Actin and myosin appear to play major roles in cell motility and changes of cell shape.

Actin is a major component of the cytoskeleton-the fibrous array that pervades almost every kind of cell and gives it a specific shape.

Nonmuscle myosin is different in sequence from muscle myosin. Rather than forming thick filaments, nonmuscle myosin tends to form dimers, interacting with cytoplasmic actin to form a loose contractile network. Organized contraction and relaxation of such networks lead to a wide variety of cell movements and responses, including amoeboid crawling.

Actin-myosin contractile complexes are also involved in cytokinesis - the division of cells in the last stages of mitosis (Figure 8.17). Myosin is essential for cytokinesis, but is not essential for the contractile process in the mitotic spindle.