Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 111

activity was present in smaller spindles, but in this case, differences in a
depolymerizing kinesin, Kif2a, were responsible. Kif2a is inhibited by the
import receptor Importin a , whose cytoplasmic levels decrease during
development as more of it partitions to a membrane fraction ( Wilbur and
Heald, 2013 ; Fig. 3.7 B). This allows cells to autonomously coordinate
spindle size independent of developmental stage, a need that arises due to
asymmetric cell divisions that occur in the embryo.
Thus, in both interspecies and developmental scaling systems, spindle
size differences are mediated, at least in part, by changes in MT stability.
Interestingly, size changes are accompanied by architectural differences.
In response to Katanin inhibition, the kinetochore fibers of X. tropicalis spin-
dles protrude through the spindle poles, indicating that Katanin not only
contributes to spindle length differences but is also required to coordinate
k-fiber and spindle MT stability in X. tropicalis ( Fig. 3.7 A; Loughlin et al.,
2011 ). Spindle assembly pathways also differ, with smaller spindles less
dependent on the RanGTP pathway ( Wilbur and Heald, 2013 ). Recent
work has also identified unique features of MT asters in the very large cells
of amphibian embryos ( Mitchison et al., 2012 ). The reworking of architec-
tural features of large MT assemblies is therefore a commonly utilized mech-
anism to adapt to varying cellular environments.
5.3. Spindle architecture and compartmentalization
Observations of the different spindle architectures that occur across phylog-
eny can be used to understand how different molecular mechanisms func-
tion in concert to establish an emergent dynamic structure such as the
spindle. These same observations can also be used to understand how sub-
cellular structures are coordinated with the cellular context to provide a crit-
ical function under different physiological conditions. Comparative analysis
may facilitate integration of molecular, cellular, and organizational informa-
tion of spindle function.
In many fungi and other nonmetazoan eukaryotes, mitotic spindle for-
mation occurs enclosed within a nuclear envelope that does not break down
during mitosis ( De Souza and Osmani, 2007 ; Fig. 3.8 A). Maintenance of the
compartmentalization of the cell provides a way to localize and concentrate
molecules important for spindle assembly and chromosome segregation and
prevents the need to sort the cytoplasm after mitosis. It may also provide a
structural component that resists forces and provides anchoring sites for pro-
teins and thereby contributes to spindle assembly. However, having spindle