Laporte et al. describe how precursors of the cytokinetic contractile ring assemble at the cell equator.

As fission yeast enter mitosis, protein clusters called cytokinesis nodes form around the cell cortex at the future site of cell division. The nodes eventually coalesce into a contractile ring to separate daughter cells, but little is known about the nodes’ initial assembly, except that their localization at the cell cortex is determined by Mid1, a homologue of the animal–cell cleavage furrow protein anillin.

Laporte et al. analyzed the composition of cytokinesis nodes in different mutant backgrounds to define two protein modules that assemble around Mid1 at the plasma membrane. The first module consisted of the myosin light chain Cdc4 and the actin-bundling IQGAP protein Rng2, which together recruited the myosin motor protein Myo2. The second module incorporated the membrane-bending F-BAR protein Cdc15. Both modules recruited the actin-nucleating formin Cdc12 into the nodes, Laporte et al. found.

The researchers then used a single-molecule imaging technique called SHREC to examine how these components are organized within each cytokinesis node. Myo2’s head domain points into the cell interior, an orientation that may help it capture and pull actin filaments nucleated from neighboring nodes as the separate clusters condense into a full contractile ring. Senior author Jian-Qiu Wu now wants to examine how the organization of the node proteins changes as they form the contractile ring. He's particularly interested in whether Myo2 polymerizes into filaments, as has been observed in the contractile ring of other organisms.

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J. Cell Biol.