Evidence for F-actin-dependent and -independent mechanisms involved in assembly and stability of the medial actomyosin ring in fission yeast

Cell division in a number of eukaryotes, including the fission yeast Schizo saccharomyces pombe, is achieved through a medially placed actomyosin-based contractile ring. Although several components of the actomyosin ring have been identified, the mechanisms regulating ring assembly are still not unde rstood. Here, we show by biochemical and mutational studies that the S.pomb e actomyosin ring component Cdc4p is a light chain associated with Myo2p, a myosin II heavy chain, Localization of Myo2p to the medial ring depended o n Cdc4p function, whereas localization of Cdc4p at the division site was in dependent of Myo2p, Interestingly, the actin-binding and motor domains of M yo2p are not required for its accumulation at the division site although th e motor activity of Myo2p is essential for assembly of a normal actomyosin ring, The initial assembly of Myo2p and Cdc4p at the division site requires a functional F-actin cytoskeleton, Once established, however, P-actin is n ot required for the maintenance of Cdc4p and Myo2p medial rings, suggesting that the attachment of Cdc4p and Myo2p to the division site involves prote ins other than actin itself.