New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 226

3.2. Kinase domain
The kinase domain and its function in mitosis is one of the most controver-
sial topics of BubR1-ology. The fact that vertebrate BubR1 has such a
domain whereas its yeast orthologue Mad3 does not suggests immediately
that it may not be critical to the SAC function. Early experiments testing
this hypothesis generated conflicting results ( Harris et al., 2005; Kops
et al., 2004 ). It is now clear, however, that the kinase activity plays little
or no part in BubR1's role as a regulator of the SAC. Later, the kinase activ-
ity was implicated in the establishment of K-MT attachments ( Elowe et al.,
2010; Guo et al., 2012; Huang et al., 2008; Rahmani et al., 2009 ). Most
recently, Suijkerbuijk et al. (2012a) have argued that vertebrate (but not Dro-
sophila ) BubR1 kinase domain may not even be functional, having acquired
a number of nonconserved amino acid substitutions at key positions in the
catalytic domain. We will come back to the kinase activity of BubR1 and its
possible functions later (see Sections 4.2 and 4.3 ).
3.3. KARD domain
The kinetochore attachment regulatory domain (KARD) was defined in
two recent studies ( Kruse et al., 2013; Suijkerbuijk et al., 2012b ). It covers
the residues 665-682, just proximal to the kinase domain, and is subject to a
number of phosphorylation. In its phosphorylated form, it binds the protein
phosphatase PP2A-B56 a , an activity of major importance for promoting
proper K-MT attachments.
3.4. Posttranslational modifications and interacting proteins
BubR1 is highly phosphorylated in mitosis ( Chan et al., 1999; Taylor et al.,
2001 ). Many sites have been identified (summarized in Table 6.1 ). Cdk1 and
Plk1 appear to be the major enzymes phosphorylating BubR1, but Mps1
and Aurora B may contribute as well. BubR1 may also autophosphorylate
( Guo et al., 2012; Mao et al., 2005 ). Many of these phosphorylations are
sensitive to MT attachment or kinetochore tension, and some have been
directly implicated in BubR1's contribution to promoting K-MT attach-
ments (see Sections 4.3.1 and 4.3.2 ).
BubR1 is also acetylated on Lysine 250 specifically at unattached kinet-
ochores in prometaphase. This acetylation appears to be important to protect
BubR1 from degradation. The same L250 has also been found to be
sumoylated (see Section 4.5 ). A list of proteins reported to directly interact
with BubR1 is provided in Table 6.2 . Some are well-established partners,
while others have not been confirmed.