New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 242

A significant step in our understanding of the role at least some of these
myriads of phosphorylations has been taken by two recent studies defining a
domain of BubR1 that interacts with the phosphatase PP2A-B56 a ( Kruse
et al., 2013; Suijkerbuijk et al., 2012b ) and is responsible for the recruitment
of a subset of PP2A-B56 a to kinetochores. PP2A-B56 a had previously been
implicated in the promotion of stable K-MT attachment by opposing the
destabilizing phosphorylations of Aurora B and Plk1 at kinetochores
( Foley et al., 2011 ). Depleting PP2A-B56 a by SiRNA profoundly reduced
K-MT attachments, but they could be substantially restored in these cells by
inhibition of Aurora B or Plk1.
The two groups identified a highly phosphorylated conserved domain of
BubR1 (residues 665-682), referred to as KARD ( Suijkerbuijk et al.,
2012b ), whose integrity is necessary for the ability of BubR1 to promote
stable K-MT attachments. Within the KARD domain, there are at least
three phosphorylation sites: two (S670 and S676) described above ( Elowe
et al., 2007, 2010; Huang et al., 2008 ) plus a third site (T680), also phosphor-
ylated by Plk1 on tensionless kinetochores, as is S676 ( Suijkerbuijk et al.,
2012b ). Expressing the triple S670A, S676A, and T680A BubR1 mutant
in HeLa cells abolished chromosome alignment, but chemically inhibiting
Aurora B activity largely restored proper chromosome alignment in these
cells. Phosphorylation of the KARD domain promoted physical interaction
between BubR1 and the B56 a regulatory subunit of PP2A ( Kruse et al.,
2013; Suijkerbuijk et al., 2012b ). Kruse et al. showed moreover that the
binding affinity of the KARD domain for B56 a increased as a function of
the number of phosphorylations. Remarkably, when a phosphomimetic
(S670D, S676D, T680D) KARD domain fragment was constitutively teth-
ered to kinetochores by fusing it to the KMN subunit Mis12, the chromo-
some alignment defects observed in BubR1-depleted cells were fully
rescued. The corresponding phospho-dead KARD fragment (S670A,

S676A, T680A) had no effect ( Suijkerbuijk et al., 2012b ).
Thus, BubR1, as the target of Plk1 and possibly other kinases, may inte-
grate several upstream signals to regulate the degree of phosphatase activity at
kinetochores as a function of the state of MT attachment at each kineto-
chore, in order to best promote the establishment of K-MT attachments.
The studies by Suijkerbuijk et al. (2012b) and Kruse et al. (2013) go a long
way to explaining how BubR1 and Plk1 may cooperate to establish and
maintain correct K-MT attachments. They also help explain the original
observation by Lampson and Kapoor (2005) that BubR1 activity opposes
Aurora B activity at kinetochores.