The Contractile Ring

The contractile ring is a structure composed of actin filaments and myosin II filaments that forms during cytokinesis to divide a cell into two daughter cells. It appears as a cleavage furrow that deepens around the cell's equator starting in anaphase. The contractile ring generates the force necessary to pinch off and separate the two new cells through coordinated contraction of its protein components.

RhoA is the chief regulator of contractile ring assembly and function. It is activated locally at the cell cortex near the cell's equator by a guanine nucleotide exchange factor that exchanges GDP for GTP. RhoA activation promotes actin filament polymerization and stimulates myosin II assembly, ensuring the contractile ring forms at the correct location and time after chromosome segregation.

The contractile ring consists of actin filaments, myosin II filaments, and septin filaments. Myosin motor activity generates contraction by moving along adjacent antiparallel actin filaments, causing them to slide past one another. This sliding action contracts the ring progressively until it pinches off two new cells. Septin filaments stabilize the ring structure during this process.

Anillin acts as the main organizer of the contractile ring by binding with actin, myosin II, membrane phospholipids, septin, and other structural and regulatory components. While RhoA activates contractile ring assembly, anillin coordinates the assembly of these diverse components into a functional contractile structure necessary for proper cell division.

The contractile ring forms after chromosome segregation during anaphase, coordinated by RhoA and anaphase spindle fibers. As the ring contracts, it progressively disassembles actomyosin filaments concomitantly, requiring fewer filaments to maintain ring thickness. During final cytokinesis stages, the contractile ring and central spindle mature to form the midbody, which completes cell separation through abscission.

Inefficient or absent contractile ring formation leads to abnormal cell division, impaired cell growth, and potential tumor formation. Proper contractile ring assembly and function are essential for successful cytokinesis and the production of two viable daughter cells with appropriate cytoplasmic content and genetic material.

RhoA activation is regulated by a guanine nucleotide exchange factor (Rho-GEF) located in the cortex region, the site of future cell division. Rho-GEF exchanges the GDP bound to inactive RhoA with GTP, activating the protein. This localized activation ensures the contractile ring forms specifically at the cell's equator where division will occur.