Cells lacking Cdc15p do not make a medial actin ring.

F-actin filaments can be induced to form by either of two pathways, but the elaborate cytoskeletal rearrangements seen in live cells imply that these mechanisms must somehow be coordinated. Now, Carnahan and Gould (page 851) provide the first evidence that a single, highly conserved protein links these two pathways during cytokinesis.

Previous work has shown that both the Arp2/3 complex and the formin family of proteins can induce F-actin nucleation, the rate-limiting step in filament formation. The authors show that in the yeast Schizosacchromyces pombe, the protein Cdc15p interacts directly with both the formin Cdc12p and an Arp2/3 complex regulator. Both Cdc12p and the Arp2/3 complex are essential for forming the cytokinetic actomyosin ring, a structure required for cell cleavage. The Cdc15p–Cdc12p complex appears in a medial structure in cells before ring formation, and Cdc15p is also required for the medial localization of Arp2/3 complex regulators. Cdc15p is highly phosphorylated in interphase, but becomes dephosphorylated early in mitosis.

The authors propose that dephosphorylation of Cdc15p allows it to associate with Cdc12p and initiate formation of a primary F-actin ring during metaphase. Arp2/3 could then join the complex, driving the maturation of the ring in late anaphase. In interphase, Cdc15p is distributed in a pattern very similar to that of actin patches, raising the possibility that it also coordinates cytoskeletal rearrangements at other times in the cell cycle.

As Cdc15p is the founding member of a highly conserved family of proteins, similar mechanisms are likely to be at work in many types of eukaryotic cells. Cdc15p may simply be a scaffold that brings Cdc12p and Arp2/3 together, or it could also have a catalytic function. ▪