Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 98

even though both bound equivalent amounts of CPC. These results indicate
that concentration and activation of Aurora B is required for full activity.
However, the downstream activities involved in MT nucleation are

unknown.
Whatever the mechanism for kinetochore-mediated nucleation of MTs,
it is clear that establishing close contact of MTs with kinetochores early dur-
ing spindle assembly likely improves the kinetics of assembly by increasing
the effective size of the kinetochore target during search-and-capture. Fur-
thermore, kinetochore-mediated nucleation may contribute directly to
k-fiber formation.

3.4. Microtubule-branching nucleation
The idea of existing MTs acting as templates for new MT nucleation and
growth stems from observations in Drosophila S2 cells and Xenopus extracts
showing that MT plus-ends, as visualized by fluorescently labeled EB1,
appear throughout the body of the spindle ( Mahoney et al., 2006;
Tirnauer et al., 2004 ), indicating nucleation at points distinct from centro-
somes or chromosomes. Furthermore, plant cortical arrays clearly generate
branched MT arrays, though the molecular mechanism of this remains
unknown. In animals, this MT-nucleating activity is mediated by an
eight-member complex called Augmin originally identified in Drosophila ,
and its depletion resulted in severe loss of spindle MTs ( Goshima et al.,
2008; Uehara et al., 2009 ). An interaction between the Augmin complex
and g -TURC was shown to be essential for its MT nucleation in human
cells, and a similar interaction is predicted for Drosophila ( Uehara et al.,
2009 ). This suggests a model in which Augmin binds to preexisting MTs
and recruits g -TURC to nucleate a new MT ( Fig. 3.4 ).
The nucleation of MTs by Augmin could function to promote spindle
assembly in two ways. First, nucleation of MTs off preexisting MTs could
quickly bolster MT mass and increase the kinetics of spindle formation,
which has experimental support in Xenopus egg extracts ( Petry et al.,
2011 ). Second, MT-branching nucleation could assist in establishing bipo-
larity by generating antiparallel overlap through nucleation of MTs along the
same axis as the existing spindle array, as polarity of newly branched MTs is
maintained ( Petry et al., 2013 ; Fig. 3.5 A). Further cross-linking of these
MTs would then provide structural stability to the spindle.
MT-branching nucleation has interesting implications for the architec-
ture of the spindle. Due to the low angle of branching and the maintenance
of MT polarity, nucleation occurring anywhere along an existingMTwould