Figure 7.

A summary of our data regarding Mid1p-dependent contractile-ring assembly in fission yeast. (A and B) Contractile rings are assembled from Cdc12p nodes and actin filaments growing in random directions from the nodes through the myosin-II motor activity. Cdc12p molecules in speckles and/or diffused in cytoplasm are gradually recruited into the division machinery as the nodes mature and condense. When the ring constricts, Cdc12p molecules are released back to the cytoplasm as the numbers of speckles increase. The stages of cytokinesis are the same as in Figure 1F. (A) The majority of cells contain neither spots nor aster/leading cable-like structures. (B) Spots in some cells travel to the division site and usually dissolve before node maturation and ring assembly. As nodes condense into a contractile ring, structures like the aster and leading cable are occasionally formed from short actin filaments but not from the spot. Around the time of cell separation, new spots may form.