structural:summarization, information:extraction, structural:expansion
Actomyosin ring and primary septum formation

Two interdependent events drive cytokinesis in S. cerevisiae. The first event is contractile actomyosin ring (AMR) constriction and the second event is formation of the primary septum (PS), a chitinous cell wall structure that can only be formed during cytokinesis. The PS resembles in animals the process of extracellular matrix remodeling. When the AMR constricts, the PS begins to grow. Disrupting AMR misorients the PS, suggesting that both have a dependent role. Additionally, disrupting the PS also leads to disruptions in the AMR, suggesting both the actomyosin ring and primary septum have an interdependent relationship.

The AMR, which is attached to the cell membrane facing the cytosol, consists of actin and myosin II molecules that coordinate the cells to split. The ring is thought to play an important role in ingression of the plasma membrane as a contractile force.

Proper coordination and correct positional assembly of the contractile ring depends on septins, which is the precursor to the septum ring. These GTPases assemble complexes with other proteins. The septins form a ring at the site where the bud will be created during late G1. They help promote the formation of the actin-myosin ring, although this mechanism is unknown. It is suggested they help provide structural support for other necessary cytokinesis processes. After a bud emerges, the septin ring forms an hourglass. The septin hourglass and the myosin ring together are the beginning of the future division site.

The septin and AMR complex progress to form the primary septum consisting of glucans and other chitinous molecules sent by vesicles from the Golgi body. After AMR constriction is complete, two secondary septums are formed by glucans. How the AMR ring dissembles remains poorly unknown.

Microtubules do not play as significant a role in cytokinesis compared to the AMR and septum. Disruption of microtubules did not significantly impair polarized growth. Thus, the AMR and septum formation are the major drivers of cytokinesis.