New Insights into the Role of BubR1 in Mitosis and Beyond - International Review of Cell and Molecular Biology - page 236

Aurora B inhibition on BubR1 kinetochore recruitment may be mediated
through Mps1.
To summarize, BubR1 recruitment to kinetochores is most likely
dependent on multiple contacts with KNL1: via its own TPR domain,
via its constitutive partner Bub3, and via its weak interaction with Bub1
(which in turn seems to be recruited to KNL1 as a complex with Bub3,
binding to phosphorylated MELT motifs). Although there is no data yet
to support it, it seems reasonable to imagine that BubR1/Bub3 may also
contact KNL1 via the MELT motifs. Of course, interactions with other
kinetochore proteins are not excluded. Multiple weak contacts may be
the best way to maintain the dynamic nature of BubR1 kinetochore levels
so that it is responsive to changes in the status of K-MT attachment.
4.2. BubR1 and assembly of the MCC
The goal of the SAC is to inhibit the ubiquitination of Cyclin B and Securin
by the APC/C cdc20 . This inhibition is mediated by the MCC, composed of
Cdc20 bound to Mad2, BubR1, and Bub3 ( Sudakin et al., 2001 ). While
some MCC may be generated in the absence of kinetochores ( Kulukian
et al., 2009 ) or in the cytoplasm ( Malureanu et al., 2009 ) (and indeed
MCC has been detected in interphase cell extracts; Sudakin et al., 2001 ),
the overwhelming preponderance of evidence argues that it is the catalytic
effect of unattached kinetochores that generates the levels of MCC required
to delay anaphase onset ( Lara-Gonzalez et al., 2012 ).
The first step in the generation of the MCC occurs through the confor-
mational activation of Mad2 that occurs at unattached kinetochores ( De
Antoni et al., 2005; Mapelli et al., 2006; Yang et al. 2008 ). Pioneering struc-
tural studies demonstrated that the kinetochore-associated Mad1-bound
form of Mad2 (called Closed or C) transiently dimerizes with a second
Mad2 molecule, normally in the inactive (Open or O) conformation in
the cytoplasm, and in doing so induces a conformational change so that it
now adopts the C form, allowing it to bind Cdc20 ( Mapelli et al., 2006;
Yang et al., 2008 ).
In the next step, BubR1-Bub3 binds the Mad2-Cdc20 complex to form
the MCC. In mammalian cells, the MCC as isolated biochemically usually
contains substoichiometric amounts of Mad2. Nevertheless in the absence of
Mad2, Cdc20 does not bind to BubR1-Bub3 in appreciable amounts,
suggesting again that Mad2 may act catalytically to promote Cdc20 binding
to BubR1 ( Kulukian et al., 2009; Nilsson et al., 2008 ). Presumably, the