Contractile ring formation

During cytokinesis the actin–myosin contractile ring forms between two daughter cells, pinching them apart. On page 391, Wu et al. demonstrate that the contractile ring in fission yeast arises from a band of nodes rather than from one single spot, as previously proposed.

Previous experiments yielded two countervailing views of how the actin–myosin ring forms. In one scenario, an actin cable was thought to extend from a single progenitor spot that contained Mid1p/anillin, a large adaptor protein known to be involved in ring formation. In the alternate scenario, Mid1p was thought to accumulate in multiple nodes that then coalesced into a ring.

Using a series of fluorescently tagged proteins, Wu et al. found that myosin-II appeared around the equator of the cell before anaphase and was concentrated in a large number of nodes that form a band around the cell. Subsequently, the nodes melded together to form the actin-myosin ring. In addition to myosin-II these nodes contained several other proteins that participate in contractile ring function, including Cdc12p/formin, Rlc1p/myosin light chain, Rng2p/IQGAP, Cdc15p/PCH, and Mid1p.

Formation of the nodes required Mid1p. In cells lacking Mid1p, Cdc12 and myosin proteins did not accumulate in the neck region. When the team inhibited actin polymerization, the nodes formed but failed to give rise to a contractile ring. The single progenitor spot was not detected under all growth conditions and cells lacking the spot formed normal contractile rings, suggesting that it is not a critical component of the structure.