Association of Actin and Myosin in the Contractile Ring a

The process of cell division culminates in cytokinesis, the actual cleavage of one cell into two. Cytokinesis is powered by the interaction of actin and myosin in the contractile ring, a specialized, transient organelle that forms immediately at the onset of cytokinesis and disappears once cleavage of the cell is complete. Because of its transient nature and small size relative to the entire cell, the contractile ring has proven difficult to study at the biochemical level. Although it is clear that the contraction of actin and myosin occurs in the contractile ring (reviewed in refs. 1 and 2), most details of their organization are still unknown. In addition, little is known about other components of the ring. These components probably include molecules that regulate the interaction of actin and myosin, those that are involved in the specific localization of actin and myosin into the ring, and those that are involved in and regulate the polymerization and depolymerization of actin and myosin in the ring. Furthermore, to have an effective cleavage, the contractile complex of actin and myosin in the contractile ring must be attached by at least one component to the plasma membrane at the cleavage furrow. To address some of these problems, we have been studying the organization of actin and myosin in the contractile ring in preparations of isolated cortices of dividing sea urchin eggs. This paper reviews our results on the morphology and localization of actin and myosin in the contractile ring and our data on the association of actin and myosin with the cortex in the cleavage furrow. To prepare isolated cortices containing contractile rings, sea urchin eggs that are just beginning to undergo cleavage are pipetted onto polylysine-coated coverslips. After a short time of attachment, the eggs are sheared with a stream of buffer (75 mM KCl, 1 mM EGTA, 1 mM MgCl,, 1 mM TAME, and 20 mM Hepes, pH 7.4). The isolated cortices can then be fixed immediately for immunofluorescence staining or they can be treated with reagents that might alter one or more ring components and then fixed and stained with the desired antibodies. Details of this procedure can be found in Schroeder and Otto?