Dual modes of cdc42 recycling fine-tune polarized morphogenesis.

In budding yeast, the highly conserved small GTPase Cdc42 localizes to the cortex at a cell pole and orchestrates the trafficking and deposition of cell surface materials required for building a bud or mating projection (shmoo). Using a combination of quantitative imaging and mathematical modeling, we elucidate mechanisms of dynamic recycling of Cdc42 that balance diffusion. Rdi1, a guanine nucleotide dissociation inhibitor (GDI), mediates a fast recycling pathway, while actin patch-mediated endocytosis accounts for a slower one. These recycling mechanisms are restricted to the same region of the nascent bud, as both are coupled to the Cdc42 GTPase cycle. We find that a single dynamic parameter, the rate of internalization inside the window of polarized delivery, is tuned to give rise to distinct shapes of Cdc42 distributions that correlate with distinct morphogenetic fates, such as the formation of a round bud or a pointed shmoo.