Fission yeast divide by constricting a myosin-rich medial ring and then septating. Defects in the septation initiation defective (Sid) group of proteins prevent both events. Sparks et al. (page 777) find that one of these proteins, Sid2p, is found at both the spindle pole body and the medial ring. Sid2p may carry a signal that anaphase is completed from the spindle pole to the medial ring, thus triggering cytokinesis.
Sid2p is at the spindle poles throughout the cell cycle, but appears at the medial ring only at the end of anaphase. Its appearance there depends on the formation of the medial ring, the presence of the medial ring protein Cdc15p, and an intact tubulin cytoskeleton. In cells that have neither microtubules nor a mitotic checkpoint, ∼10-fold less Sid2p reaches the medial ring, probably by diffusion.
In wild-type cells Sid2p...
