Transient septin sumoylation steers a Fir1-Skt5 protein complex between the split septin ring

Muller et al. uncover a role for septin sumoylation. Sumyolated septins recruit the Fir1-Skt5 complex to the septin hourglass. During cytokinesis, Fir1-Skt5 is released from the septins into the newly created space between the two rings. There, Fir1-Skt5 stimulates chitin synthase III for septum formation. Ubiquitylation and phosphorylation control composition and architecture of the cell separation machinery in yeast and other eukaryotes. The significance of septin sumoylation on cell separation remained an enigma. Septins form an hourglass structure at the bud neck of yeast cells that transforms into a split septin double ring during mitosis. We discovered that sumoylated septins recruit the cytokinesis checkpoint protein Fir1 to the peripheral side of the septin hourglass just before its transformation into the double-ring configuration. As this transition occurs, Fir1 is released from the septins and seamlessly relocates between the split septin rings through synchronized binding to the scaffold Spa2. Fir1 binds and carries the membrane-bound Skt5 on its route to the division plane where the Fir1-Skt5 complex serves as receptor for chitin synthase III.

Automated summary

Muller et al. have discovered the role of septin sumoylation in cell separation. Sumoylated septins recruit the Fir1-Skt5 complex, which stimulates chitin synthase III for septum formation. This study sheds light on the significance of septin sumoylation in cell separation, which was previously unclear.