Passengers operate the brakes

Passengers operate the brakes M embers of the chromosome passenger complex (CPC) regulate kinesin motors to control how fast the mitotic spindle elongates , Rozelle et al. report. The CPC consists of the mitotic kinase Aurora B and its associated regulatory proteins. The complex localizes to kinetochores at the beginning of mitosis and regulates their attachment to the mitotic spindle. The CPC moves to the central spindle in anaphase, but the complex's precise function at this stage of mitosis remains unclear. Rozelle et al. examined the anaphase spindles of budding yeast lacking various components of the CPC. Although spindles grew normally in cells lacking Aurora B itself, spindle poles moved apart more slowly in the absence of the regulatory passenger proteins Sli15, Bir1, and CBF3. Spindle microtubules are slid apart by the kinesins Cin8 and Kip1, yet deleting either of these motor proteins accelerated spindle elongation in CPC-mutant yeast, indicating that they can also act as brakes to limit microtubule movements. The CPC may switch the motors' activities by rearranging them on the anaphase spindle—more kinesin accumulated at the spindle midzone in the absence of the CPC regulatory subunits—or by infl uencing their posttranslational modifi cation. CPC mutants took longer to complete mitosis because their slowly elongating spindles activated a signaling pathway called the mitotic exit network, which maintains cells in anaphase. Senior author Kenneth Kaplan now wants to investigate how slow-growing spindles activate this pathway and to determine whether the CPC modulates spindle length in order to ensure that chromosome segregation is complete before the cell divides in two. W est et al. reveal the diversity of endoplasmic reticulum (ER) structures present in yeast and describe how these structures pass from the mother cell into the bud. The peripheral ER is a continuous network of membrane tubules and cisternae that extends from the nuclear envelope. West et al. looked at ER structure using electron tomog-raphy to reconstruct the organelle's three-dimensional organization. The researchers saw three types of ER. ER tubules branched from the nuclear envelope throughout the cytoplasm, connecting distinct ER domains and contacting other organelles. A single central cisterna stretched from the nuclear envelope toward the developing bud. The rest of the ER was tightly associated with the plasma membrane as a network of both tubules and cisternae. Plasma membrane– associated ER was unable to enter the bud directly. Instead, the central cisternal and tubular ER domains extended …