Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 105

other motors that typically slide against antiparallel MTs will encounter a
much higher density of parallel MTs at the poles, where it may act instead
as a cross-linker ( Uteng et al., 2008; Wittmann et al., 1998 ). By promoting
dynamic transport of factors throughout the spindle, motors provide an
additional layer of regulation of spindle assembly and maintenance factors.
Finally, a large number of motor proteins have been identified that bind
not only to MTs but also to chromosomes. Despite common localization,
the functions of these varied proteins appear to be nonredundant. The
kinesin-10 member Xkid facilitates the congression of chromosomes and
their alignment at the metaphase plate ( Funabiki and Murray, 2000 ). While
some kinesin-4 members also contribute to chromosome alignment, inter-
estingly, Xklp1 has also been shown to regulate MT density in the spindle
( Castoldi and Vernos, 2006; Vernos et al., 1995 ). The many roles of chro-
mokinesins indicate that MT-chromosome attachments are not only impor-
tant for physically tethering and separating the chromosomes, but they
actually help establish and maintain the spindle structure as well.
5. VARIATIONS ON A THEME: TAILORING
SPINDLE ARCHITECTURE
Subcellular organization must accommodate the variety of cellular
structures and lifestyles found among eukaryotic species. Similarly, within
an organism, specialization of cell types involves changes to cell shape and
function that may require alteration of subcellular structures. To maintain
accurate chromosome segregation in a variety of contexts, the spindle has
adapted to accommodate cell size, shape, dynamics, and contents
( Fig. 3.6 ). How then does the underlying architecture of morphologically
distinct spindles facilitate chromosome segregation and yet allow unique
functional elaborations? Can the presence or levels of specific spindle pro-
teins, domains, or activities be correlated with spindle architectures across
phylogenies or upon cellular specialization? While genomic and expression
data that can assess changes in individual spindle-related proteins is pouring
in, cell biological assessment of spindle architectures is lacking to generate
comprehensive comparisons. However, some correlations of morphology
with genomic data are available and can provide some insight into unique
features of spindle architecture across phylogeny, and development of sys-
tems to specifically study the molecular mechanisms driving different spindle
architectures is progressing.