Tension between two kinetochores suffices for their bi-orientation on the mitotic spindle

Abstract

Any which way but loose back-to-back arrangement of kinetochores is not needed for correct alignment on the mitotic spindle, as shown by Hilary Dewar, Tomoyuki Tanaka (University of Dundee, UK), and colleagues. Sister chromatids are glued together by cohesin such that their kinetochores face opposing poles. This arrangement might thus prevent both chromatids from attaching to spindle microtubules from the same pole. But Tanaka's group shows that even when geometry fails, a tension-sensitive mechanism fixes any mistakes. The authors messed with the usual geometry in two ways. First, they confronted yeast cells with a nonreplicating dicentric minichromosome. Its two kinetochores are not held in any fixed relative orientation , yet were efficiently attached to opposing poles. Second, normal chromosomes in cohesin mutants (which have attachment defects), were roughly linked by inhibiting topoisomerase II. This restored bipolar attachments. Thus, any connection that can produce tension is enough to ensure biorientation. The tension-sensitive correction depends on the Ipl1 kinase, whose mammalian homo-logue, Aurora B, prevents monopolar attachments. This suggests that Ipl1 activity knocks off attachments until tension somehow stops it-perhaps by turning off the kinase, turning on a counteracting phosphatase, or pulling substrates away from the kinase. A dynamite intercellular highway ong, delicate tubules are a new trade route for the intercellular exchange of goods, as shown by Amin Rustom, Hans-Hermann Gerdes (University

An ATM- and ATR-dependent checkpoint inactivates spindle assembly by targeting CEP63

“…At this point, kinetochores are attached to the positive ends of microtubules and chromosome movement is thought it be coupled directly with microtubule dynamics. The turnover of kinetochore-microtubule interactions promoted by Aurora B has been proposed as a mechanism to ensure chromosome bi-orientation (Tanaka et al, 2002;Hauf et al, 2003;Dewar et al, 2004;Lampson et al, 2004). It is generally thought that only when biorientation is achieved, the process that is referred to as chromosome congression takes place, in which chromosomes are directed towards the central metaphase plate (Rieder and Salmon, 1994).…”

Chromosome bi-orientation on the mitotic spindle

“…As a control, we also created an unreplicated monocentric chromosome that was the same as the unreplicated dicentric chromosome except that the former did not have the second centromere regulated by the GAL1-10 promoter. We observed the motion of these two kinds of minichromosomes with time-lapse microscopy in metaphase and anaphase (Dewar et al 2004). The unreplicated monocentric chromosomes always stayed in the vicinity of one of two SPBs and did not spend any appreciable time in the space between two SPBs.…”

“…The corollary is that geometry of two kinetochores is dispensable for their efficient bi-orientation on the mitotic spindle (although this does not exclude the possibility that the geometry mechanism functions redundantly to facilitate bi-orientation of sister centromeres in mitosis). (Dewar et al 2004) is illustrated. ARS, DNA replication origin; RS, recombination site; P MET3 , MET3 promoter; P GAL , GAL1-10 promoter; Met, methionine; Gal, galactose; Glc, glucose.…”

“…If the tension-dependent mechanism works to facilitate bi-orientation in mitosis, we can predict that any type of connections between two kinetochores (including connections that do not support backto-back geometry of the two kinetochores) will suffice for their bi-orientation, so long as these connections generate tension upon bi-orientation of the two kinetochores. To test this, we studied the behaviour of the two kinetochores on an engineered, unreplicated dicentric circular minichromosome in budding yeast (figure 2; Dewar et al 2004). The two kinetochores are not sisters born by DNA replication, and therefore should not have back-to-back geometry.…”

“…Nonetheless, the two kinetochores are connected by chromatin, on which tension could be applied if they bi-orient on the spindle. Because such minichromosomes cannot be propagated, we needed to establish an induction system by which we can make them from minichromosomes that are propagated stably (figure 2; Dewar et al 2004). The original minichromosome carried two centromeres; the first one was constitutively active and the second one was conditionally inactivated owing to its juxtaposition to the GAL1-10 promoter (Hill & Bloom 1987).…”