11 research outputs found
Microtubule dynamics and kinetochore function
Chromosome alignment during mitosis is frequently accompanied by a dynamic
switching between elongation and shortening of kinetochore fibers (K-fibers) that connect
kinetochores and spindle poles [1, 2]. In higher eukaryotes, mature K-fibers consist of 10–30
kinetochore microtubules (kMTs) whose plus-ends are embedded in the kinetochore [1-3]. A
critical and long-standing question is how the dynamics of individual kMTs within the K-fiber
are coordinated [1-5]. We have addressed this question by using electron tomography to
determine the polymerization/depolymerization status of individual kMTs in the K-fibers of
PtK1 and Drosophila S2 cells. Surprisingly, we find that the plus-ends of two-thirds of kMTs
are in a depolymerizing state, even when the K-fiber exhibits net tubulin incorporation at the plus
end [6-8]. Furthermore, almost all individual K-fibers examined had a mixture of kMTs in the
polymerizing and depolymerizing states. Therefore, although K-fibers elongate and shrink as a
unit, the dynamics of individual kMTs within a K-fiber are not coordinated at any given moment.
Our results suggest a novel control mechanism through which attachment to the kinetochore
outer plate prevents shrinkage of kMTs. We discuss the ramifications of this new model on the
regulation of chromosome movement and the stability of K-fibers
The Drosophila kinetochore
Drosophila melanogaster is a widely used model organism for the molecular dissection of mitosis in animals. However, despite the popularity of this system, no studies have been published on the ultrastructure of Drosophila kinetochores and kinetochore fibers (K-fibers) in somatic cells. To amend this situation, we used correlative light (LM) and electron microscopy (EM) to study kinetochores in cultured Drosophila S2 cells during metaphase, and after colchicine treatment to depolymerize all microtubules (MTs). We find that the structure of attached kinetochores in S2 cells is indistinct, consisting of an amorphous inner zone associated with a more electron-dense peripheral surface layer that is approximately 40-50 nm thick. On average, each S2 kinetochore binds 11+/-2 MTs, in contrast to the 4-6 MTs per kinetochore reported for Drosophila spermatocytes. Importantly, nearly all of the kinetochore MT plus ends terminate in the peripheral surface layer, which we argue is analogous to the outer plate in vertebrate kinetochores. Our structural observations provide important data for assessing the results of RNAi studies of mitosis, as well as for the development of mathematical modelling and computer simulation studies in Drosophila and related organisms
The coupling between sister kinetochore directional instability and oscillations in centromere stretch in metaphase PtK1 cells
High-resolution kinetochore tracking reveals that oscillations in centromere stretch occur at twice the frequency of kinetochore oscillations because of the nonlinear kinetics of kinetochore directional instability, which produce poleward and away-from-the-pole kinetochore oscillations
Finding the middle ground: how kinetochores power chromosome congression
ISSN:1420-682XISSN:1420-907