New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 240

authors tested a mutation at BubR1 residue D882 (which is also a conserved,
kinase-critical residue) to glutamine, and a truncation mutant lacking the
entire kinase domain. Neither mutant affected the protein's overall stability,
and both mutants still provided cells with normal SAC function and K-MT
interactions. Finally, the authors failed to detect any catalytic activity asso-
ciated with the kinase domain expressed in vitro . Based on their combined
phylogenetic biological analysis, Suijkerbuijk et al. postulated that BubR1
catalytic domain retains no activity ( Suijkerbuijk et al., 2012a ).
The caveat of this study is that all their BubR1 kinase assays were carried
out only with the catalytic domain of BubR1 (residues 721-1044 of human
BubR1) expressed and purified from bacteria. Therefore, it cannot be
entirely excluded that the full-length BubR1 may be necessary for detectable
catalytic activity, or that specific cofactors (such as CenpE) or posttransla-
tional modifications are required in order to exhibit kinase activity.

4.3. BubR1 and K MT attachment

Establishing stable amphitelic K-MT attachment is critical for metaphase
chromosome alignment. During mitosis, the kinetochores initially interact
with the MTs of captured by the mitotic spindle through a lateral or side-on
microtubule attachment. They then move toward the pole by Dynein-
mediated transport ( Rieder and Alexander, 1990 ). During this poleward
transport, kinetochores switch from lateral to the load-bearing end-on
attachment (via binding to the Ndc80 complex) necessary for chromosome
congression to the metaphase plate and later segregation to the poles at ana-
phase ( Tanaka and Desai, 2008 ). While various events that regulate K-MT
attachment and congression are still not fully understood, several lines of
evidence described below demonstrate that BubR1 is

-

important for

this process.
Ditchfield et al. (2003) and then Lampson and Kapoor (2005) established
that BubR1 has a role in promoting K-MT attachment and chromosome
alignment. BubR1-depleted cells treated for 2 h with the proteasome inhib-
itor MG132 (which blocks anaphase onset even in the absence of a func-
tional SAC) mostly accumulated in a prometaphase-like state with many
misaligned kinetochores. In contrast, Mad2-depleted cells so treated were
mostly found in a metaphase configuration with properly aligned chromo-
somes. These results suggested that BubR1 was promoting proper chromo-
some attachment, in addition to its role in the SAC. Moreover, the
attachment defects seen in BubR1-depleted cells were partially rescued