Talk:2010 Lecture 12

From CellBiology

Influence of lever structure on myosin 5a walking PNAS article EM Images.

JCB Movies

  • Microtubule and actin movements are coordinated Fluorescent speckle microscopy (FSM) allowed Salmon et al. to examine both microtubules (MTs) and filamentous actin (f-actin) in migrating newt cells. F-actin exhibited four zones of dynamic behavior: rapid retrograde flow in the lamellipodium, slow retrograde flow in the lamellum, anterograde flow in the cell body, and no movement in the convergence zone between the lamellum and cell body. MTs moved at the same trajectory and velocity as f-actin in the cell body and lamellum, but not in the lamellipodium or convergence zone. MTs grew along f-actin bundles, and quiescent MT ends moved in association with f-actin bundles. Thus f-actin movements have a profound effect on MTs in migrating cells, and MTs and f-actin may bind to one another in vivo.
  • Actin-dependent picket fences slow diffusion in the plasma membrane Fujiwara et al. track the movement of single phospholipid molecules in the plasma membrane. Over the short-term, these molecules appear to stay within defined compartments, but after an average of 11ms they hop to an adjacent compartment. Over even longer time periods (an average of 0.33 s), the lipids hop between larger compartments. Similar compartments are not apparent on vesicles, as trajectories are less closely apposed. Trajectories within a single compartment are not significantly slowed relative to diffusion rates in vesicles, so the delays in hopping between compartments must explain the lower overall diffusion rate for lipids in cellular membranes. The compartmentalization depends on the actin-based membrane skeleton, but not on the extracellular matrix, extracellular domains of membrane proteins, or cholesterol-enriched rafts. The authors propose that various transmembrane proteins anchored to the actin-based membrane skeleton meshwork act as rows of pickets that temporarily confine phospholipids.
  • An actin fragment that relaxes myofibroblasts Myofibroblasts are specialized fibroblasts that can contract to aid in wound healing, possibly by using stress fibers containing Œ±-smooth muscle actin (Œ±-SMA). Hinz et al. join the N-terminal sequence of Œ±-SMA to a fusion peptide. Application of the resulting protein to myofibroblasts relaxes the cells reversibly. Such an agent may be useful in treating fibrocontractive diseases.
  • A Rop GTPase controls pollen tube tip growth Fu et al. show that Rop1At, a Rop GTPase belonging to the Rho family, controls actin dynamics and thus pollen tube tip growth in tobacco plants.
  • Myosin recruitment drives the distribution of nuclei in fly embryos As nuclei divide in the early fly embryo, they are actively distributed along the long axis of the embryo. Royou et al. show that the cortical contractions that drive this are accompanied by periodic accumulation of myosin to the cortex. Recruitment occurs at the end of telophase, correlated with the drop in cdc2/cyclin B activity, and results in a cortical contraction during interphase.
  • A sensor for the activity and abundance of MLCK Chew et al. construct a sensor that can read out both the abundance (shown as peak height) and activity (red is inactive and blue is active) of myosin light chain kinase (MLCK), a protein that activates myosin during nonmuscle cell contraction. The sensor has a Ca2+/calmodulin binding site placed between two added fluorescent domains. When MLCK is activated by the binding of Ca2+/calmodulin, FRET between the two fluorescent domains is disrupted. In contracting cells, MLCK is recruited to and activated along contracting stress fibers (also visible in a second cell). MLCK is also activated in the lamella of motile and stationary cells, and at the cleavage furrow during cytokinesis.
  • MLCK stimulates rapid contraction; Rho kinase stimulates sustained contraction Katoh et al. report that the calcium-dependent myosin light chain kinase (MLCK) triggers rapid stress fiber contraction, whereas Rho-kinase elicits sustained contraction, which is necessary for maintaining stress fibers, focal adhesions, and cytoplasmic tension. The authors separate the effects of these two contractile systems by preparing contractile fibers either in glycerol (which maintains both contractile systems) or Triton X-100 (which removes the Rho-kinase system).

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