The waistline in Schizosaccharomyces pombe is positioned by a belt-like ring of mid1p at the cell's midpoint. Mid1p, located in the plasma membrane, then recruits myosin and other contractile ring proteins to separate the cell into two.
Padte and colleagues predicted by computer simulation that mid1p is in the middle because its diffusion to the poles is forbidden. They found that, in yeast cells lacking a polar kinase called pom1p, mid1p was no longer in a band around the middle, but was instead spread out. This in turn caused misplaced or multiple myosin contractile rings to form and the yeast to divide asymmetrically.
Spreading of mid1p in pom1p-deficient cells was only seen in one direction, however. Although pom1p is located at both poles, it is usually enriched at one. What prevents mid1p from creeping toward the other pole is currently under investigation.