Septin- und Aktomyosindynamik während der Zellteilung von Ustilago maydis

Septins are conserved GTP-binding proteins essential for cytokinesis in animal and fungal cells. The filament-forming septins were discovered in the yeast Saccharomyces cerevisiae where they undergo a complex structural reorganization during budding growth. Septin filaments align in longitudinal orientation along the mother-bud axis and form an hourglass- or collar-like structure. During cytokinesis the septin collar is converted into two rings with circumferential orientation of filaments. The molecular mechanism of this dynamic structural transition is yet unknown. We could show that septin collar-to-ring transition in budding cells of the dimorphic fungus Ustilago maydis involves disassembly and reassembly of septin filaments. We used a chemical genetic approach to arrest dividing cells with a stable septin collar. We observed that at this stage the essential myosin light chain Cdc4 but not the FCH domain protein Cdc15 is associated with the septin collar. Inhibitor release results in instantaneous disassembly of the septin collar, while Cdc4 recruits Cdc15 from the cytoplasm to form the contractile actomyosin ring. Reassembly of septin filaments into a ring-like structure occurred upon constriction of the actomyosin ring. We propose that septin filaments exert a dual function in budding cells. Septin filaments in longitudinal orientation confer mechanical stability of the mother-bud neck while circumferential orientation of septin filaments is required for their function during cytokinesis.