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kinetochore or k-fibers
. K-fiber MTs connect to duplicated chromosomes so
that one copy (called a sister chromatid) is attached to each pole of the spin-
dle. The connection occurs at the kinetochore, a macromolecular MT
attachment site formed during mitosis at the centromere of each sister chro-
matid (
Jensen, 1982; Rieder, 2005
). An examination of a small variety of
spindles using electron microscopy, and more recently electron tomogra-
phy, has identified some structural features of k-fibers, while cell-based
observations and perturbations have provided insight into the dynamics of
k-fiber MTs. In some fungi, such as
Saccharomyces cerevisiae
and
Ashbya gossipi
,
k-fibers consist of one or two MTs that extend from the spindle pole to the
kinetochore (
Gibeaux et al., 2012; Winey et al., 1995
), while in most spe-
cies, k-fibers are bundles of 20-30 MTs (
McEwen et al., 1997; Rieder,
1981
). In Ptk1 cells, individual k-fiber MTs may span the entire half spindle
length but are bundled with many shorter overlapping MTs (
Rieder, 1981
).
Holocentric chromosomes have the unusual property of assembling kinet-
ochore complexes and attaching to MTs along their entire length. Although
best characterized in the nematode
Caenorhabditis elegans
, holocentric chro-
mosomes are found in a broad range of plant and animal species (
Melters
et al., 2012
). Despite variation in their number and organization, a common
feature of k-fiber MTs is that they are resistant to treatments that depolymer-
ize other spindle MTs, such as cold temperature or low doses of
MT-depolymerizing drugs. The extensive bundling of k-fibers by MT
cross-linking proteins likely increases the stability of their component
MTs. The functional effect of k-fiber MT stability may be to provide a rigid
connection to chromosomes so that forces are transmitted efficiently and not
lost on bending or splaying of k-fiber MTs. A fascinating feature of k-fiber
MTs is that their dynamics appear coordinated, and end-on kinetochore
attachment is maintained even though the associated MTs are growing or
shrinking (
Guo et al., 2013
). In vertebrate somatic cells, once sister chroma-
tids are attached to opposite poles, the sister chromatid pair oscillates as one
k-fiber grows and the other shrinks, congressing to build a “metaphase
plate” in which chromosomes are aligned in the center and poised to seg-
regate. K-fiber MTs are therefore absolutely essential for spindle function,
and this population of MTs is found in some form in all spindles.
A second population of MTs within the spindle constitutes those that are
not part of a k-fiber, referred to here as
spindle MTs
. These MTs may be short
and reside within a single half of the spindle or long enough to cross over the
center of the spindle to interact with oppositely oriented MTs in an antipar-
allel fashion (termed interpolar MTs). Spindle MTs are less stable than
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