Fig. 10. A model for F-actin ring formation in the wild-type S. pombe cell. F-actin cables are elongated between both ends of the cell during interphase. As positional signal(s) is generated from unseparated SPBs and/or nucleus at pre-prophase, formation of an aster-like structure is initiated from a spot during prophase, mediated by Cdc12 or other unknown proteins. This process includes branching of the F-actin cable, which is closely associated with the SPBs. During metaphase, the aster formation and accumulation of the F-actin cables progress at the medial region. A leading F-actin cable extends from the aster and encircle the cytoplasm at the equator. Thus a primary F-actin ring is formed. Cdc12 may be involved in this primary ring formation. A thick F-actin ring is formed during anaphase by packing and fusion of the accumulated F-actin cables with the leading cable. Cdc15 may play a role in this process. During cytokinesis, the F-actin ring initiates contraction being regulated by the Spg1-Cdc7-Sid2 pathway, and then F-actin cables reappear.