The origin of septin ring size control in budding yeast

The size of organelles and cellular structures needs to be tightly controlled and adjusted to the overall cell size to ensure cell function. A prominent example of a self-assembly process forming a structure whose size scales with cell size is Cdc42-driven cell polarization and subsequent septin ring formation in Saccharomyces cerevisiae . Despite extensive research, the mechanisms that determine Cdc42 cluster and septin ring size are still unclear. Combining computational modeling, live-cell imaging and genetic manipulations, we show here that positive feedback in the polarization pathway, together with the amount of polarity proteins increasing with cell size, can account for the increase of the Cdc42 cluster area with cell size, and as a consequence also the scaling of the septin ring diameter. We demonstrate that in bni1Δ cells, which have a larger septin ring compared to wild-type cells but a similarly sized Cdc42-GTP cluster, disruption of F-actin cable assembly and polarization toward the bud site results in diffuse exocytosis, causing septin ring enlargement. Furthermore, we find that in cells with impaired negative feedback in the polarization pathway, the Cdc42-GTP cluster area expands compared to wild-type cells, while the septin ring diameter remains mostly unchanged. This decoupling can be partially explained by a high recruitment rate of septin via polarity factors. Taken together, we provide insights into the origin of septin ring size control, in particular the scaling with cell volume, by integrating new experimental findings and mechanistic models of budding yeast polarization.