Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 104

The major plus-end-directed motor in the spindle is the kinesin-5 mem-
ber Eg5/BimC/Kinesin spindle protein. Eg5 is a homotetramer capable of
binding and cross-linking twoMTs ( Cole et al., 1994 ). For antiparallel MTs,
which Eg5 seems to preferentially bind, the motor functions to slide both
MTs with their minus-ends outward away from the center of the spindle
( Fig. 3.5 E). This establishes a basic plus-end in, minus-end out organization
of MTs as well as aligning and cross-linking them in an orientation along the
same axis. Disruption of Eg5 by the small molecule inhibitor monastrol
results in collapse of the spindle into a monopole structure in Xenopus egg
extracts ( Kapoor et al., 2000 ). In mammalian cells, Eg5 is required to estab-
lish bipolarity of the spindle but not to maintain it ( Kollu et al., 2009 ), since
the kinesin-12 HKlp2 shares this function ( Vanneste et al., 2009 ). Addition-
ally, Eg5 sliding of MTs toward the pole is thought to contribute to MT flux,
as the inhibition of Eg5 eliminates flux in Xenopus egg extract spindles
( Miyamoto et al., 2004 ). However, Eg5 inhibition in mammalian cells does
not drastically change the flux rate ( Cameron et al., 2006 ). This difference may
reflect alternative spindle architectures in which the extent of the antiparallel
MT overlap in Xenopus is much higher. Clearly, Eg5 and its orthologs play a
critical role in establishing the antiparallel array of spindle MTs and therefore
the spindle architectural features and functions associated with it.
The dominant minus-end-directed motors in the spindle are dynein and
kinesin-14 class members (including Ncd, HSET, XCTK2). Between two
antiparallel MTs, minus-end movement functions to oppose the MT
motion of kinesin-5s. However, one MT could also act as cargo, transported
toward the minus-end of another MT, thereby clustering the minus-ends of
MTs together. These motors therefore have a critical role in focusing MTs
into a coalesced pole ( Fig. 3.5 F). Drosophila containing disruptions in Ncd, a
kinesin-14, have defects in spindle pole integrity ( Endow et al., 1994 ), and
pole formation is completely abrogated upon dynein inhibition in the
centrosome-independent, self-organizing pathway of spindle assembly
around DNA-coated beads in Xenopus egg extracts ( Heald et al., 1996 ).
In addition to its role in MT organization, dynein also retains its capacity
to carry cargoes to minus-ends and the spindle poles, spatially localizing cer-
tain activities. Some dynein cargoes are thought to help form and maintain
the spindle pole, such as NuMA, which stabilizes the pole structure through
assembly of multiple MT-binding domains upon formation of higher order
oligomers ( Merdes et al., 2000 ; Fig. 3.5 F). Dynein also carries a number of
motors to the pole, like Eg5 and Xklp2, the outcome of which could alter
motor activity depending on local MT organization. For example, Eg5 or