Actin: Roles in Cell Division

Actin is a major component of the cytoarchitecture and plays significant roles in cell migration and cell division. It accomplishes its function over cell division in association with myosin. The most dramatic example of actin-myosin contraction in nonmuscle cells is provided by Cytokinesis—the division of a cell into two following mitosis. Toward the end of mitosis in animal cells, a contractile ring consisting of actin filaments and myosin II assembles just underneath the plasma membrane. Its contraction pulls the plasma membrane progressively inward, constricting the center of the cell and
pinching it in two. Interestingly, the thickness of the contractile ring remains constant as it contracts, implying that actin filaments disassemble as contraction proceeds. The ring then disperses completely following cell division. In nonmuscle cells actin-myosin contraction is regulated primarily by phosphorylation of one of the myosin light chains, called the regulatory light chain. Phosphorylation of the regulatory light chain in these cells has at least two effects: It promotes the assembly of myosin into filaments, and it increases myosin catalytic activity, enabling contraction to proceed. The enzyme that catalyzes this phosphorylation, called myosin light-chain kinase, is itself regulated by association with the Ca²⁺-binding protein calmodulin. Increases in cytosolic Ca²⁺ promote the binding of calmodulin to the kinase, resulting in phosphorylation of the myosin regulatory light chain. Increases in cytosolic Ca²⁺ are thus responsible, albeit indirectly, for activating myosin in nonmuscle cells.