18.15: The Contractile Ring
Contractile rings are composed of microfilaments and are responsible for separating the daughter cells during cytokinesis. Contractile ring assembly proceeds along with other cell cycle events; however, very few mechanistic details are known about the timing and coordination of the contractile rings with the cell cycle.
A small GTPase, RhoA, controls the function and assembly of the contractile ring. RhoA belongs to the Ras superfamily of proteins. The activation of formins by RhoA promotes actin filament formation, whereas the activation of multiple protein kinases by RhoA stimulates the myosin II assembly and contraction. The kinases phosphorylate the myosin light chain and stimulate filament formation and motor activity. In addition to actin and myosin II (actomyosin), septin filaments are also involved in contractile ring formation. Septin filaments stabilize the contractile ring and play an important role in yeast cytokinesis.
The activation of RhoA is regulated by a guanine nucleotide exchange factor (Rho-GEF). This protein is found in the cortex region, which is the site of future cell division. The inactive form of RhoA is bound to GDP. Rho-GEF exchanges the GDP bound to RhoA with GTP. The binding of GTP activates RhoA, which in turn triggers the formation of contractile rings.
RhoA also regulates the activity of the scaffold protein anillin, an essential player in contractile ring formation. While RhoA is considered the principal activator for the assembly of the contractile ring, anillin acts as the main organizer for the ring by binding with actin, myosin II, membrane phospholipids, septin, and other structural and regulatory components involved in contractile ring formation.
The continuous shrinkage of the contractile ring means it progressively needs a smaller number of actomyosin filaments to form a ring of the same thickness; therefore, concomitant disassembly of the actomyosin filaments occurs as the ring contracts. During the final stages of the cytokinesis, the contractile ring and the central spindle containing compact microtubules matures to form the midbody and the midbody ring. The midbody ring then carries out the abscission of the parent cell, resulting in the formation of two daughter cells.