New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 243

4.3.2 Role of the kinase activity in K MT attachments

Numerous studies have reported a role for the kinase activity of BubR1 in
promoting proper K-MT attachment. Bearing in mind the possibility
raised by Suijkerbuijk et al. (2012a) that BubR1 is a pseudokinase, we nev-
ertheless review the evidence to the contrary. Mutation of BubR1 residues
T792 and T1008 (putative targets of Plk1) to the phosphomimetic T792E
and T1008E mutants not only exhibited stronger autophosphorylation
activity in vitro than wild type but also rescued the attachment defects
observed in cells depleted of BubR1 ( Matsumura et al., 2007 ). However,
they saw no rescue when the phosphomimetic mutations were combined
with a KD mutation. These data suggested that Plk1 phosphorylation
enhanced BubR1 kinase activity and that this kinase activity is necessary
to promote kinetochore attachments.
The MT plus-end associated (

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TIP) ( Green et al., 2005 )EB1andthe
tumor suppressor APC (adenomatous polyposis coli) form a complex that
modulates MT plus-end dynamics. They are found on attached kineto-
chores ( Green et al., 2005 ), and depletion of either one leads to chromo-
some missegregation in mammalian cells ( Draviam et al., 2006; Green
et al., 2005 ) and to chromosome alignment defects in Xenopus egg extracts
( Zhang et al., 2007 ). Xenopus BubR1 was reported to directly interact
with the APC/EB1 complex, and purified recombinant BubR1 was
shown to phosphorylate the APC subunit in vitro .Moreover,thisphos-
phorylation was required for the recruitment of APC to kinetochores
( Zhang et al., 2007 ).
Although these data suggest that BubR1 kinase activity contributes to
the development of stable K-MT attachments, there are also examples in
the literature where a BubR1 KD mutant appeared to have full or near full
kinetochore attachment activity ( Elowe et al., 2007; Malureanu et al., 2009 ).
These different results may be due to differences in the degree of endoge-
nous BubR1 depletion, differences in instability of the assayed KD mutant,
or perhaps differences in expression levels of the mutant transgenes.
In Drosophila , however, the kinase appears to be functional, and the
genetic tools available in this organism allowed ( Rahmanietal.,2009 )
to assay the mitotic phenotype of a bubR1-KD mutant allele (K1204A)
expressed under its own promoter in bubR1 1 null mutant flies in vivo .
The bubR1-KD mutants proved to be viable and fertile, but in mitosis
the cells were slow to establish bipolar spindles, K-fibers tended to be thin-
ner, chromosome congression was often delayed, and alignment on the
metaphase plate was unstable, all of which are consistent with the notion

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