In vitro contraction of cytokinetic ring depends on myosin II but not on actin dynamics

Mabuchi, Balasubramanian and colleagues develop a system to study the contraction of the fission yeast contractile ring in vitro . They identify components required for this process and find that myosin ATPase activity, but not actin disassembly, is needed. Cytokinesis in many eukaryotes involves the contraction of an actomyosin-based contractile ring 1 , 2 . However, the detailed mechanism of contractile ring contraction is not fully understood. Here, we establish an experimental system to study contraction of the ring to completion in vitro . We show that the contractile ring of permeabilized fission yeast cells undergoes rapid contraction in an ATP- and myosin-II-dependent manner in the absence of other cytoplasmic constituents. Surprisingly, neither actin polymerization nor its disassembly is required for contraction of the contractile ring, although addition of exogenous actin-crosslinking proteins blocks ring contraction. Using contractile rings generated from fission yeast cytokinesis mutants, we show that not all proteins required for assembly of the ring are required for its contraction in vitro . Our work provides the beginnings of the definition of a minimal contraction-competent cytokinetic ring apparatus.