The metaphase spindle at steady state – Mechanism and functions of microtubule poleward flux

Mitotic Acetylation of Microtubules Promotes Centrosomal PLK1 Recruitment and Is Required to Maintain Bipolar Spindle Homeostasis

“…Images were recorded every 10 s as the position of the red photoactivated tubulin moved with the flux. MT flux was readily seen in siLuc cells, and the average speed was calculated to be around 0.8 µm/min, in agreement with published reports [55] (Figure 5a top panels, quantification on the right). In contrast, for the bipolar and monopolar spindles analyzed in siATAT1 cells, flux was hardly observed.…”

Augmin-dependent microtubule self-organization drives kinetochore fiber maturation in mammals

“…Partial HAUS6 depletion by RNAi over 36 h (note that optimal HAUS6 depletion over 72 h completely disrupts k-fiber formation) significantly increased the half-life of both KT and non-kMTs ( Figure 5 J). In parallel, by measuring the velocity by which the photoactivation mark on spindle MTs moved relative to the metaphase plate (i.e., underwent poleward flux) ( Barisic et al., 2021 ), we found that it was reduced by ∼40% after partial HAUS6 depletion ( Figure 5 K). Overall, these data indicate that augmin promotes KT and non-kMT turnover, while assisting poleward flux in metaphase cells.…”

“…One possibility is that augmin-mediated MT amplification promotes the long-term survival of kMTs that slide poleward, directly or indirectly facilitating tubulin incorporation at the KTs. Alternatively, augmin might promote flux through its role in the formation/amplification of interpolar MTs, which are critical mechanical elements necessary for poleward flux ( Barisic et al., 2021 ). In support of the second hypothesis, we found that augmin was also required for interpolar MT formation, a property that appears to be conserved in human cells ( Manenica et al., 2020 ).…”

Chromosome clustering in mitosis by the nuclear protein Ki-67

“…Ki-67 appears to have roles in both interphase and mitosis as its cellular distribution dramatically changes during the cell cycle [ 20 ] ( Figures 1A and 2D ). In interphase, it seems to be required for heterochromatin organisation [ 5 ] and for the localisation of the nucleolar organising regions [ 24 , 25 ]. In mitosis, Ki-67 is essential for the formation of the perichromosomal layer, and the equal segregation of several proteins associated with this compartment that will then become part of the nucleolus [ 18 ].…”

“…In prometaphase, microtubule dynamics (rapidly growing and shrinking microtubules) lead to the capture of chromosomes at the kinetochores and the assembly of a bi-polar spindle begins ( Figure 1A , panel 2). As prometaphase progresses, the chromosomes are pulled in opposite directions by microtubules connected to opposite spindle poles, thus leading to chromosome oscillations, until the pole-directed forces are balanced [ 5 ]. Sister chromatids do not break apart during this process because they are firmly held together by cohesin molecules present at the centromeres.…”