New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 244

that the BubR1 kinase activity is needed in Drosophila for correct
K-MT attachments.
4.3.3 BubR1, CENP-E, and autophosphorylation
Another study reporting a kinase activity for BubR1 found that it phos-
phorylated itself on residue T608 at unattached kinetochores in human
cells and that this autophosphorylation was dependent on the presence
of CENP-E ( Guo et al., 2012 ). CENP-E is a kinesin-like protein required
for efficient capture and stabilization of microtubules by the kinetochores;
depletion of CENP-E reduces the number of bound microtubules, slows
chromosome congression, and consequently prolongs mitosis ( Abrieu
et al., 2000; Weaver et al., 2003 ). Using purified recombinant
components, CENP-E was previouslyshowntobindtoBubR1andstim-
ulate its kinase activity in vitro ( Mao et al., 2005 ). CENP-E-dependent
BubR1 autophosphorylation at unattached kinetochores was reported
to be important for a full-strength functional SAC to prevent single
chromosome loss ( Guoetal.,2012 ). Moreover, in cells expressing a non-
phosphorylatable T608A BubR1 mutant or in cells depleted of CENP-E,
Guo et al. (2012) observed misaligned chromosomes and a reduction of
Aurora B-mediated Ndc80 phosphorylation at kinetochores. Finally, the
incidence of polar chromosomes observed in CENP-E-depleted cells
was reduced by expressing a T608E phosphomimetic BubR1 mutant,
thereby suggesting that BubR1 activity and its CENP-E dependent
autophosphorylation are important for accurate chromosome segregation.
4.4. The mitotic timer
BubR1 is involved in three functions during mitosis: the SAC, the spindle
function, and the timing function. During cell division, the timing of mitotic
progression needs to be regulated in order to assure accurate chromosome
segregation. Meraldi et al. (2004) defined basal mitotic timing as the mini-
mum time interval that elapses between NEB and anaphase onset when
there are no unattached kinetochores (i.e., without any additional SAC-
induced delay). Meraldi et al. found that Mad2 and BubR1, but not other
tested SAC proteins, were required to maintain a normal minimum mitotic
duration in HeLa cells. Later, Mps1 was also shown to be required for the
timer ( Maciejowski et al., 2010 ). Depleting any one of these proteins
shortens this minimum interval. The timer is probably an entity similar to
or identical to the MCC, possibly corresponding to the MCC generated
during interphase, and present at the start of mitosis. This timer presumably