The Role of Actin and Myosin in Non-muscle Cells

Non-muscle cells contain smaller, less organized F-actin and myosin-II contractile bundles compared to muscle fibers. These bundles form stress fibers or adherence belts held together by accessory proteins like fascin, filamin, and fimbrin. While the contraction mechanism using ATP hydrolysis is similar to actin and myosin in muscle contraction, non-muscle bundles serve distinct cellular functions including cell attachment and division.

In macrophages, F-actin and myosin arrange into compact bundles called stress fibers that rearrange during phagocytosis to engulf and internalize foreign particles and pathogens. Stress fibers also bind with integrins connected to the extracellular matrix, helping cells attach to surrounding surfaces and regulate mechanosensitive machinery essential for cell movement and response.

In epithelial cells, F-actin and myosin-II form contractile bundles arranged parallel to the cell membrane as circumferential or adherence belts. These belts connect adjacent adherens junctions and help maintain the stability and structural integrity of epithelial tissues. They also regulate apical constriction of polarized epithelial cells, controlling cell shape and organization.

During cytokinesis, septin rings recruit actin and myosin to form a temporary contractile ring structure at the cell's middle. This ring-like formation divides the cytoplasm to create two daughter cells. The contractile ring represents a major step in cytokinesis, using the same actin-myosin contraction mechanism to physically separate dividing cells.

Non-muscle contractile bundles are held together by accessory proteins including fascin, filamin, and fimbrin, depending on their cellular location. These proteins organize and stabilize the formation of higher-order actin filaments into functional stress fibers and adherence belts. The composition and arrangement of accessory proteins determine bundle structure and mechanical properties.

The formation and contractile ability of actomyosin bundles in non-muscle cells are regulated by phosphorylation, a post-translational modification that controls protein activity. This regulatory mechanism allows cells to dynamically assemble and disassemble contractile structures in response to cellular signals. Phosphorylation-based regulation enables rapid adaptation of cell shape and function during processes like migration and division.

Stress fibers located near cell edges work together with focal adhesions to anchor cells to the substratum and regulate mechanosensitive machinery. These structures sense mechanical forces from the extracellular environment and transmit signals that influence cell morphogenesis and behavior. The stress fiber-focal adhesion complex is essential for cell attachment, migration, and response to mechanical stimuli.