Certain myosin isoforms (i.e. myosin II) form bipolar assemblies via the extended coiled-coil domains in the heavy chains (see also “thick filaments”). Actin “thin filaments” with opposite polarity associate with thick filaments to create contractile bundles that can be found in both muscle and nonmuscle cells. The concerted movement of the myosin heads generates the forces needed for contraction and causes the adjacent actin filaments to ‘slide’ past each other. Evidence for the sliding mechanism of force generation, comes largely from in vitro reconstitution studies, particularly the sliding filament assay created by Kron and Spudich . This assay and subsequent variations of it demonstrated the ability of myosin to bind and slide individual actin filaments  and have further elucidated the biophysical properties of this action .
Contractile bundles vary in thickness and have been shown to contain anywhere between 10 to 300 individual actin filaments . These bundles are stabilized throughout the bundle and at the filament ends by a number of accessory proteins (e.g. α-actinin, titin, components of the focal adhesion complex). Contractile bundles are also crucial for generating both traction and protrusion forces in motile cells and they are involved during cell division (e.g. cytokinesis).
For studies which investigate the role of cell contraction and motility, contractile force can be inhibited using small molecules such as blebbistatin . This is a cell-permeable, highly specific small-molecule inhibitor of myosin II Mg-ATPase activity  that is used for investigating the role of myosin II in cell contraction and motility. Blebbistatin inhibits both myosin isoforms IIA, IIB, and skeletal muscle myosin II but has little effect on smooth muscle myosin II and myosins I, Myosin V, and Myosin X .
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 . 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 . It has been shown that myosins in general are required for this process to facilitate filopodial retraction .
Myosin II specifically, has been associated with F-actin retraction in several cell types including neurons , fibroblasts  and keratocytes , 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 . 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 . It was consequently proposed that the rate of retrograde movement is reduced as the arcs contact focal adhesions near and within the lamella .
Numerous motor proteins exist and each possess unique characteristics that allow them to facilitate different processes and functions. Even within the myosin superfamily variation exists in structure and function of each member. Not only can motor proteins translocate along microfilaments, but they can induce movement in the filament itself. It is this property that gives rise to the contractile properties of skeletal muscle.
In skeletal muscle cells, myosin II forms only thick filaments that are arranged within a scaffold of actin thin filaments (along with numerous other proteins). These form the higher order fibrous structures known as sarcomeres. Each sarcomere contains numerous repeating units of interlinked thick and thin filaments, and the opposite orientation of the myosin heads causes adjacent actin filaments to slide past each other during muscle contraction. Each sarcomere is ~2 µm long in resting muscle, but this length is shortened by as much as 70% after muscle contraction. Muscle contraction is regulated by calcium levels  and by the troponin regulatory system. Although actin subunits continue to turn-over at both ends of the thin filament, this exchange is relatively slow, making the actin filaments in sarcomeres relatively more stable when compared to the actin filaments found in other cell types.
In non-muscle cells, myosin II associates with actin filaments to form contractile structures known as stress fibers along the lower surfaces where the cell is anchored to its substrate. In epithelial cells, contractile bundles are also prominent in the adhesion belt (aka adherens belt), which helps to maintain the stability and integrity of epithelial cell sheets. The contractile bundles in nonmuscle cells are similar to skeletal muscle fibers, but they are smaller (~0.4 µm in fibroblasts), less organized, and they contain different accessory proteins .
Historically speaking, the mechanism of actomyosin contraction for nonmuscle actin was examined using amoebae proteins Dictyostelium, Acanthamoeba) because the actin is very similar to muscle actin ; these initial studies showed the rate of ATP hydrolysis by myosin (and hence myosin movement) varies directly with the actin concentration . Further studies using isolated stress fibers from fibroblasts confirmed that stress fibers are contractile and shorten by as much as 25% . Myosin II bundle formation and contractile activity in nonmuscle cells is regulated by phosphorylation .
Non-muscle myosin II isoforms have a similar structure and function to their muscle equivalents. However, their interaction with actin serves to generate cellular forces rather than muscular contraction. During non-muscle actomyosin contractility, non-muscle myosin II uses energy from ATP hydrolysis to slide the actin filament to produce contractile force, and these forces have been implicated in multiple cell functions, such as cell adhesion, establishing cell polarity, and cell migration . During cell division, actomyosin contractility regulates forces on the nucleus which affect DNA synthesis and chromatin organization, and is also required for formation and contraction of the mitotic spindle .
The varied functions associated with actomyosin contractility require the involvement of many proteins other than actin and myosin. Data mining of the literature has revealed the comprehensive network of proteins that regulate actomyosin contractility, termed the ‘contractome’. A total of 100 contractome proteins were identified, comprising of 97 proteins and 3 cofactors. After organizing these proteins based on their primary function, the three biggest functional groups were serine/threonine phosphorylation regulators, primarily kinases (27 proteins), scaffolding proteins (24 proteins), and regulators of actin dynamics (12 proteins).
Using a protein interaction database to probe the contractome for interactions, researchers have been able to isolate the major features of the network. The Rho family of small GTPases activates serine/threonine kinases, and this activation is propagated by self-phosphorylation. The serine/threonine kinases activate regulators of actin dynamics, myosin phosphatase, and the myosin light chain. The scaffold group of proteins act as connectors, forming a link between actin and myosin. Scaffolding proteins also bind to the serine/threonine kinases, RhoGTPases and its RhoGEF and RhoGAP regulators, and regulate the myosin heavy chain. While the contractome demonstrates the complexity of actomyosin contractility, the initial data also reveals certain common functional processes and regulatory pathways .