Scd1 diffuses end to end along the cytoplasm to facilitate Cdc42 activation and bipolar growth

The conserved GTPase Cdc42 is a major regulator of polarized growth in most eukaryotes. In Schizosaccharomyces pombe , Cdc42 activity displays anticorrelated oscillatory dynamics between the growing ends enabling bipolarity. Cdc42 at each end is activated only when the opposite end loses activity. This suggests that a regulator of Cdc42 likely travels end-to-end to activate Cdc42. The oscillatory dynamics between the growing ends have also been observed in Cdc42 activator Scd1, its scaffold Scd2. It is unclear how these proteins move between the ends to facilitate bipolarity. We find that Scd1 does not travel between the cell ends via actin-mediated delivery. Instead, we show that Scd1 is mostly cytoplasmic and diffuses between the cell ends. The rate of diffusion is not entirely proportional to increasing the mass of Scd1 and cells lacking the inhibitor Pak1 kinase show decreased diffusion. Moreover, we show that Scd1 diffuses at a much faster rate compared to its scaffold Scd2. These findings suggest that Scd1 diffusion is not random and is regulated by Pak1 kinase. We find that decreasing the rate of diffusion disrupts Cdc42 oscillatory dynamics and results in monopolarity. Our results show that end-to-end Scd1 diffusion drives Cdc42 oscillatory dynamics and regulates cell polarity.