What is actomyosin?2018-02-06T10:20:28+00:00

What is Actomyosin?

Actomyosin refers to the actin-myosin complex that forms within the cytoskeleton. Actomyosin is inherently contractile, with the myosin motor protein able to pull on actin filaments. This property gives rise to contractile fibers that form the basis of skeletal muscle, and even in non-muscle cells, enable cell motility and force generation at the sub-cellular level.

Actin filament networks, both within filopodia and lamellipodia, are highly dynamic structures. This characteristic is exemplified in the retrograde motion that is intrinsic to the ‘treadmilling’ mechanism of filament formation. Further to this motion, a number of cellular processes such as filopodial retraction and lamellipodial/lamellal contractions, rely on the rearward movement of the whole filament network or large filament bundles. The retrograde motion of actin treadmilling may play a minor role in aiding these processes, however additional factors are required.

One class of proteins that has been implicated in the translocation of F-actin is the myosin motor protein family. It remains unclear which isoforms contribute to this process in specific situations and to what extent [1][2][3][4]. Each member of the myosin family possesses unique structural and functional properties, such as their step size, that determines their ability to engage in F-actin translocation [5]. It has been shown that myosins in general are required for this process to facilitate filopodial retraction [2].

Myosin II specifically, has been associated with F-actin retraction in several cell types including neurons [1], fibroblasts [6] and keratocytes [7], with particular emphasis on its role in the lamella and lamellipodia. Initially Myosin II was believed to influence F-actin dynamics and motility from within the lamella, as it had not been observed at the leading edge however it was recently observed within lamellipodia as protrusion reaches its peak, just prior to retraction [8]. This study postulates that myosin II is responsible for the formation and retrograde movement of actin arcs – bundles that form parallel to the leading edge and possibly contact multiple focal adhesions. This movement originates at the lamellipodium and moves rearward into the lamellum, producing a single continuously flowing actin network in the form of arcs [8]. It was consequently proposed that the rate of retrograde movement is reduced as the arcs contact focal adhesions near and within the lamella [8].

Video. Sliding Filament: sliding filament theory of muscle contraction - created by Sara Egner as part of UIC's biomedical visualization program **Some of you have noticed that there is a mispronunciation in this animation. It's true. ATP stands for adenosine triphosphate, and ADP for adenosine diphosphate. Please don't be confused on my account. ~Sara (anatomyandart.com/blog)
 [Video was uploaded to YouTube by avice01 and created by Sara Egner using electron micrography from P M Motta, P M Andrews, K R Porter and J Vial.]

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  1. Medeiros NA, Burnette DT, and Forscher P. Myosin II functions in actin-bundle turnover in neuronal growth cones. Nat. Cell Biol. 2006; 8(3):215-26. [PMID: 16501565]
  2. Lin CH, Espreafico EM, Mooseker MS, and Forscher P. Myosin drives retrograde F-actin flow in neuronal growth cones. Neuron 1996; 16(4):769-82. [PMID: 8607995]
  3. Brown J, and Bridgman PC. Role of myosin II in axon outgrowth. J. Histochem. Cytochem. 2003; 51(4):421-8. [PMID: 12642620]
  4. Diefenbach TJ, Latham VM, Yimlamai D, Liu CA, Herman IM, and Jay DG. Myosin 1c and myosin IIB serve opposing roles in lamellipodial dynamics of the neuronal growth cone. J. Cell Biol. 2002; 158(7):1207-17. [PMID: 12356865]
  5. Kress H, Stelzer EHK, Holzer D, Buss F, Griffiths G, and Rohrbach A. Filopodia act as phagocytic tentacles and pull with discrete steps and a load-dependent velocity. Proc. Natl. Acad. Sci. U.S.A. 2007; 104(28):11633-8. [PMID: 17620618]
  6. Verkhovsky AB, Svitkina TM, and Borisy GG. Myosin II filament assemblies in the active lamella of fibroblasts: their morphogenesis and role in the formation of actin filament bundles. J. Cell Biol. 1995; 131(4):989-1002. [PMID: 7490299]
  7. Svitkina TM, Verkhovsky AB, McQuade KM, and Borisy GG. Analysis of the actin-myosin II system in fish epidermal keratocytes: mechanism of cell body translocation. J. Cell Biol. 1997; 139(2):397-415. [PMID: 9334344]
  8. Burnette DT, Manley S, Sengupta P, Sougrat R, Davidson MW, Kachar B, and Lippincott-Schwartz J. A role for actin arcs in the leading-edge advance of migrating cells. Nat. Cell Biol. 2011; 13(4):371-81. [PMID: 21423177]