Global sourcing for ring construction

The fission yeast cytokinetic ring is formed from actin filaments nucleated throughout the cell, Huang et al. reveal.

Most eukaryotic cells generate a contractile actomyosin ring to separate their daughter cells at the end of mitosis. How actin filaments assemble into the ring is incompletely understood, however, mainly because it is difficult to label all the actin-based structures in a living cell without compromising actin function. In fission yeast, the cytokinetic ring is largely thought to assemble from actin filaments nucleated by the formin Cdc12p at punctate “nodes” around the cell equator. However, using an improved actin-binding probe called lifeact, Huang et al. found that Cdc12p nucleated actin filaments all around the cortex of mitotic fission yeast and that many of these filaments moved to the cell equator and incorporated into the cytokinetic ring.

Fission yeast ring assembly proceeds via two different pathways, relying on either the anillin protein Mid1p or the membrane-binding F-BAR protein Cdc15p. Cdc12p still formed actin filaments in the absence of these two proteins, indicating that these assembly pathways act downstream of actin nucleation. Huang et al. found that two myosin motor proteins—Myo2p and Myo51p—helped move nonmedial actin cables toward the cell equator so that they could join up with filaments nucleated from medial nodes.

Senior author Mohan Balasubramanian says that several animal cell types also appear to build their cytokinetic rings from actin filaments nucleated throughout the cell. He now wants to investigate how the myosin motors move actin filaments toward the cell equator.