Asymmetry in cytokinesis

ells exposed to repeated stretching along a unidirectional axis reinforce their actin stress fibers and reorient them to run perpendicular to the direction of strain. The two aspects of the response are regulated independently, report Yoshigi et al. (page 209). Numerous tissues are exposed to repeated mechanical strain and respond by C After stretching (bottom), re-localization of zyxin to stress fibers helps reinforce them. reorganizing their cytoskeletons. Researchers hypothesize that stretch-responsive channels or cell adhesion sites may detect the mechanical force and control downstream changes. When cells were exposed to mechanical stress, zyxin, a prominent component of focal adhesions, relocalized to actin stress fibers. Zyxin redistribution occurred only when cells were grown on a matrix that allowed integrin engagement, but blocking stretch-activated channels had no effect on zyxin mobilization. Zyxin movement also caused a similar relocalization from focal adhesions to stress fibers of vasodilator-stimulated protein (VASP), which promotes actin polymerization. In mouse fibroblasts lacking a functional zyxin gene, VASP local-ization did not shift in response to cyclic stretch, nor did the actin fibers become thickened, as compared with wild-type cells. Significantly, the fibers in the mutant cells were reoriented normally in response to stretch, illustrating that reorientation and reinforcement are mechanistically distinct processes. It is not yet clear whether zyxin is a mechanosensor itself or is an early protein in the response pathway, but the results do show that focal adhesions are a key factor for detecting and responding to mechanical stress. wo minus end–directed motors, Ncd (Kinesin-14) and dynein, have complementary capture-and-transport roles that shape the T Without Ncd (middle), K-fibers don't focus; but without dynein (right), they are not transported toward centrosomes. spindle during mitosis, report Goshima et al. on page 229. Spindle microtubules emanate from the centrosomes or, as long thick ropes called K-fibers, from kineto-chores. Previous work showed that two minus end–directed molecular motors—dynein and Ncd—are required to intertwine the two types of microtubules and bring them into the familiar diamond shape. In cells lacking either motor, the microtubules splay apart, failing to gather their minus ends near the centrosomes. Using video microscopy with GFP-labeled components, FRAP, and RNAi, the team found that the motors have some functional redundancy but also have a preference for jobs. Loss of Ncd results primarily in a loss of K-fiber focusing, whereas loss of dynein compromises the pulling of K-fibers toward centrosomes. Ncd apparently acts as a dynamic cross-linking protein …