An introduction to actomyosin[Edit]
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].
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].