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The Role of Myosin II in Cell Locomotion 
Myosin II is the so-called "classical" myosin, being the first type of myosin to be discovered.  Myosin II forms bipolar filaments that interact with actin filaments to produce contraction.  Myosin II was first isolated from muscle but is also found in non-muscle cells, and it is especially enriched in highly motile cell types such as amoebae.  Partly because myosin II was at first the only type known at the time it was hypothesized by many that myosin II played a pivotal role in the motility of cells.
 

Myosin II is not actually required for cell locomotion (but it helps).
Using the haploid amoeba Dictyostelium discoideum two groups (
Knecht & Loomis, 1987; ) simultaneously discovered that  cells in which myosin II function was obliterated could still locomote.  This was a major surprise as many had assumed that myosin II was likely to provide the motive force. 

 

Retraction of adhesion - A role for Myosin II?

References:-

Cheung, A., Dantzig, J. A., Hollingworth, S., Baylor, S. M., Goldman, Y. E., Mitchison, T. J. & Straight, A. F. (2001) A small-molecule inhibitor of skeletal muscle myosin II., Nature Cell Biol. 4, 83-88.

Eddy, R. J., Pierini, L. M., Matsumura, F. & Maxfield, F. R. (2000) Ca2+-dependent myosin II activation is required for uropod retraction during neutrophil migration., J.Cell Sci. 113, 1287-1298.

Höner, B., Citi, S., Kendrick-Jones, J. & Jockusch, B. M. (1988) Modulation of cellular morphogenesis and locomotory activity by antibodies against myosin, J.Cell Biol. 107, 2181-2189.

Knecht, D. A. & Loomis, W. F. (1987) Antisense RNA inactivation of myosin heavy chain gene expression in Dictyostelium discoideum., Science. 236, 1081-1085.

Kolega, J. (1998) Cytoplasmic dynamics of myosin IIa and IIb: spacial 'sorting' of isoforms in locomoting cells, J.Cell Sci. 111, 2085-2095.

Kolega, J. & Taylor, D. L. (1993) Gradients in the concentration and assembly of myosin II in living fibroblasts during locomotion and fiber transport, Mol.Biol.Cell. 4, 819-836.

Reines, D. & Clarke, M. (1985) Immunological analysis of the supramolecular structure of myosin in contractile cytoskeletons of Dictyostelium amoebae., J.Biol.Chem. 260, 14248-14254.

Sinard, J. H. & Pollard, T. D. (1989) Microinjection into Acanthamoeba castellanii of monoclonal antibodies to myosin-II slows but does not stop cell locomotion., Cell Motility Cytoskeleton. 12, 42-52.

Svitkina, T. M., Verkovsky, A. B., McQuade, K. M. & Borisy, G. G. (1997) Analysis of the actin-myosin II system in fish epidermal keratocytes: Mechanism of cell body translocation., J.Cell Biol. 139, 397-415.

Yumura, S. & Uyeda, T. Q. P. (1997) Myosin II can be localized to the cleavage furrow and to the posterior region of Dictyostelium amoebae without control by phosphorylation of myosin heavy and light chains. Cell Mot.Cytoskel. 36, 313-322.

 
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