Pinched off

During the final stage of cell division, a short-lived contractile ring constricts the cellular membrane and eventually separates the dividing cell in two. Although this “molecular muscle’s” composition, mainly actin and myosin, is similar to its skeletal counterpart, the force-producing mechanism is fundamentally different.

Combining traditional imaging and genetic approaches with a novel quantitative microscopy model, Stowers Investigator Rong Li, PhD, and her colleagues revealed that the depolymerization of actin filaments combined with actin cross-linkers—which act like pawls on a ratchet—and not the sliding myosin motors credited with contracting skeletal muscle is the main driving force behind the tightening of the actomyosin ring that completes cell division in budding yeast.

In muscle cells, the so-called motor domain of myosin binds actin and generates tension through a “power-stroke” mechanism fueled by the energy released from ATP hydrolysis. However, budding yeast genetically engineered to lack the motor domain of myosin suffers only minor ill effects. “It had long been known that the contractile ring is made up of actin and myosin, the same molecules that allow our muscles to contract,” explains Li, who led the team. “As a result, it was logical to assume that these intracellular structures work the same way.”

Their study breaks new ground in the field of cytokinesis and provides new insight into the contraction mechanisms of actomyosin structures in non-muscle cells, which play an important role in cell division but also in many other processes such as cell shape changes, cell adhesion and motility.

The study was published in the June 15, 2012, issue of Developmental Cell.