A TOG Protein Confers Tension Sensitivity to Kinetochore-Microtubule Attachments

Abstract

The development and survival of all organisms depends on equal partitioning of their genomes during cell division. Accurate chromosome segregation requires selective stabilization of kinetochore-microtubule attachments that come under tension due to opposing pulling forces exerted on sister kinetochores by dynamic microtubule tips. Here, we show that the XMAP215 family member, Stu2, makes a major contribution to kinetochore-microtubule coupling. Stu2 and its human ortholog, ch-TOG, exhibit a conserved interaction with the Ndc80 kinetochore complex that strengthens its attachment to microtubule tips. Strikingly, Stu2 can either stabilize or destabilize kinetochore attachments, depending on the level of kinetochore tension and whether the microtubule tip is assembling or disassembling. These dichotomous effects of Stu2 are independent of its previously studied regulation of microtubule dynamics. Altogether, our results demonstrate how a kinetochore-associated factor can confer opposing, tension-dependent effects to selectively stabilize tension-bearing attachments, providing mechanistic insight into the basis for accuracy during chromosome segregation.

Anaphase A: Melting Microtubules Move Chromosomes toward Spindle Poles

“…However, the performance achieved in laser trap tip-coupling assays already provides a reasonably good match to physiological conditions. Native budding yeast kinetochore particles remain attached to dynamic microtubule tips for 50 min on average while continuously supporting 5 pN of tension [43,135]. These statistics compare favorably with the total duration of budding yeast mitosis, which is typically < 1 hr, and with the estimated levels of kinetochore force in this organism, 4 to 6 pN [69].…”

“…Considering that curled protofilaments are much less prominent at assembling tips in vitro [144], and that the conformational wave mechanism is based on curled protofilaments, a purely conformational wave-based coupler would be expected to detach more quickly during assembly than during disassembly. But just the opposite is true: The reconstituted couplers usually detach far less quickly from assembling tips [43,135,171].…”

“…Reconstituted tip-couplers made from various combinations of kinetochore subcomplexes [119,154] and from native kinetochore particles [43,135,171,172] can also maintain persistent, tensionbearing attachments to assembling tips (e.g., see Figure 7). Their assembly-coupled movement in vitro is analogous to situations in vivo when kinetochores move anti-poleward in association with growing microtubule tips, such as during pre-anaphase chromosome oscillations, or during transient reversals of anaphase A chromosome-to-pole movement.…”

“…Tension applied through Dam1c-based tipcouplers [204] or through native yeast kinetochore particles [43,135] promotes net growth of the attached microtubule. Tension speeds tip assembly, slows disassembly, inhibits switches from growth to shortening ('catastrophes'), and promotes the resumption of growth ('rescues') [43,135]. The effect of tension on catastrophe frequency is especially dramatic: At modest concentrations of free tubulin, the growth of a bare microtubule tip will typically persist for only a few minutes before a catastrophe occurs.…”

“…Another class of molecules likely to contribute are microtubule plus end-binders, such as those of the TOG (tumor overexpressed gene) family. TOG family proteins (Stu2 in budding yeast, XMAP215 in Xenopus, and chTOG in humans) localize to kinetochores [102,[128][129][130][131][132][133][134] and contribute directly to tip-coupling in vitro [135,136]. The knockdown phenotypes for these plus end-binders, and for kinetochore motors, are often complex, suggesting roles in multiple different aspects of mitosis and making it difficult to assess specifically their roles in kinetochore tip-coupling in vivo.…”