Distinct Cdc42 protein levels differentially regulate polarized growth and cell fusion in Schizosaccharomyces pombe

The conserved Cdc42 GTPase is a key driver of symmetry breaking and polarized growth, forming zones of activity that locally recruit effectors to organize the cytoskeleton and polarize secretion. Here, we show that Cdc42 also functions in cell-cell fusion during Schizosaccharomyces pombe sexual reproduction, but concentrates at the fusion site through mechanisms distinct from those proposed in Saccharomyces cerevisiae. Notably, the cdc42-mCherrySW allele, which is functional for cell polarization and has been used across organisms for dynamic studies, exhibits a strong fusion defect. These cells block fusion before cell wall digestion but after actin fusion focus formation, indicating that Cdc42 is required to translate the vesicle cluster into polarized cargo delivery. We trace the defect to instability of Cdc42-mCherrySW and demonstrate that mating and cell fusion require higher Cdc42 protein levels than mitotic polarized growth. Remarkably, by constructing an allelic series driving Cdc42 expression over a 5-fold range, we discover that mitotic polarized growth responds linearly to Cdc42 protein levels, whereas mating exhibits a sharp switch-like response. We further trace this all-or-none response to pheromone-induced polarized growth. Thus, polarized growth in response to intrinsic or extrinsic cues exhibits distinct requirements to Cdc42 protein levels.