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First published online December 20, 2007
doi: 10.1242/10.1242/jcs.007112

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Nonmuscle myosin II moves in new directions

Laboratory of Molecular Cardiology, Genetics and Developmental Biology Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD 20892, USA

View larger version (28K): [in this window] [in a new window]   Fig. 1. (A) Diagram of a myosin molecule showing the globular head region, the -helical coiled-coil rod and the short non-helical tail (NHT). The subfragment-1 (S-1), rod and heavy meromyosin (HMM) proteolytic domains are also indicated. (B) An example of a bipolar filament, which is formed by interaction among the rod domains. (C) Regulation of myosin activity by phosphorylation of MLC20 of NMII by MLCK and Rho kinase. Whereas MLCK can only phosphorylate MLC20, Rho kinase can phosphorylate MLC20 and also a subunit of myosin phosphatase (MYPT1). Phosphorylation by Rho kinase activates myosin and inactivates MYPT. Both result in an increase in phosphorylated MLC20 and activation of myosin.

View larger version (114K): [in this window] [in a new window]   Fig. 2. Cell polarization due to rearrangement of the actomyosin cytoskeleton. (A) An example of apical-basal cell polarity, such as that shown by epithelial cell layers in the intestines and skin. Myosin is concentrated at the apical side of the cell, where it might participate in assembly and disassembly of the apical junction complex (green rectangles), which tightly links cells, or in the process of apical constriction, which alters the shape of the cell and its tissue layer. (B) Diagram of cell migration and the establishment of front-to-back polarity. Extension of the lamellipodium at the front (right) of the migrating cell is due to addition of actin monomers to the branching actin network. The migrating cell attaches to and detaches from the surface via integrin clusters. Focal complexes form at the cell front and mature into focal adhesions owing to NMII contractility. Activation of NMII contributes to detachment of the rear (left) of the moving cell. Arrows indicate the direction of movement.

View larger version (97K): [in this window] [in a new window]   Fig. 3. Diagram showing a structural role for nonmuscle myosin II in maintaining cell adhesion. The extracellular domains of cadherin molecules interact with those of the neighboring cell. Bipolar filaments of myosin bind to actin, which, in turn, interacts with the catenins, which are shown interacting with the cytoplasmic portion of cadherins (green rectangles). The nature of the interaction between the catenin and associated proteins and actin is unclear at present (see text). As outlined above, NMII plays a role in both maintaining and disassembling the adhesion junction complex. (Right) A similar type of role can be visualized for NMII with regard to focal adhesions through which the cell interacts with the extracellular matrix (ECM). Among the various proteins binding to the integrins are -actinin, talin, focal adhesion kinase, vinculin and paxillin. (Bottom) NMII also serves as a scaffold for kinases and small GTP-binding molecules (as well as molecules that have yet to be identified), which regulate its activity and its interaction with actin and other molecules such as microtubules. The diagram omits numerous actin-bundling and regulatory proteins.

View larger version (160K): [in this window] [in a new window]   Fig. 4. Immunofluorescence confocal images of mouse spinal cords. (a) Cross-section at E12.5 of a wild-type (B+/B+) spinal cord stained for NMIIB (green) and N-cadherin (red). (b) Magnified view of a hypomorphic Arg709Cys NMIIB-mutant (BCN/BCN) spinal cord at E12.5 stained as in panel a. Note that the neuroepithelial cells have invaded and obstructed the spinal canal in the mutant mouse. (c) Apical border of the cells lining the spinal canal in a B+/B+ mouse at E11.5, showing a mesh-like adhesion structure stained for NMIIB (green) and β-catenin (red). (d) Spinal canal of an NMIIB-ablated (B–/B–) mouse at E11.5, showing collapse of the apical mesh-like structure, with neuroepithelial cells protruding into the canal, stained for NMIIB (green, which is not seen in the B–/B– mouse) and N-cadherin (red). These panels are reproduced with permission (Ma et al., 2007).