Tenascin-X and the pick of destiny

Spindle microtubules sustain the tension C hacón et al. reveal how the mitotic spindle maintains a constant level of tension on correctly attached chromosomes. When sister chromatids attach to microtubules emanating from opposite spindle poles, the chromatin that surrounds and connects the chromatids' centromeres is thought to come under tension, generating a mechanical signal that permits the cell to separate the chromosomes and exit mitosis. But it is unclear how much tension pericentromeric chroma-tin experiences and how cells cope with fl uctuations in this force. By measuring the inherent stiffness of pericentromeric chromatin and how much it became stretched during metaphase, Chacón et al. calculated that each budding yeast pericentromere experiences around 5 pN of tension when correctly attached to the mitotic spindle, more than enough to activate downstream signaling events. The researchers then investigated how peri-centromeric tension was affected by changes in pericentromere structure. Pericentromeres were much more flexible in yeast lacking the DNA-packaging enzyme topoisomerase II, but they still experienced 5 pN of tension during metaphase because the mitotic spindle contrived to pull sister centromeres further apart than normal. The spindle achieved this feat by growing longer than it did in wild-type cells, while simultaneously shortening the kinetochore microtubules that directly attach to chromosomes. Modeling experiments suggested that changes in pericentro-meric tension can induce compensatory changes in kinetochore microtubule dynamics, which could help prevent natural variations in pericentromere structure from falsely activating checkpoint pathways that delay mitotic exit. Senior author Melissa Gardner now wants to investigate how tension regulates kineto-chore microtubules and to determine how much pericentromeric tension can decrease without activating the spindle checkpoint. W ang et al. reveal how two proteins cluster the myosin motor Myo51 to promote assembly of the fi ssion yeast cytoki-netic ring. Fission yeast assemble a con-tractile ring from myosin II–containing precursor nodes scattered around the cell equator. The motor protein captures actin fi laments nucleated from neighboring nodes and pulls the structures together until they coalesce into a compact actomyosin ring. The type V myosin Myo51 also contributes to ring formation by promoting the delivery of additional actin fi laments to the cell equator. But Myo51's precise function and how it is regulated during cytoki-nesis remain unclear. Wang et al. identifi ed two coiled-coil proteins, which they named Rng8 and Rng9, that colocalized with Myo51 on actin cables and the cytokinetic ring. Deleting Rng8 or Rng9 largely abolished Myo51's recruitment to …