New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 241

by the inhibition of Aurora B ( Ditchfield et al., 2003; Lampson and Kapoor,
2005 ), suggesting that BubR1 may stabilize K-MT attachments partly by
counteracting (directly or indirectly) the activity of Aurora B at the kinet-
ochores (see Section 4.3.1 ).

4.3.1 Phosphorylation of BubR1 and K MT attachments

BubR1 is known to be hyperphosphorylated during mitosis ( Table 6.1 ), and
several studies have shown that BubR1 is a substrate of Plk1 in vitro and
in vivo in human cells ( Elowe et al., 2007; Matsumura et al., 2007;
Suijkerbuijk et al., 2012b ) and in Xenopus egg extracts ( Wong and Fang,
2007 ). Matsumura et al. (2007) showed that BubR1 is phosphorylated
in vitro by Plk1 on two sites T792 and T1008. A BubR1 phosphomimetic
mutant (T792E, T1008E) rescued the chromosome alignment defects of
cells depleted of both Plk1 and BubR1, whereas the nonphosphorylatable
T792A and T1008A mutants did not. Another study ( Elowe et al., 2007 )
reported that BubR1 was phosphorylated by Plk1 on residue S676 and that
this phosphorylation was dependent on a priming phosphorylation by Cdk1
on residue T620. BubR1 T620A mutant cells also displayed chromosome
attachment defects. Huang et al. (2008) identified four additional phosphor-
ylated sites (S435, S543, S670, and S1043), and cells expressing BubR1
S670A mutant again displayed attachment defects. They reported that these
four sites were not phosphorylated by Plk1, and suggested that Mps1 may be
the kinase. Elowe et al. (2010) subsequently identified two more sites (S574
and S720). When all five sites (S543, S574, S670, S720, and S1043) were
mutated to alanine, the resulting BubR1 mutant was profoundly defective
in chromosome attachment to the spindle. They also reported that sites
S543, S670, and S1043 were direct targets of Cdk1 rather than Mps1.
Importantly, none of these studies revealed any role for these phosphoryla-
tion events on the SAC function of BubR1.
The degree of phosphorylation of some of these sites appears to corre-
late with the status of kinetochore attachment. All these sites were
phosphorylated on unattached kinetochores. Both S670 ( Huang et al.,
2008 ) and S676 ( Elowe et al., 2007 ) were dephosphorylated on bioriented
kinetochores, but upon treatment with taxol (which relaxes kinetochore
tension attached kinetochores) phosphorylation levels on S676 rose again,
whereas S670 phosphorylation remained low. Thus, BubR1 may be
differentially phosphorylated depending on whether the kinetochores
are under tension or not.

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