM localises between the centriole pair of the mother centrosome in G1 phase
to negatively regulate centrosome replication (Okuda et al.
2000
; Grisendi et al.
2005
). In late G1 phase, NPM is phosphorylated by CDK2-cyclin E on T199. This
triggers its dissociation from the centrosome and its relocalisation to the nucleus
and the start of centrosome replication (Tokuyama et al.
2001
) (Fig.
11.3
). NPM
phosphorylation by CDK2-cyclin E therefore functions as a licensing factor for
centrosome replication (Okuda et al.
2000
). CDK2-cyclin A can also phosphory-
late NPM on T199 in vitro, suggesting that NPM phosphorylation continues
through S phase and may ensure that centrosome replication is not initiated a
second time once cyclin E has been degraded (Tokuyama et al.
2001
).
CDK1-cyclin B was shown to phosphorylate NPM on two alternative sites
(T234 and T237) in vitro and it is possible that phosphorylation of these are
involved in the recruitment of NPM to the centrosomes during mitosis, in prep-
aration
for
the
centrosome
replication
during
the
next
cell
division
cycle
(Tokuyama et al.
2001
).
11.2.2.2 Mps1
The Mps protein kinases were first identified in yeast as temperature-sensitive
mutants that were defective in the replication of the spindle pole, thus resulting in the
formation of monopolar spindles during mitosis (Winey et al.
1991
). Rather than
arresting in metaphase, these mutants were found to continue cycling and segregate
their DNA inappropriately, thus identifying a second role for Mps in the mitotic
spindle assembly checkpoint (SAC) (Weiss and Winey
1996
; Abrieu et al.
2001
).
In addition to its mitotic roles, mammalian Mps1 was subsequently found to
play a role in centrosome replication (Fisk and Winey
2001
). Mouse Mps1
(mMps1) was found to localise to the centrosome in interphase as well as during
mitosis. Overexpression of mMps1 caused centrosome re-replication in S phase
arrested NIH3T3 cells and overexpression of a kinase-dead form (mMps1-KD)
blocked centrosome replication (Fisk and Winey
2001
). Initial functional analyses
of human Mps1 (hMps1) did not support a direct role for this kinase in centrosome
replication (Stucke et al.
2002
). However, subsequent studies in human cells
revealed that overexpression of hMps1 in S phase-arrested U2OS cells results in
centrosome re-replication, while overexpression of a hMps1-KD blocked centro-
some replication in a number of human cell lines (Fisk et al.
2003
; Kanai et al.
2007
). Recently, hMps1 overexpression was found to promote centrosome repli-
cation through phosphorylation of the structural centriole component centrin 2
(Yang et al.
2010
). Mps1 phosphorylation of centrin 2 on three threonine residues
(T45, T47, T118) stimulates the formation of new centrioles (Yang et al.
2010
).
Mps1 itself is regulated by phosphorylation. Inhibition of CDK2 activity in S
phase by treatment with chemical inhibitors of CDK2, resulted in loss of the
centrosomal localisation of Mps1 and blocked centrosome re-replication induced
by S phase arrest (Fisk and Winey
2001
). Further examination of the role of CDK2
in Mps1 regulation at the centrosome revealed that CDK2 functions to promote the
