Roles of Hof1p, Bni1p, Bnr1p, and Myo1p in cytokinesis in Saccharomyces cerevisiae

Cytokinesis in Saccharomyces cerevisiae occurs by the concerted action of t he actomyosin system and septum formation. Here we report on the roles of H OF1, BNI1, and BNR1 in cytokinesis, focusing on Hof1p. Deletion of HOF1 cau ses a temperature-sensitive defect in septum formation. A Hof1p ring forms on the mother side of the bud neck in G2/M, followed by the formation of a daughter-side ring. Around telophase, Hof1p is phosphorylated and the doubl e rings merge into a single ring that contracts slightly and may colocalize with the actomyosin structure. Upon septum formation, Hof1p splits into tw o rings, disappearing upon cell separation. Hof1p localization is dependent on septins but not Myo1p. Synthetic lethality suggests that Bni1p and Myo1 p belong to one functional pathway, whereas Hof1p and Bnr1p belong to anoth er. These results suggest that Hof1p may function as an adapter linking the primary sept-um synthesis machinery to the actomyosin system. The formatio n of the actomyosin ring is not affected by bni1 Delta, hof1 Delta, or bnr1 . However, Myo1p contraction is affected by bni1 Delta but not by hof1 Delt a or bnr1 Delta. Ln bni1 Delta cells that lack the actomyosin contraction, septum formation is often slow and asymmetric, suggesting that actomyosin c ontraction may provide directionality for efficient septum formation.