Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 116

sliding motor. This is consistent with measurements of MT flux by speckle
microscopy ( Yang et al., 2008 ) but leaves open the role of MT
depolymerases of the kinesin-13 family in driving flux that has been indi-
cated in human cell lines ( Ganem et al., 2005 ). In the model by Loughlin
et al., the transport of MTs is homogeneous throughout the spindle, and
MTs depolymerize as they reach the pole through the action of a specific
MT-destabilizing activity, replicating the proposed involvement of
kinesin-13's in MT flux but not recapitulating the slowed flux near the
poles. The slightly different clustering and nucleation mechanisms could
account for the different requirements for spindle pole formation and there-
fore spindle length control. Ultimately, the models are not mutually exclu-
sive and both rely on modeling of a minus-end-directed motor based on
dynein. It is likely that both mechanisms work to drive spindle architecture
into a robust regime capable of maintaining steady-state structure and
generating precise function, despite fluctuating conditions.
5.5. Future prospects for determining spindle architecture
With the development of super-resolution light microscopy and additional
technical advances, it may soon become feasible to distinguish individual
MTs and small bundles within the densely packed spindle. Even if MT orga-
nization cannot be resolved precisely, other spindle components should be
amenable to super-resolution techniques. Analysis of components that are
crucial to spindle MT organization will make it possible to infer important
aspects of spindle architecture. For instance, several proteins have recently
been identified that bind the minus-ends of MTs in addition to the well-
established g -TuRC ( Goodwin and Vale, 2010; Meunier and Vernos,
2011 ). Super-resolution investigations with these proteins will test current
predictions of the distribution of MT minus-ends in the spindle, which have
not yet been observed directly. Quantification of the number of MTs in the
kinetochore fiber has already been estimated through electron microscopy
data, but the dynamic process of partitioning k-fiber MTs from the rest of
the spindle MTs remains unknown. Finally, measurements of strength and
stability of the spindle—the functional consequences of different architec-
tural arrangements of MTs—are only just beginning and should provide
substantial insight into the mechanisms of chromosome segregation.
Other novel methods, especially those based on quantitative analyses,
may provide unique insights into spindle architecture, much like the laser
ablation studies that were used to probe MT length and organization.