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

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. Subsequent de-sumoylation and synchronized binding to the scaffold Spa2 relocate Fir1 in a seamless transition between the split septin rings. Fir1 binds and carries Skt5 on its route to the division plane where the Fir1-Skt5 complex serves as receptor for chitin synthase III. We propose that the opposite positioning of the sumoylated septins and Spa2 creates a tension across the ring that upon de-sumoylation tunnels the membrane-bound Fir1-Skt5 complex through a transiently permeable septin diffusion barrier.