Chromosome segregation errors: a double-edged sword - TI Pharma

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Besides Borealin and Knl1 116,120-122 , other downstream targets of Mps1 have been described as well,
but many remain controversial: some substrates have been implicated in the role of Mps1 in spindle
pole duplication (reviewed in 159 ), others, such as Ndc80, Mad1 or CENP-E have, until now, been
identified as Mps1 substrates in only one model organism 141,160,161 . Mps1 has also been implicated in
activating DNA damage signaling through phosphorylation of Chk2 162 , BLM 163 and p53 164 . In addition,
DNA damage-induced expression of p53 was also shown to suppress expression of various mitotic
checkpoint proteins, including Mps1 165 . This is in contrast to DNA damage-induced Chk2 activation,
which resulted in stabilization of Mps1 protein 166 . Although a link could exist between Mps1 and
the DNA damage checkpoint response, clear significance and direct mechanisms remain unknown.
Contrary to its downstream substrates, auto-phosphorylation of Mps1 has been described in multiple
organisms on conserved residues (reviewed in 167 ). The catalytic activity of Mps1 is restricted to its
C-terminus 123,168,169 and although in vitro reactivity towards tyrosine residues has been observed
168,170 170-175 , only threonine or serine residues were found to be phosphorylated by Mps1 in vivo .
Threonine676 and Threonine686, two of the identified auto-phosphorylation sites on hMps1, reside
in the catalytic C-terminus 170-172 and were found to play an important role in acquiring full kinase
activity 170-172 . Substitution of Thr686 to alanine renders Mps1 almost completely kinase dead 170-
172 170 172 , whereas substituting Thr676 to an alanine resulted in a 1.4- to 5-fold decrease in hMps1
kinase activity. Interestingly, expressing the T676A mutant of Mps1 in human cells affected mitotic
checkpoint function and resulted in mild defects in mitotic progression 171,172 . Since only structures
of inactive Mps1 protein have been resolved 123,168,169,176 , it remains difficult to determine how these
phosphorylations precisely affect its catalytic activity in vivo.
2.5 Cytokinesis: full separation of daughter cells
When the duplicated chromatin has moved to opposite sides of the mother cell, cytokinesis will finish
cell division by partitioning the remaining cellular material into two daughter cells. Cytokinesis in
animal cells is initiated during anaphase, when the mitotic spindle reorganizes into the central spindle,
a dense array of overlapping microtubules in between the two DNA packs. The central spindle forms
as a consequence of the decline in Cyclin B1-Cdk1 activity, which leads to stabilization of MTs and
association of several microtubule-binding proteins (reviewed in 177 ). MTs of the central spindle are
being bundled by proteins such as PRC1 178 and the central spindlin complex, consisting of MKLP-1 and
the Rho-family GTPase-activating protein Cyk-4 179,180 . Cyk-4 recruits the RhoGEF Ect2 to the central
spindle 181 , an event that is regulated through phosphorylation of Cyk-4 by polo-like kinase 1 (Plk1)
182 . PRC1 is thought to be the main docking factor for Plk1 on the spindle midzone 183 , but additional
binding sites have been reported as well 184 . Another important complex involved in central spindle
formation is the aforementioned chromosomal passenger complex (CPC). CPC members relocalize
from centromeres to the central spindle in anaphase, which requires the two kinesins MKLP-1 and
MKLP-2 in mammalian cells 185,186 . The CPC functions in central spindle formation by phosphorylating
PRC1 187 and MKLP-1 16 , but might also directly mediate MT-bundling by functioning as a structural
component 177 . Central spindle assembly provides cues to initiate cleavage furrow formation at the
plasma membrane by promoting concentration and activation of the small GTPase RhoA at the
equatorial cortex 181,188,189 . The RhoA pathway induces cleavage furrow formation by stimulating the
assembly of the actomyosin ring through activation of ROCK kinase 190 and nucleation of actin filaments
191-193 . Subsequent contraction of the actomyosin ring induces ingression of the plasma membrane
and actual division of the cytoplasm. Although extremely important in cell division, the actual
mechanism for force-generation within this contractile ring is not well understood (reviewed in 194 ).
Cytokinesis is completed when abscission occurs, the process in which the two plasma membranes are
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