Polarity determinants Tea1p, Tea4p, and Pom1p inhibit division-septum assembly at cell ends in fission yeast.

Correct positioning of the cell-division plane is crucial for cell function in all organisms. The fission yeast Schizosaccharomyces pombe divides by utilizing an actomyosin-based contractile ring and is an attractive model for the study of cytokinesis. The metazoan anillin-related protein Mid1p stimulates medial assembly of the division septum by recruiting actomyosin-ring components to the medial cortex. Here, we describe an inhibitory mechanism, involving the cell-end-localized polarity determinants Tea1p, Tea4p/Wsh3p, and Pom1p (tip complex), which prevents division-septum assembly at the cell ends. While Mid1p and the tip complex are dispensable for cell viability, their simultaneous loss leads to lethality. The FER/CIP homology protein Cdc15p, which organizes the actomyosin ring and cell membranes during cytokinesis, is a candidate for regulation by the tip complex. Since dual regulation of division-site placement is also seen in nematodes, such regulation might be a general feature of eukaryotic cytokinesis.